Corowa Howlong Mulwala Flood Study Report

FEDERATION COUNCIL

COROWA, HOWLONG AND MULWALA FLOOD STUDY

DRAFT FINAL REPORT

Level 2, 160 Clarence Street Sydney, NSW, 2000

Tel: (02) 9299 2855

Fax: (02) 9262 6208

Email: wma@wmawater.com.au

Web: www.wmawater.com.au

COROWA, HOWLONG AND MULWALA FLOOD STUDY

Project Corowa, Howlong and Mulwala Flood Study Project

Client

Cover Photo: John Foord Bridge (Bridge Road) over the Murray River at Corowa, looking upstream, November

This document, ‘Corowa, Howlong and Mulwala Flood Study’ (2024), is licensed under the Creative Commons Attribution 4.0 Licence, unless otherwise indicated.

Please give attribution to: © Federation Council (2024)

We also request that you observe and retain any notices that may accompany this material as part of the attribution.

Notice Identifying Other Material and/or Rights in this Publication:

The author of this document has taken steps to both identify third-party material and secure permission for its reproduction and reuse. However, please note that where these third-party materials are not licensed under a Creative Commons licence, or similar terms of use, you should obtain permission from the rights holder to reuse their material beyond the ways you are permitted to use them under the Copyright Act 1968. Please see the Table of References at the rear of this document for a list identifying other material and/or rights in this document.

Further Information

For further information about the copyright in this document, please contact:

Federation Council

100 Edward St, Corowa NSW 2646 council@federationcouncil.nsw.gov.au (02) 6033 8999

DISCLAIMER

The Creative Commons Attribution 4.0 Licence contains a Disclaimer of Warranties and Limitation of Liability. In addition: This document (and its associated data or other collateral materials, if any, collectively referred to herein as the ‘document’) were produced by WMAwater for Federation Council only. The views expressed in the document are those of the author(s) alone, and do not necessarily represent the views of the Federation Council. Reuse of this study or its associated data by anyone for any other purpose could result in error and/or loss. You should obtain professional advice before making decisions based upon the contents of this document.

COROWA, HOWLONG AND MULWALA FLOOD STUDY

2.1.3.

3.1.2. Murray River Flood Plain Management Study (Gutteridge Haskins Davey 1986).......................................................................................................9

3.1.3. Yarrawonga Weir Review of Flood Security (Australian Dams Alliance, 1999).....................................................................................................10

3.1.4. Report for Howlong Drainage

(GHD 2008) .................................10

3.1.5. Floodplain Risk Management Study – Corowa, Howlong & Mulwala and Floodplain Risk Management Plan – Corowa, Howlong & Mulwala (URS 2009) 11

3.1.6. Murray River Regional Flood Study Dicks/Seppelts levees to downstream of the Ulupna Creek confluence (Water Technology 2011)....................12

3.1.7. Albury City to Greater Hume Murray River Flood Study (GHD 2012)........13

3.1.8. Corowa Shire Local Flood Plan (NSW SES 2015)....................................13

3.1.9. Yarrawonga Weir Flood Incident Management Plan (MDBA 2016) 14

3.1.10. Albury Floodplain Risk Management Study and Plan (WMAwater 2016)14

3.1.11. South Corowa Stormwater Drainage Strategy (Spiire 2018) 14

6.6.

6.7.

7.3.1.

8.2.2.

8.4.3.

APPENDIX

APPENDIX E.

APPENDIX

APPENDIX

APPENDIX H.

APPENDIX I. MULWALA DESIGN FLOOD MAPS

APPENDIX J. DESIGN FLOOD RESULT TABLES

APPENDIX K. DESIGN FLOOD RESULT GRAPHS

APPENDIX L. SENSITIVITY RESULTS

Table

Table

Table

Table

Table 5: Murray River Historic Events and Design Estimates (Gutteridge Haskins Davey 1986) for Corowa and Yarrawonga ..........................................................................................................10

Table 6: Murray River Historic Events and Design Estimates (URS 2009) for Corowa..............11

Table 7: Murray River Historic Events and Design Estimates (URS 2009) for Yarrawonga Weir12

Table

Table

Table

Table

Table

Table

Table

Table

Table

Table 21: Comparison of peak levels and flows at the gauges for the Murray River 1992 event57

Table 22: Comparison of peak levels and flows at the gauges for the Murray River 1996 event59

Table 23: Comparison of peak levels and flows at the gauges for the Murray River 2016 event61

Table 24: Comparison of peak levels and flows at the gauges

Table

Table

Table

Table 31: Floods at Hume Reservoir

Table 44: Corowa Comparison of FFA Flows (ML/d) and Levels (m) ........................................94

Table 45: Corowa Gauge Design Flood Levels (m) 95

Table 46: Peechelba Design Peak Flows (ML/d) with GEV and LPIII Distributions 98

Table 47: Adopted Peechelba Design Peak Flows (ML/d) 99

Table 48: Peechelba and Wangaratta Comparison of FFA Flows (ML/d)..................................99

Table 49: Yarrawonga Comparison of Annual Peak Flows (ML/d) for Significant Events (19051979) 101

Table 50: 1917 Event modelling Results 103

Table 51: Yarrawonga Historic Event Gauge Heights (m) 104

Table 52: Yarrawonga 1% AEP Flow Estimate Sensitivity to 1917 Flow and Corresponding High Flow Censoring Threshold (ML/d) 104

Table 53: Large flood events at Yarrawonga (pre-weir) and corresponding flows for the Murray River and Ovens River 105

Table 54: Yarrawonga Design Peak Flows (ML/d) with GEV and LPIII Distributions 105

Table 55: Adopted Yarrawonga Design Peak Flows (ML/d) ....................................................106

Table 56: Yarrawonga Annual Exceedance Probability of Historic and Calibration Events 106

Table 57: Yarrawonga Comparison of FFA Flows (ML/d) 107

Table 58: Yarrawonga Gauge Design Flood Levels (m) 108

Table 59: Coincidence of Murray River and Ovens River Floods ............................................109

Table 60: Adopted Peak Design Inflows for the Murray River TUFLOW Model 110

Table 61: Summary of downstream catchment areas and contributing peak PMF flows rates 113

Table 62: Design rainfall depths (mm) at the centroid of the Howlong catchment (including Majors Creek) 116

Table 63: Design rainfall depths (mm) at the centroid of the Corowa catchment

Table 64: Design rainfall depths (mm) at the centroid of the Mulwala

Table 65: GSDM

Table

Table 67: Adopted

Table

Table

Table

Table

Table

Table

Table

Table

Table

Table

Table 88: Summary of Estimated Overland Flow Flood Damages at Corowa..........................172

Table 89: Summary of Estimated Murray River Flood Damages at Mulwala 173

Table 90: Summary of Estimated Overland Flow Flood Damages at Mulwala 173

APPENDIX E

Table E1: Doctors Point AMS (ML/d)

Table E2: Corowa AMS (ML/d) ................................................................................................E.4

Table E3: Ovens River at Peechelba AMS (ML/d)....................................................................E.6

Table E4: Murray River at Yarrawonga AMS (ML/d)

APPENDIX J

Table J1: Peak Flood Levels (mAHD) at Key Locations

Table J2: Peak Flood Depths (m) at Key Locations

Table J3: Peak Flows (m3/s) at Key Locations

LIST OF PHOTOGRAPHS

Photo 1: Flood Marks on concrete pole outside Council Chambers at Corowa 27

Photo 2: Howlong Aerial Imagery in 2010 (top) and 2018 (bottom) (Source: Nearmap) 81

Photo 3: Corowa Aerial Imagery in 2010 (top) and 2018 (bottom) (Source: Nearmap)..............82

LIST OF DIAGRAMS

Diagram 1: Histogram of the difference between the 2020 LiDAR dataset and the SCIMS points 18

Diagram 2: Available daily rainfall gauge records for the study area .........................................23

Diagram 3: Assumed Yarrawonga Weir Gate Operations in the TUFLOW model 43

Diagram 4: Radar image of the storm over Corowa at 12:49am on 30th January 2022 (Source: theweatherchaser.com) 49

Diagram 5: 2016 event recorded flow hydrographs at Doctors Point and Corowa.....................51

Diagram 6: Inflow hydrographs for the 1974 flood event ...........................................................52

Diagram 7: Inflow hydrographs for the 1975 flood event 54

Diagram 8: Inflow hydrographs for the 1992 flood event 56

Diagram 9: Inflow hydrographs for the 1996 flood event 58

Diagram 10: Inflow hydrographs for the 2016 flood event .........................................................60

Diagram 11: Comparison actual and simulate Yarrawonga Weir gate operations for the October 2016 event 62

Diagram 12: Comparison of recorded flow hydrographs on the Murray River and its tributaries, in additional to daily rainfall for the November 2022 event 64

Diagram 13: Inflow hydrographs for the 2022 flood event .........................................................65

Diagram 14: Comparison actual and simulate Yarrawonga Weir gate operations for the November 2022 event 67

Diagram 15: Comparison of the 1996 and 2016 rating curves at Doctors Point gauge 77

Diagram 16: Comparison of the 1996 and 2016 rating curves at Corowa gauge 77

Diagram 17: Estimated 2016 flow hydrographs at Doctors Point using both the 1996 and 2016

Diagram 18: Doctors Point Gauge Rating Curves and Gaugings

Diagram 19: Corowa Gauge Rating Curves and Gaugings

Diagram 20: Adopted Ovens River flow correlation between Wangaratta and Peechelba 97

Diagram 21: Murray River Design Flood Hydrographs for the 20% AEP to 0.2% AEP Events 112

Diagram 22: Ovens River Design Flood Hydrographs for the 20% AEP to 0.2% AEP Events 112

Diagram 23: Schematic of PMF Inflows for the Murray River

Diagram 24: Design Flood Hydrographs for the PMF Event

Diagram 25: Temporal Pattern Bins

Diagram 26: Design modelling techniques for an ensemble of temporal patterns (Ball et al 2019)

Diagram 27: Yarrawonga Weir Operation in the 1% AEP event

Diagram 28: Yarrawonga Weir Stage-Discharge Curve Comparison

Diagram 29: General flood hazard vulnerability curves (Source: Ball et al 2019)

Diagram 30: Flow Chart for Determining Flood Emergency Response Classifications (Commonwealth of Australia 2017)

Diagram 31: Flood Damages Categories (including damage and losses from permanent inundation) 164

Figure 1: Study Area - Overview

Figure 2: Study Area - Howlong

Figure 3: Study Area - Corowa

Figure 4: Study Area - Mulwala

Figure 5: LiDAR Data Extents

LIST OF FIGURES

Figure 6: Comparison of LiDAR data with survey

Figure 7: Gauge Data

Figure 8: Council Flood Data

Figure 9: Community Questionnaire Responses

Figure 10: Community Drop-In session Responses

Figure 11: Howlong Subcatchments

Figure 12: Corowa Subcatchments

Figure 13: Mulwala Subcatchments

Figure 14: TUFLOW Model Domains

Figure 15: Murray River Model Terrain

Figure 16: Howlong Model Terrain

Figure 17: Corowa Model Terrain

Figure 18: Mulwala Model Terrain

Figure 19: Murray River Model Surface Roughness

Figure 20: Howlong Model Surface Roughness

Figure 21: Corowa Model Surface Roughness

Figure 22: Mulwala Model Surface Roughness

Figure 23: Murray River Model Hydraulic Structures

Figure 24: Howlong Model Hydraulic Structures

Figure 25: Corowa Model Hydraulic Structures

Figure 26: Mulwala Model Hydraulic Structures

Figure 27: Cumulative Rainfall for the March 2012 Storm - Howlong

Figure 28: Rainfall Burst Intensities and Frequencies for the March 2012 Storm - Howlong

Figure 29: Cumulative Rainfall for the March 2012 Storm - Mulwala

Figure 30: Rainfall Burst Intensities and Frequencies for the March 2012 Storm - Mulwala

Figure 31: Cumulative Rainfall for the January 2022 Storm - Corowa

Figure 32: Rainfall Burst Intensities and Frequencies for the January 2022 Storm - Corowa

Figure 33: FFA at Doctors Point demonstrating the effect of censoring low flows

Figure 34: FFA at Doctors Point demonstrating the effect of censoring high flows

Figure 35: FFA at Doctors Point demonstrating the effect of probability distributions

Figure 36: Adopted FFA at Doctors Point

Figure 37: Adopted FFA at Corowa

Figure 38: Adopted FFA at Peechelba

Figure 39: Adopted FFA at Yarrawonga

Figure 40: Howlong Critical Duration – 20% AEP Event

Figure 41: Howlong Adopted Storm Peak Level versus Critical Duration Mean Peak Level – 20% AEP Event

Figure 42: Howlong Critical Duration – 5% AEP Event

Figure 43: Howlong Adopted Storm Peak Level versus Critical Duration Mean Peak Level – 5% AEP Event

Figure 44: Howlong Critical Duration – 1% AEP Event

Figure 45: Howlong Adopted Storm Peak Level versus Critical Duration Mean Peak Level – 1% AEP Event

Figure 46: Howlong Critical Duration – PMF Event

Figure 47: Corowa Critical Duration – 20% AEP Event

Figure 48: Corowa Adopted Storm Peak Level versus Critical Duration Mean Peak Level – 20% AEP Event

Figure 49: Corowa Critical Duration – 5% AEP Event

Figure 50: Corowa Adopted Storm Peak Level versus Critical Duration Mean Peak Level – 5% AEP Event

Figure 51: Corowa Critical Duration – 1% AEP Event

Figure 52: Corowa Adopted Storm Peak Level versus Critical Duration Mean Peak Level – 1% AEP Event

Figure 53: Corowa Critical Duration – PMF Event

Figure 54: Mulwala Critical Duration – 20% AEP Event

Figure 55: Mulwala Adopted Storm Peak Level versus Critical Duration Mean Peak Level – 20% AEP Event

Figure 56: Mulwala Critical Duration – 5% AEP Event

Figure 57: Mulwala Adopted Storm Peak Level versus Critical Duration Mean Peak Level – 5% AEP Event

Figure 58: Mulwala Critical Duration – 1% AEP Event

Figure 59: Mulwala Adopted Storm Peak Level versus Critical Duration Mean Peak Level – 1% AEP Event

Figure 60: Mulwala Critical Duration – PMF Event

Figure 61: Howlong Reporting Locations

Figure 62: Corowa Reporting Locations

Figure 63: Mulwala Reporting Locations

Figure 64: Howlong Property Database – Event Flooded Above Floor due to Murray River

Figure 65: Corowa Property Database – Event Flooded Above Floor due to Murray River

Figure 66: Mulwala Property Database – Event Flooded Above Floor due to Murray River

Figure 67: Howlong Property Database – Event Flooded Above Floor due to Local Overland

Figure 68: Corowa Property Database – Event Flooded Above Floor due to Local Overland

Figure 69: Mulwala Property Database – Event Flooded Above Floor due to Local Overland

APPENDIX D: CALIBRATION RESULTS

Figure D1: Murray River 1974 Event Calibration to Gauge Levels

Figure D2: Murray River 1974 Event Calibration to Flood Data

Figure D3: Murray River 1975 Event Calibration to Gauge Levels

Figure D4: Murray River 1975 Event Calibration to Flood Data

Figure D5: Murray River 1992 Event Calibration to Gauge Levels

Figure D6: Murray River 1992 Event Calibration to Flood Data

Figure D7: Murray River 1996 Event Calibration to Gauge Levels

Figure D8: Murray River 1996 Event Calibration to Flood Data

Figure D9: Murray River 2016 Event Calibration to Gauge Levels

Figure D10: Murray River 2016 Event Calibration to Flood Data

Figure D11: Murray River 2016 Event Calibration to Photos – Corowa Caravan Park

Figure D12: Murray River 2016 Event Calibration to Photos – Corowa Terrace

Figure D13: Murray River 2016 Event Calibration to Photos – Corowa Bindaree Holiday Park

Figure D14: Murray River 2022 Event Calibration to Gauge Levels

Figure D15: Murray River 2022 Event Calibration to Flood Data

Figure D16: Corowa January 2022 Event Calibration to Flood Marks

Figure D17: Corowa January 2022 Event Calibration to Questionnaire Responses

Figure D18: Corowa January 2022 Event Calibration to Drop-in Session Responses

Figure D19: Corowa January 2022 Event Calibration to SES Assessments

Figure D20: Corowa January 2022 Event Calibration to Council Service Requests

Figure D21: Corowa January 2022 Event Calibration to Drone Photography – Industrial Area

Figure D22: Corowa January 2022 Event Calibration to Drone Photography – Aerodrome

Figure D23: Corowa January 2022 Event Calibration to Drone Photography – Skehans Lane

Figure D24: Howlong March 2012 Event Results

Figure D25: Mulwala March 2012 Event Results

Figure D26: Murray River 1917 Event Modelling

APPENDIX F: MURRAY RIVER DESIGN FLOOD MAPS

Figure F1: Peak Flood Depth and Level – 20% AEP Event

Figure F2: Peak Flood Depth and Level – 10% AEP Event

Figure F3: Peak Flood Depth and Level – 5% AEP Event

Figure F4: Peak Flood Depth and Level – 2% AEP Event

Figure F5: Peak Flood Depth and Level – 1% AEP Event

Figure F6: Peak Flood Depth and Level – 0.5% AEP Event

Figure F7: Peak Flood Depth and Level – 0.2% AEP Event

Figure F8: Peak Flood Depth and Level – PMF Event

Figure F9: Peak Flood Velocity – 20% AEP Event

Figure F10: Peak Flood Velocity – 10% AEP Event

Figure F11: Peak Flood Velocity – 5% AEP Event

Figure F12: Peak Flood Velocity – 2% AEP Event

Figure F13: Peak Flood Velocity – 1% AEP Event

Figure F14: Peak Flood Velocity – 0.5% AEP Event

Figure F15: Peak Flood Velocity – 0.2% AEP Event

Figure F16: Peak Flood Velocity – PMF Event

Figure F17: Peak Hydraulic Hazard – 5% AEP Event

Figure F18: Peak Hydraulic Hazard – 1% AEP Event

Figure F19: Peak Hydraulic Hazard – PMF Event

Figure F20: Hydraulic Categories – 1% AEP Event

Figure F21: Flood Emergency Response Classification of Communities

Figure F22: Preliminary Flood Planning Area

Figure F23: Preliminary PMF Extent for Flood Planning Controls

Figure F24: Flood Planning Constraint Categories

APPENDIX G: HOWLONG DESIGN FLOOD MAPS

Figure G1: Peak Flood Depth and Level – 20% AEP Event

Figure G2: Peak Flood Depth and Level – 10% AEP Event

Figure G3: Peak Flood Depth and Level – 2% AEP Event

Figure G4: Peak Flood Depth and Level – 5% AEP Event

Figure G5: Peak Flood Depth and Level – 1% AEP Event

Figure G6: Peak Flood Depth and Level – 0.5% AEP Event

Figure G7: Peak Flood Depth and Level – 0.2% AEP Event

Figure G8: Peak Flood Depth and Level – PMF Event

Figure G9: Peak Flood Velocity – 20% AEP Event

Figure G10: Peak Flood Velocity – 10% AEP Event

Figure G11: Peak Flood Velocity – 2% AEP Event

Figure G12: Peak Flood Velocity – 5% AEP Event

Figure G13: Peak Flood Velocity – 1% AEP Event

Figure G14: Peak Flood Velocity – 0.5% AEP Event

Figure G15: Peak Flood Velocity – 0.2% AEP Event

Figure G16: Peak Flood Velocity – PMF Event

Figure G17: Peak Hydraulic Hazard – 5% AEP Event

Figure G18: Peak Hydraulic Hazard – 1% AEP Event

Figure G19: Peak Hydraulic Hazard – PMF Event

Figure G20: Hydraulic Categories – 5% AEP Event

Figure G21: Hydraulic Categories – 1% AEP Event

Figure G22: Hydraulic Categories – PMF Event

Figure G23: Flood Emergency Response Classification of Communities

Figure G24: Preliminary Flood Planning Area

Figure G25: Preliminary PMF Extent for Flood Planning Controls

Figure G26: Flood Planning Constraint Categories

APPENDIX H: COROWA DESIGN FLOOD MAPS

Figure H1: Peak Flood Depth and Level – 20% AEP Event

Figure H2: Peak Flood Depth and Level – 10% AEP Event

Figure H3: Peak Flood Depth and Level – 2% AEP Event

Figure H4: Peak Flood Depth and Level – 5% AEP Event

Figure H5: Peak Flood Depth and Level – 1% AEP Event

Figure H6: Peak Flood Depth and Level – 0.5% AEP Event

Figure H7: Peak Flood Depth and Level – 0.2% AEP Event

Figure H8: Peak Flood Depth and Level – PMF Event

Figure H9: Peak Flood Velocity – 20% AEP Event

Figure H10: Peak Flood Velocity – 10% AEP Event

Figure H11: Peak Flood Velocity – 2% AEP Event

Figure H12: Peak Flood Velocity – 5% AEP Event

Figure H13: Peak Flood Velocity – 1% AEP Event

Figure H14: Peak Flood Velocity – 0.5% AEP Event

Figure H15: Peak Flood Velocity – 0.2% AEP Event

Figure H16: Peak Flood Velocity – PMF Event

Figure H17: Peak Hydraulic Hazard – 5% AEP Event

Figure H18: Peak Hydraulic Hazard – 1% AEP Event

Figure H19: Peak Hydraulic Hazard – PMF Event

Figure H20: Hydraulic Categories – 5% AEP Event

Figure H21: Hydraulic Categories – 1% AEP Event

Figure H22: Hydraulic Categories – PMF Event

Figure H23: Flood Emergency Response Classification of Communities

Figure H24: Preliminary Flood Planning Area

Figure H25: Preliminary PMF Extent for Flood Planning Controls

Figure H26: Flood Planning Constraint Categories

APPENDIX I: MULWALA DESIGN FLOOD MAPS

Figure I1: Peak Flood Depth and Level – 20% AEP Event

Figure I2: Peak Flood Depth and Level – 10% AEP Event

Figure I3: Peak Flood Depth and Level – 2% AEP Event

Figure I4: Peak Flood Depth and Level – 5% AEP Event

Figure I5: Peak Flood Depth and Level – 1% AEP Event

Figure I6: Peak Flood Depth and Level – 0.5% AEP Event

Figure I7: Peak Flood Depth and Level – 0.2% AEP Event

Figure I8: Peak Flood Depth and Level – PMF Event

Figure I9: Peak Flood Velocity – 20% AEP Event

Figure I10: Peak Flood Velocity – 10% AEP Event

Figure I11: Peak Flood Velocity – 2% AEP Event

Figure I12: Peak Flood Velocity – 5% AEP Event

Figure I13: Peak Flood Velocity – 1% AEP Event

Figure I14: Peak Flood Velocity – 0.5% AEP Event

Figure I15: Peak Flood Velocity – 0.2% AEP Event

Figure I16: Peak Flood Velocity – PMF Event

Figure I17: Peak Hydraulic Hazard – 5% AEP Event

Figure I18: Peak Hydraulic Hazard – 1% AEP Event

Figure I19: Peak Hydraulic Hazard – PMF Event

Figure I20: Hydraulic Categories – 5% AEP Event

Figure I21: Hydraulic Categories – 1% AEP Event

Figure I22: Hydraulic Categories – PMF Event

Figure I23: Flood Emergency Response Classification of Communities

Figure I24: Preliminary Flood Planning Area

Figure I25: Preliminary PMF Extent for Flood Planning Controls

Figure I26: Flood Planning Constraint Categories

APPENDIX K: DESIGN FLOOD RESULT GRAPHS

Figure K1: Design Flood Event Profiles for the Murray River

Figure K2: Howlong-Burrumbuttock Road (H01)

Figure K3: Kywong-Howlong Road south (H04)

Figure K4: Riverina Highway at Majors Creek (H05)

Figure K5: Riverina Highway at Fairway Close (H06)

Figure K6: Larmer Street (H10)

Figure K7: Lowe Street (H11)

Figure K8: Holbeach Street (H13)

Figure K9: Townsend Street north (H14)

Figure K10: Jude Street (H15)

Figure K11: Hammer Street (H16)

Figure K12: Honour Avenue at Header World Avenue (C01)

Figure K13: Whitehead Street Wetland (C03)

Figure K14: Banksia Street north (C05)

Figure K15: Banksia Street south (C06)

Figure K16: Pinot Crescent (C07)

Figure K17: Sophia Close (C08)

Figure K18: Federation Avenue north (C09)

Figure K19: Nixon Street (C10)

Figure K20: Federation Avenue south (C11)

Figure K21: Martin Street (C12)

Figure K22: Roden Street (C14)

Figure K23: Boral Circuit (C19)

Figure K24: Dominion Road (C20)

Figure K25: Noresman Road (C21)

Figure K26: Tocumwal Road east (M01)

Figure K27: Corowa Road north (M02)

Figure K28: Corowa Road south (M03)

Figure K29: North Road (M04)

Figure K30: Lakeview Circuit (M05)

Figure K31: Savernake Road (M06)

Figure K32: Hicks Street (M07)

Figure K33: Edward Street (M08)

Figure K34: Jasmine Circuit (M09)

Figure K35: Payne Street (M10)

Figure K36: Havelock Street (M11)

Figure K37: Erne Street (M12)

Figure K38: Centenary Circuit (M13)

Figure K39: Lucan Street (M14)

Figure K40: Manners Street (M15)

Figure K41: Melbourne Street (M19)

Figure K42: Gulai Road north (M20)

Figure K43: Gulai Road south (M21)

Figure K44: Wandoo Circuit (M22)

APPENDIX L: SENSITIVITY RESULTS

Figure L1: Sensitivity No Initial Loss Howlong – 1% AEP Event

Figure L2: Sensitivity No Initial Loss Corowa – 1% AEP Event

Figure L3: Sensitivity No Initial Loss Mulwala – 1% AEP Event

Figure L4: Sensitivity Double Initial Loss Howlong – 1% AEP Event

Figure L5: Sensitivity Double Initial Loss Corowa – 1% AEP Event

Figure L6: Sensitivity Double Initial Loss Mulwala – 1% AEP Event

Figure L7: Sensitivity -50% Continuing Loss Howlong – 1% AEP Event

Figure L8: Sensitivity -50% Continuing Loss Corowa – 1% AEP Event

Figure L9: Sensitivity -50% Continuing Loss Mulwala – 1% AEP Event

Figure L10: Sensitivity +50% Continuing Loss Howlong – 1% AEP Event

Figure L11: Sensitivity +50% Continuing Loss Corowa – 1% AEP Event

Figure L12: Sensitivity +50% Continuing Loss Mulwala – 1% AEP Event

Figure L13: Sensitivity -20% Catchment Lag Corowa – 1% AEP Event

Figure L14: Sensitivity +20% Catchment Lag Corowa – 1% AEP Event

Figure L15: Sensitivity -20% Mannings Roughness Murray River – 1% AEP Event

Figure L16: Sensitivity -20% Mannings Roughness Howlong – 1% AEP Event

Figure L17: Sensitivity -20% Mannings Roughness Corowa – 1% AEP Event

Figure L18: Sensitivity -20% Mannings Roughness Mulwala – 1% AEP Event

Figure L19: Sensitivity +20% Mannings Roughness Murray River – 1% AEP Event

Figure L20: Sensitivity +20% Mannings Roughness Howlong – 1% AEP Event

Figure L21: Sensitivity +20% Mannings Roughness Corowa – 1% AEP Event

Figure L22: Sensitivity +20% Mannings Roughness Mulwala – 1% AEP Event

Figure L23: Sensitivity No Blockage Murray River – 1% AEP Event

Figure L24: Sensitivity No Blockage Howlong – 1% AEP Event

Figure L25: Sensitivity No Blockage Corowa – 1% AEP Event

Figure L26: Sensitivity No Blockage Mulwala – 1% AEP Event

Figure L27: Sensitivity High Blockage Murray River – 1% AEP Event

Figure L28: Sensitivity High Blockage Howlong – 1% AEP Event

Figure L29: Sensitivity High Blockage Corowa – 1% AEP Event

Figure L30: Sensitivity High Blockage Mulwala – 1% AEP Event

Figure L31: Sensitivity Downstream Boundary Slope x0.5 Murray River – 1% AEP Event

Figure L32: Sensitivity Downstream Boundary Slope x0.5 Corowa – 1% AEP Event

Figure L33: Sensitivity Downstream Boundary Slope x2 Murray River – 1% AEP Event

Figure L34: Sensitivity Downstream Boundary Slope x2 Corowa – 1% AEP Event

Figure L35: Sensitivity Climate Change Murray River 0.5% AEP Event Versus 1% AEP Event

Figure L36: Sensitivity Climate Change Howlong 0.5% AEP Event Versus 1% AEP Event

Figure L37: Sensitivity Climate Change Corowa 0.5% AEP Event Versus 1% AEP Event

Figure L38: Sensitivity Climate Change Mulwala 0.5% AEP Event Versus 1% AEP Event

Figure L39: Sensitivity Climate Change Murray River 0.2% AEP Event Versus 1% AEP Event

Figure L40: Sensitivity Climate Change Howlong 0.2% AEP Event Versus 1% AEP Event

Figure L41: Sensitivity Climate Change Corowa 0.2% AEP Event Versus 1% AEP Event

Figure L42: Sensitivity Climate Change Mulwala 0.2% AEP Event Versus 1% AEP Event

Figure L43: Sensitivity Murray River Historic Conditions Mannings Roughness– 1% AEP Event

Figure L44: Sensitivity Murray River Historic Conditions No Bridges – 1% AEP Event

Figure L45: Sensitivity Murray River Historic Conditions No Bridges or Embankments – 1% AEP Event

Figure L46: Sensitivity Murray River Historic Conditions No Bridges or Embankments or Weir (asis) – 1% AEP Event

Figure L47: Sensitivity Murray River Historic Conditions No Bridges or Embankments or Weir (Grassed) – 1% AEP Event

Figure L48: Sensitivity Murray River Historic Conditions No Bridges or Embankments or Weir (Vegetated) – 1% AEP Event

Figure L49: Sensitivity Murray River Historic Conditions 1917 (Grassed) – 1% AEP Event

Figure L50: Sensitivity Murray River Historic Conditions 1917 (Vegetated) – 1% AEP Event

Corowa, Howlong and Mulwala Flood Study

FOREWORD

The NSW State Government’s Flood Prone Land Policy, contained in the Flood Risk Management Manual (NSW Department of Planning and Environment 2023), provides a framework to ensure the sustainable use of floodplain environments. The Policy is specifically structured to provide solutions to existing flooding problems in rural and urban areas. In addition, the Policy provides a means of ensuring that any new development is compatible with the flood hazard and does not create additional flooding problems in other areas.

Under the Policy, the management of flood liable land remains the responsibility of local government. The State Government subsidises flood mitigation works to alleviate existing problems and provides specialist technical advice to assist Councils in the discharge of their floodplain management responsibilities.

The Policy provides for technical and financial support by the Government through five sequential stages:

This document constitutes the first and second stages of the management process for Federation Council. It presents a compilation of the data collected and has defined flood behaviour and flood risk for the towns of Howlong, Corowa and Mulwala.

This study was commissioned under the 2005 NSW Floodplain Development Manual (NSW Government 2005), however, it is recognised that the 2023 Flood Risk Management Manual (NSW Department of Planning and Environment 2023) was gazetted while the project was in progress. While the study was undertaken in accordance with the 2005 manual, there are elements that are consistent to both the 2005 and 2023 manuals. Where appropriate, the 2023 manual is referenced where project methodology or outputs are consistent with the new 2023 manual

Corowa, Howlong and Mulwala Flood Study

ACKNOWLEDGEMENTS

This study was undertaken by WMAwater Pty Ltd, on behalf of Federation Council. Federation Council has prepared this document with financial assistance from the NSW Government through its Floodplain Management Program. The document does not necessarily represent the opinions of the NSW Government or the Department of Climate Change, Energy, the Environment and Water

A number of organisations and individuals have contributed both time and valuable information to this study. The assistance of the following in providing data and/or guidance to the study is gratefully acknowledged:

• Corowa, Howlong and Mulwala Floodplain Risk Management Committee

• Residents of the study area

• Federation Council

• Department of Climate Change, Energy, the Environment and Water

• NSW State Emergency Service

Introduction

Corowa, Howlong and Mulwala Flood Study

EXECUTIVE SUMMARY

Federation Council (Council) engaged WMAwater to undertake the Corowa, Howlong and Mulwala Flood Study. The objective of this study is to improve understanding of flood behaviour and impacts to better inform management of flood risk in the study area. The study area includes the towns of Howlong, Corowa and Mulwala, as well as the intervening reaches of the Murray River. Both local overland flooding and mainstream flooding from the Murray River were investigated in this study.

Background

The towns are located on the banks of the Murray River, downstream of Hume Dam. Yarrawonga Weir impounds the Murray River and forms Lake Mulwala at the downstream end of the Murray River reach investigated in this study. The Murray River floodplain is typically well-defined, with the towns generally elevated above the floodplain. The town of Howlong consists of relatively flat, undulating terrain that results in low points that are susceptible to ponding of runoff. Majors Creek is a tributary of the Murray River and flows across a wide floodplain primarily to the west of the town. Corowa is situated relatively high above the Murray River, however, a lower terrace area is flood prone. Parts of the Corowa township are located on a small hill, although the land surrounding this hill is very flat – particularly to the north west where Corowa Airport is located. Mulwala is located on the banks of Lake Mulwala and is low-lying and very flat. It is susceptible to ponding of local runoff that is difficult to drain effectively.

The towns are affected by two separate flood mechanisms – Murray River flooding and local overland flooding. These generally occur independently of each other due to the storm mechanisms required to produce the relevant flooding and the difference in timing. Large Murray River flood events have occurred in 1867, 1870, 1917, 1974, 1975, 2016 and most recently 2022. Local overland events have occurred in 2012 and 2022. Both flood mechanisms were investigated in this study.

Available Data

As part of the data collection, WMAwater received the previous studies undertaken for the area and for the wider Murray River catchment as well as stormwater infrastructure from Council’s database, design drawings and a field survey. Topographic information was received from aerial, ground and bathymetric surveys. Gauge data was collected, including rainfall and stream flow data. Information about previous flood events in the area was obtained from Council which included surveyed flood marks, drone photography, service requests and SES assessments.

Community Consultation

At the commencement of the project, the community were informed of the study and provided the opportunity to contribute their observations of flooding within the catchment. A total of 36 responses to a community questionnaire were obtained, with approximately 80% indicating that they were affected by overland flooding. Most respondents referenced the Corowa overland flood event that occurred in January 2022. A community drop-in session was also held in each of the towns to gather information. The largest attendance was at Corowa, given the sessions were held

Howlong and Mulwala Flood Study

approximately one month after the January 2022 flood event in Corowa. Additional information was obtained from residents via these sessions.

Apart from residents with a creek at the rear of their property (or front, that is crossed by a driveway bridge/culvert), the majority of respondents described overland flow flood behaviour (i.e. the flood waters were not confined to a defined creek or channel). A large proportion of these also mention blockage of street drainage pits as contributing to flooding. The most common flood events mentioned were the recent events of 2020, 2021 and 2022.

Model Development

The models developed to simulate overland flood behaviour in the study area consist of a twostage process:

1. Hydrologic modelling using WBNM to convert rainfall to runoff

2. Hydraulic modelling using TUFLOW to estimate overland flow distributions, flood depths, levels and velocities.

Both of these models were developed for each of the towns to simulate overland flow flooding. Subcatchments were delineated to trapped low points, stormwater infrastructure or flow paths. Subcatchments were assigned an impervious fraction and a typical catchment lag factor was adopted in the WBNM models.

The TUFLOW hydraulic models cover each of the towns. The models consist of a 2 m by 2 m regular grid. The best available terrain and structure data was incorporated into the model, along with model adjustments to ensure that hydraulic features (including gutters and channels) were adequately represented. The simulated runoff hydrographs from the WBNM models were applied to the TUFLOW model as inflows.

A TUFLOW hydraulic model was also developed for the Murray River, consisting of a 20 m grid with sub-grid sampling. Topographic features such as embankments were enforced in the model and key structures including bridges and Yarrawonga Weir were included in the model. Inflows were derived from upstream stream gauge data for the Murray River and Ovens River.

Model Calibration

The flood events of 1974, 1975, 1992, 1996, 2016 and 2022 were utilised to undertake calibration of the Murray River model. Gauge data, flood marks, photography and observations were used to calibrate the model. It was found that two sets of roughness values were needed – one representing conditions prior to 2016, and another set with a higher floodplain roughness for the 2016 and 2022 events. This was validated through a detailed investigation of the change in the Corowa rating curve that occurred around 2016. The model results fit well with the observed data, including gauge levels and flows across all six events.

The January 2022 overland event was used to calibrate the Corowa models, with these parameters being applied to the Howlong and Mulwala models (due to lack of sufficient data to undertake model calibration in those towns). The model results for Corowa generally fit with the observed flood behaviour, although there is a slight overestimation of flood levels compared to surveyed levels. Calibration results are contained in Appendix D

Flood Frequency Analysis

Corowa, Howlong and Mulwala Flood Study

A flood frequency analysis (FFA) was undertaken at Doctors Point (409017), Corowa (409002) and Yarrawonga (409025). The adopted approach typically consisted of obtaining an annual maximum series of flows from the available gauge data, application of low flow censoring (30,000 ML/d), application of high flow censoring (for the 1867, 1870 and 1917 historic events) and utilising a Log Pearson Type III probability distribution. This resulted in 1% AEP peak flow of 250,000 ML/d at Doctors Point and Corowa, and 390,000 ML/d at Yarrawonga. FFA was also undertaken at Peechelba (403241) for the Ovens River. The peak flow rates from these analyses were used to derive inflows for the Murray River model.

Design Flood Modelling

Design flood modelling was undertaken in accordance with Australian Rainfall and Runoff (ARR) 2019 guidelines. For the Murray River, the 2016 event hydrograph was scaled using the peak flows derived from the FFA. For the probable maximum flood event (PMF), a Hume Dam design outflow hydrograph was utilised from a previous study.

For the local overland models, design rainfalls from the Bureau of Meteorology were adopted in the WBNM models. ARR 2019 requires an ensemble of temporal patterns to be run for each duration and these were simulated in the hydrologic and hydraulic model. The critical storm duration (duration that produces the highest flood level) was determined based on the mean of the 10 temporal patterns for each duration The storm duration was typically long (approximately 24 hours) for the towns due to the flood storage that dominates most of the towns. There were very few areas that are conveyance driven, where a shorter storm duration dominates.

The design flood events simulated were the 20%, 10%, 5%, 2%, 1%, 0.5% and 0.2% annual exceedance probability (AEP) events in addition to the PMF, for both the Murray River and local overland events.

Design Flood Results

Design flood depths, levels, velocities, hydraulic hazard and hydraulic categories were mapped and are provided in Appendix F for the Murray River, Appendix G for Howlong local overland flooding, Appendix H for Corowa local overland flooding and Appendix I for Mulwala local overland flooding. Flood results were also tabulated and plotted at key road crossings, with results presented in Appendix J and Appendix K

At Howlong, the Murray River only affects the town in the PMF event. In local overland events, low lying portions of the town are affected, including areas around Jude Street and Clarke Street. Majors Creek inundates a large portion of the floodplain to the east of the town and begins to impact the town in the 2% AEP event. Detailed results are provided at the gauges and structures crossing river. An investigation of model results for Yarrawonga Weir and Lake Mulwala was also undertaken.

At Corowa, the Murray River affects the lower terrace in frequent events and only affects the main town in the PMF event. In local overland events, trapped low points are affected, including Header World Avenue, the Whitehead Street Wetlands, Sophia Close and Pinot Crescent, around Corowa Airport, north of Nixon Street (near Cemetery Road) and isolated areas in south Corowa.

Corowa, Howlong and Mulwala Flood Study

At Mulwala, the Murray River begins to affect the town in the 2% AEP event, with breakout flows through the town occurring in the 1% AEP event. In local overland events, water begins ponding in frequent events around the new development north of Tocumwal Road, Payne Street and Hicks Street, Manners Street and Nyncoola Circuit. There are areas to the west of the town affected by trapped low points.

Flood emergency response planning was considered for each of the towns, for both Murray River and local overland flooding, including property inundation, road inundation and emergency response classification.

A flood planning area was defined for both the Murray River and local overland events and advice was provided considering the new modelling of overland flow affectation of the towns. Flood planning constraint categories were also mapped.

Sensitivity Analysis

A sensitivity analysis (Appendix L) was undertaken for key modelling parameters by varying the adopted values and assessing the change in peak flood levels. Peak flood levels are relatively insensitive to changes throughout the urban areas. Sensitivity analysis was also conducted on Lake Mulwala and the constrictions that structures such as bridges and the weir have on Murray River flood levels at Mulwala.

Economic Impacts of Flooding

A flood damage assessment was undertaken to determine the economic impact of flooding. A property database was developed with an estimation of floor levels. Flood damage curves were applied that estimate the cost of damage for a certain depth of inundation. Average annual damages (AAD) were estimated for each of the towns, with separate damage calculations undertaken for the Murray River and local overland events. At Howlong, the Murray River only causes damage in rare events, leading to a low AAD of approximately $130,000. For overland events, a large number of properties are estimated to be inundated, resulting in an AAD of $2.6M. At Corowa, the Murray River inundation of the lower terrace in frequent events primarily affects caravan parks, however, there is a damage cost associated with this. The AAD was estimated to be approximately $10.2M. Overland flow affectation had an AAD of approximately $1.8M. At Mulwala the Murray River breaks out in the 2% AEP event and flood damages were estimated to be $3.3M. For overland flooding, the AAD was approximately $2.2M.

1. INTRODUCTION

Corowa, Howlong and Mulwala Flood Study

The towns of Corowa, Howlong and Mulwala are situated on the banks of the Murray River on the southern border of NSW. The towns lie within the Federation Council (Council) Local Government Area (LGA). Council, with assistance from the NSW Department of Climate Change, Energy, the Environment and Water (DCCEEW) commissioned WMAwater to undertake a flood study for these towns, as well as the intervening reaches of the Murray River. The study area can be seen in Figure 1

Flood modelling tools were developed that can be used by Council for decision-making about land-use planning, and in future studies to assess the effectiveness of potential measures to reduce flood risk. The models were calibrated using observations from historical floods, and used to estimate the impacts of flooding for a range of standardised “design” flood probabilities. This modelling was completed in accordance with the guidelines in Australian Rainfall and Runoff 2019 (Ball et al. 2019).

Flooding within each of the towns can occur from either local rainfall runoff or Murray River flooding This study has investigated both mechanisms of flooding. A comprehensive flood study for these towns has not previously been undertaken.

1.1. Scope and Objectives

There is a need to define the flood behaviour for the towns of Corowa, Howlong and Mulwala, for both riverine and local overland flooding. The objective of this study is to improve understanding of flood behaviour and impacts, and better inform management of flood risk in the study area. It will provide a foundation for development of a robust floodplain risk management plan. Federation Council is responsible for managing flood risk as outlined in the NSW Flood Risk Management Manual (NSW Department of Planning and Environment 2023). This study will enable Council to:

• Understand the current flood risk across the catchment

• Provide up to date flood data for all end users

• Enable future development planning

• Assess cumulative impacts of future development

• Assess the effectiveness of potential flood mitigation measures

• Inform emergency management and planning (in collaboration with NSW State Emergency Service)

Design flood events to be defined include the 20%, 10%, 5%, 2%, 1%, 0.5%, 0.2% Annual Exceedance Probability (AEP) events and the Probable Maximum Flood (PMF) or Extreme Flood event across the study area. It involved the following broad tasks:

• Collection of data and information relevant to the study

• Preparation of hydrologic and hydraulic models capable of defining the flood behaviour

• Calibrate the hydrologic and hydraulic models using available data from recent flood events

• Simulate design flood behaviour across the study area for a range of probabilities

• Undertake sensitivity analysis of adopted model parameters

• Interpretation and presentation of model results to describe and categorise flood behaviour and hazard

• Provide information relating to the consequences of flooding, emergency response, land use planning and cumulative flood impacts of development

1.2. Report Outline

This report documents the data, methodology and outputs from the study. The structure of the report is as follows:

Section 1: provides an introduction to the study

Section 2: provides background information for the study

Section 3: outlines the available data used in the study

Section 4: outlines the community consultation undertaken

Section 5: describes the development of the hydrologic model

Section 6: describes the development of the hydraulic model

Section 7: provides details of model calibration

Section 8: details the flood frequency analysis

Section 9: outlines design flood event modelling

Section 10: presents flood model results

Section 11: presents the results of the sensitivity analysis

Section 12: outlines the flood damage assessment

Section 13: lists references used in this report

2. BACKGROUND

2.1. Study Area

The study area consists of the towns of Corowa, Howlong and Mulwala, as well as the intervening reaches of the Murray River (Figure 1). The study focusses on flood affectation within NSW, although Murray River flooding also affects land within Victoria (as the southern bank of the Murray River forms the boundary between the two states in this area). Flooding on the Victorian side of the Murray River is shown for illustrative purposes only and are not ratified by the relevant Victorian authorities. The Murray River and each of the towns is discussed in the following sections.

2.1.1.Murray River

The Murray River rises at approximately 1,430 mAHD in the Australian Alps, between Mount Pilot and Forest Hill on the NSW/Victorian border and flows in a westerly direction approximately 2,530 km to enter the Southern Ocean through Lake Alexandrina in South Australia (Gutteridge Haskins and Davey et al. 1986). The river is managed by the Murray Darling Basin Authority (MDBA). The Murray River is heavily regulated, with fourteen (14) weirs and three (3) large dams controlling water levels and river flows (MDBA 2020) The Hume Dam is located approximately 10 km east of Albury and controls Murray River flows downstream of it, including through the study area. The dam was constructed over a seventeen (17) year period, commencing in late 1919. At the downstream end of the study area, Yarrawonga Weir controls water levels in Lake Mulwala and flows downstream of it.

The Murray River catchment area to key locations is shown in Table 1 There are a number of tributaries that join the Murray River between Hume Dam and Yarrawonga Weir, with the primary ones being the Kiewa River (confluence between Hume Dam and Albury-Wodonga, upstream of the study area) and the Ovens River (confluence just upstream of Lake Mulwala, within the study area).

1 Catchment areas based on WaterNSW gauge station data

2 Measured on GIS using the NSW/Victoria border

The rural areas between the towns are predominately agricultural in nature, with cropping and grazing the main pursuits. The area is serviced by the Riverina Highway from Albury to just north of Corowa, and Spring Drive from Corowa to Mulwala. These main roads generally follow the top of the high bank that has contained Murray River flooding in recorded historic events. There are

Table 1: Murray River Catchment Details

several crossings of the Murray River downstream of Albury:

• River Road crossing at Howlong

• Federation Way at the northern end of Corowa

• Bridge Road (John Foord Bridge) at Corowa/Wahgunyah

• Melbourne Street viaduct crossing Lake Mulwala at Mulwala/Yarrawonga

• The Rock Oaklands Railway crossing Lake Mulwala

• Yarrawonga Weir (including Weir Road)

2.1.2.Howlong

Howlong is the first town located on the Murray River downstream of Albury-Wodonga, approximately 30 km to the west. It has a population of 2,777 with 1,275 private dwellings (2016 census, ABS 2021) There is significant development pressure in Howlong at the present time. The town in shown in Figure 2. The Riverine Highway runs through the town, including along the town’s main street (Hawkins Street), providing the primary access to the east and west. River Road provides access from the town to the south, across the Murray River and is the first crossing of the Murray downstream of Albury-Wodonga.

The town is bounded to the south by the Black Swan Anabranch (and Black Swan Lagoon), with the main Murray River channel located less than a kilometre to the south of this. The anabranch joins the Murray River in the vicinity of Memorial Park, located on the western edge of Howlong. The Murray River floodplain is naturally confined with steep banks, with the town of Howlong typically being approximately 10 m above the Murray River water level in normal conditions, at an elevation of approximately 145-150 mAHD. This results in a well-defined floodplain approximately 3 km wide at this location.

The terrain in Howlong is ‘terraced’, with a flat lower terrace located adjacent to the Murray River floodplain and a more elevated and undulating section to the north and west. In the lower terrace, the terrain is very flat, at an elevation of approximately 145-145.3 mAHD, with a flow path through the middle of it (termed the Victoria Street flow path) that drains to the Murray River floodplain in the vicinity of Hume Street. The upper areas of the town are more undulating, although still relatively flat. There is a remnant flow path that is now a trapped low point on the northern side of the town, in the vicinity of Jude Street. The stormwater network in Howlong generally consists of a kerb and gutter system with pits and pipes, or roadside table drains. Runoff from the town area, including the trapped low point, is collected by the stormwater network and discharged to the Murray River floodplain to the south and west.

Majors Creek is a tributary of the Murray River, with a catchment area of approximately 160 km2 Majors Creek runs in a southerly direction, just to the east of Kywong-Howlong Road on the edge of the town.

2.1.3.Corowa

Corowa is located approximately 50 km west of Albury-Wodonga on the northern bank of the Murray River. It has a population of 5,482 with 2,741 private dwellings (2016 census, ABS 2021).

Howlong and Mulwala Flood Study

The town is shown in Figure 3 The main thoroughfare through the town consists of Honour Avenue and Federation Avenue, running in a northeast-southwest direction. The main street of the town is Sanger Street, running in a north-south direction from Honour Avenue/Federation Avenue to Bridge Road. Bridge Road crosses the Murray River via the historic John Foord Bridge (timber and iron truss bridge constructed in 1892), providing access to the Victorian town of Wahgunyah, located on the southern bank of the Murray River. At the north-eastern end of Corowa, Federation Way provides a second crossing of the Murray River at Corowa, which was completed in 2005. Between Corowa and Wahgunyah, the Murray River is naturally constricted to less than 700 m wide.

The town is located on a small hill, with three ‘terraces’ as the elevation reduces down to the Murray River. The lower terrace is approximately 3-5 m above the normal Murray River water level (at approximately 132-134 mAHD), where the Ball Park Caravan Park, Corowa Civic Bowls Club, Council’s Corowa Office, Corowa Caravan Park and a variety of sporting facilities are located. The second level is at an elevation of approximately 137-138 mAHD and consists of two portions of the residential area of the town, in the vicinity of the lower portion of Federation Avenue and the area south of Brocklesby Street The third level is located at an elevation of 140-141 mAHD, consisting of the area between Nixon Street and Lawrence Street. The rest of the town is located on relatively steep slopes of the hill, with the peak being in the vicinity of Corowa High School.

Most of the town’s streets have a kerb and gutter system, with an underground stormwater network. These typically discharge directly to the Murray River, with two exceptions. The area on the western side of the hill drain to the west, with piped and overland flows making their way to an open channel that runs around the western, eastern and southern sides of Corowa Airport. This channel flows to the south and discharges into the Murray River. The area on the northern side of the hill drains to a natural trapped low point where the Whitehead Street Wetlands are located.

2.1.4.Mulwala

Mulwala is located approximately 80 km west of Albury-Wodonga and is situated on the northern bank of Lake Mulwala, which is formed by Yarrawonga Weir impounding the Murray River (see Section 3.5 for further information). It has a population of 2,161 with 1,328 private dwellings (2016 census, ABS 2021). The town is shown in Figure 4. Melbourne Street is the main road through the town, running parallel to the bank of Lake Mulwala. The Victorian town of Yarrawonga is located on the southern bank of Lake Mulwala. The Yarrawonga Weir bridge (Weir Road) was permanently closed to traffic on 15 October 2021 (MDBA 2021a), leaving the Melbourne Street viaduct crossing Lake Mulwala as the only vehicle crossing between Mulwala and Yarrawonga. The Rock Oaklands Railway also crosses the lake immediately upstream of Yarrawonga Weir. The railway line runs through the town and forms a hydraulic control. Mulwala Canal, a large irrigation canal, runs in a westerly direction and divides the town into two sections.

The entire town is very flat, generally being between 126 and 126.5 mAHD. The stormwater network in Mulwala generally consists of a kerb and gutter system with pits and pipes, or roadside table drains. These typically discharge into small detention basins or directly to Lake Mulwala Given the flat nature of the town, the basins require pumps to drain water. The main town area east of the railway line and the areas north of Mulwala Canal drain to Lake Mulwala, while areas

to the west of the railway line and south of Mulwala Canal drain to two artificial channels located immediately to the south of Bayly Street. These channels discharge to the Murray River floodplain downstream of Yarrawonga Weir. To the north-west of the town (south of the Mulwala Canal), there is a large Department of Defence site. The southern portion of the site drains south toward the LakeMulwala and the Murray River, while the northern portion of the siteconsists of undulating terrain with trapped low points that generally drain to the north toward Mulwala Canal.

2.2. Flood Mechanisms

Flooding in each of the towns can occur from two mechanisms:

Murray River Flooding: The towns can be impacted by rising Murray River water levels. In particular, the lower terrace levels of Corowa can be affected by relatively frequent Murray River flood events. The lower terrace level of Howlong can also be affected, although only in rarer events. Yarrawonga Weir maintains fairly consistent water levels in Lake Mulwala, and hence Mulwala is less susceptible to flooding from the Murray River in frequent events. The storage available within Lake Mulwala also contribute to a lower flood risk for the town of Mulwala for these events.

Local Overland Flooding: The towns can also be impacted by intense rainfall events that cause significant runoff in excess of the stormwater network capacity. This is exacerbated by the relatively flat nature of many of the towns. The ponding of water from local runoff and well as overland flow paths can affect each of the towns. The Majors Creek catchment is located to the north of Howlong and has a catchment area of approximately 160 km2. The main creek flows south, just to the east of Howlong. This is only significant local creek that impacts any of the towns. Flooding due to Majors Creek is considered in this study and is contained within the ‘local overland’ flood model results as Majors Creek flooding is more closely aligned with flooding due to local runoff rather than the Murray River.

The two flood mechanisms typically occur at different times. Murray River flooding is controlled by outflows from Hume Dam. While the primary role of Hume Dam is water conservation, it also provides some flood mitigation benefit for downstream areas. Inflows into the dam are driven by rainfall over the 15,000 km2 upstream catchment. Flood events occur from widespread rainfall events that occur over several days or more. Outflows from the dam are controlled by MDBA and are based on the following priorities (MDBA 2021b):

1. Protect the structural integrity and safety of the dam; then

2. Maximise water availability (i.e. fill the storage to at least 99% of capacity prior to any ensuing drawdown to meet downstream needs); and then

3. Limit flood damage to downstream communities and increase benefits to the environment and public amenity.

Given this, the passing of floodwaters from Hume Dam will be dependent on how full the dam is. An incoming flood may be entirely captured by the dam, or may be entirely passed by the dam, depending on how much storage is available. The Bureau of Meteorology (BoM) will issue flood forecast information and flood warnings based on forecast rainfall and dam releases. The travel time of a flood peak from Hume Dam to the downstream towns is estimated in Table 2. There is typically adequate information to predict the flood magnitude and arrival time at each of the towns

Corowa, Howlong and Mulwala Flood Study

with reasonable accuracy. Peaks of major floods generally last a few days, although the river can often remain in flood (above the minor flood level) for several months. It should also be noted that flooding on the Murray River can also be driven by large inflows from the Kiewa and Ovens Rivers, which can occur independently to Hume Dam spills. Flows from these catchments is also considered when BoM issues flood forecasts and flood warnings, with similar lead times.

Table 2: Typical Flood Peak Travel Times from Hume Dam

1 Based on October 1975 flood. This is also confirmed in travel time estimations in the Corowa Shire Local Flood Plan (30-34 hrs to Corowa and an additional 24 hours to Mulwala, NSW State Emergency Service 2015), and the average travel time from Albury to Yarrawonga of historic events of 2.7 days in the Murray River Flood Plan Management Study (Gutteridge Haskins Davey 1986)

In contrast, local overland flooding typically occurs from local rainfall bursts over the towns. The same storm is unlikely to produce flooding by both mechanisms at the same time

2.3. Historical Flooding

Major flood events recorded on the Murray River in the past have included 1867, 1870, 1917, 1931, 1956, 1974, 1975, 1992 and most recently 2016 and 2022 A rank of major flood events for this reach of the Murray River are presented in Table 3. These flood events are based on recorded flood levels at the Corowa Gauge (GS 409002). Each of the top ten events reached above 7.5 m at the gauge. The gauge has been in operation since 1909 and hence does not include the flood events of the 1800’s. Comparisons were also made with recorded flood levels at the Howlong gauge (GS 409037), which confirmed the relative magnitude of events within the gauged period from 1967, with the exception of the 2016 event, which was slightly larger than the 1970 event at the Howlong gauge.

Table 3: Recorded Murray River Flood Events at Corowa

a The WaterNSW dataset has a gauge height of 7.9 m recorded, although this is assumed to be a daily average (since only one value is recorded per day prior to 1967). Gutteridge Haskins Davey (1986) indicated the gauge height for 1917 reached 8 8 m, making it the largest gauged event at Corowa.

Corowa, Howlong and Mulwala Flood Study

b The WaterNSW dataset has a gauge height of 7.86 m recorded, although this is assumed to be a daily average (since only one value is recorded per day prior to 1967). Gutteridge Haskins Davey (1986) indicated the gauge height for 1931 reached 7.9 m.

c These flood event ranks were swapped when assessing flood levels at the Howlong gauge (GS 409037)

Historic evidence suggests that the floods in the 1800’s have magnitudes as shown in Table 4, with approximate Average Recurrence Intervals (ARI) estimated.

Table 4: Murray River Flood Events prior to gauge data Year Approximate ARI1

1867 90

1870 150

1 Victorian State Government 2018

Comments

Fourth highest known at Albury and second highest at Echuca (Victorian State Government 2018, Gutteridge Haskins Davey 1986)

Largest known flood throughout the entire Murray catchment, record flood heights in Echuca and Mildura (Victorian State Government 2018). URS (2009) suggests that the 1917 flood was slightly higher than the 1870 flood at and above Corowa.

The largest events to occur in this stretch of the Murray River from Hume Dam to Yarrawonga Weir in recorded history are 1870 (the largest), 1867 and 1917. Due to the occurrence of these events prior to reliable gauges being installed, there is uncertainty regarding the magnitude of the events. However, what is apparent is that these are the largest events to occur in recorded history. These events occurred prior to the construction of major infrastructure such as Hume Dam and Yarrawonga Weir, and as such, understanding the magnitude of these events in the current day floodplain conditions is difficult to determine. Further discussion on Murray River events and flows is contained in Section 8.

3. AVAILABLE DATA

3.1. Previous Studies

A number of previous studies have been completed for the towns of Corowa, Howlong and Mulwala. These studies typically focus on drainage within the towns and are over a decade old, and hence their relevance to the current study is limited. The most relevant study is the Corowa, Howlong and Mulwala Floodplain Risk Management Study and Plan (URS 2009). In addition to these, there are a number of studies available for the Murray River that focus on the entire river system, or more localised reaches of the river upstream or downstream of the current study area. A summary of the studies made available to WMAwater is provided below (in chronological order).

3.1.1.Howlong Drainage Study (Gutteridge Haskins Davey 1985)

This study (Gutteridge Haskins Davey 1985) investigated the topography and drainage features of Howlong. The study identified the large external drainage catchments located to the northeast and east of the town, and urban development occurring within depressions and gullies throughout the town as contributing factors to overland flooding. A drainage strategy was developed consisting of the diversion or retardation of external catchment flows and internal drainage design standards of 1 in 5 years for piped flows and 1 in 50 years for surface flows (primarily via roads).

3.1.2.Murray River Flood Plain Management Study (Gutteridge Haskins Davey 1986)

The detailed report for this study (Gutteridge Haskins Davey 1985) identified areas impacted by flooding on the Murray River floodplain between Lake Hume to the South Australian border. The study considered historic flood inundation along with previous and proposed flood plain works to identify impacted areas and prioritise areas for further investigation. The report details the information available for the historic events of 1867, 1870, 1916, 1917, 1931, 1956, 1974 and 1975. Flood frequency analysis (FFA) indicated the results outlined in Table 5. The associated flood atlas mapped the 1975 event from Lake Hume to Yarrawonga.

Corowa, Howlong and Mulwala Flood Study

Table 5: Murray River Historic Events and Design Estimates (Gutteridge Haskins Davey 1986) for Corowa and Yarrawonga

1 Using a gauge zero level of 126.015 mAHD, as per the report. Design levels based on 1985 rating tables.

2 Using a gauge zero level of 115.035 mAHD, as per the report. Design levels based on 1985 rating tables.

3 The adopted 1% AEP was the 1917 flood level

4 The report contains discussion in Appendix C for potential issues with historic flows. The official flow records for 1917 are 306,000 ML/day, but contains discussion for this to be lower, at 272,000 ML/day. These both differ to the tabulated values in the body of the report. This is similar for the 1867 and 1870 events – these were listed as 245,000 ML/day and 247,000 ML/day respectively in the Appendix.

Note: NA is not available (as stated in the report), while - denotes that it is not reported

3.1.3.Yarrawonga Weir Review of Flood Security (Australian Dams Alliance, 1999)

The Australian Dams Alliance (1999) prepared a review of the flood security of Yarrawonga Weir for Goulburn-Murray Water, detailing the expected performance of the weir during significant flood events. Extracts of this report were provided to WMAwater. The review focussed on the security of the weir structure and potential flooding impacts upstream and downstream of the weir were not considered. The review found that under free flow conditions, the weir is capable of passing 270,000 ML/d at the full supply level (FSL) of 124.9 mAHD) and a design discharge of 345,000 ML/d at the design flood level of 125.85 mAHD. A peak discharge of 445,000 ML/d was estimated when the flow reaches the bottom of the raised gates (127.0 mAHD), assuming no outflanking flows. In events larger than this, the structural integrity of the weir and embankment (when overtopped) may be compromised.

3.1.4.Report for Howlong Drainage Strategy (GHD 2008)

This study (GHD 2008) further expanded on the previous drainage study, noting that the drainage issues and recommended strategy were still relevant at the time of the report. It identified that its

Corowa, Howlong and Mulwala Flood Study

implementation was limited, however, primarily due to funding. The report recommended communication of the strategy to improve awareness, construction of priority capital works and a progressive implementation of the strategy for future developments and investigations

3.1.5.Floodplain Risk Management Study – Corowa, Howlong & Mulwala and Floodplain Risk Management Plan – Corowa, Howlong & Mulwala (URS 2009)

This study was completed in 2009 on behalf of Corowa Shire Council (now Federation Council), consisting of a Floodplain Risk Management Study (FRMS) and Floodplain Risk Management Plan (FRMP). The FRMS report addresses the flooding, environmental and planning issues associated with the management of flood prone land within the towns of Corowa, Howlong and Mulwala, as well as the rural lands along the Murray River between these towns. The study did not undertake any flood modelling, but instead relied on historic flood data to estimate Murray River flood levels at each of the towns for various return periods. The study did not investigate local overland flooding The report indicated that there were no local flooding problems in the towns. The study also investigated the consequences of failure of Hume Dam by adopting 1999 estimates from NSW State Water hydraulic modelling.

The most useful information from the study is its assessment of historic flood events. The data for historic events (as reported) as well as estimates of the design flood levels up to the probable maximum flood (PMF) are summarised in Table 6 and Table 7 for Corowa and Yarrawonga Weir, respectively. The study adopted a 100 year ARI flood level of 7.138 m and PMF level of 12.168 m for the Howlong gauge (using a gauge zero of 132.382 mAHD according to WaterNSW).

Table 6: Murray River Historic Events and Design Estimates (URS 2009) for Corowa

Comments

Adopted level at the time of URS (2009) report, from Gutteridge Haskins Davey (1986)

This is above the WaterNSW gauge data by almost 1 m

Similar to the level estimated in Gutteridge Haskins Davey (1986) report

50

5

From Gutteridge Haskins Davey (1986)

From Gutteridge Haskins Davey (1986)

gauge data

From the Report on Delineation of Flood Frequency Zones in the River Murray Flood Plains

Corowa, Howlong and Mulwala Flood Study

Table 7: Murray River Historic Events and Design Estimates (URS 2009) for Yarrawonga Weir

500

Estimated in URS (2009) report using data from Water Technology (2011) for U/S and from Gutteridge Haskins Davey (1986) for D/S 200

Estimated in URS (2009) report using data from Water Technology (2011) for U/S and from Gutteridge Haskins Davey (1986) for D/S 100

From Gutteridge Haskins Davey (1986)

From Gutteridge Haskins Davey (1986)

From Gutteridge Haskins Davey (1986)

The FRMP recommended the following high priority flood risk mitigation measures:

• Management of assets

• Local Environment Plan (LEP) and Development Control Plan (DCP) updates

• Local flood plan updates

• Community awareness program

• Obtain updated dambreak modelling and revise Flood Planning Levels (FPLs) as required

3.1.6.

Murray River Regional Flood Study Dicks/Seppelts levees to downstream of the Ulupna Creek confluence (Water Technology 2011)

This study (Water Technology 2011) covers the Murray River from approximately 20 km downstream of Yarrawonga Weir. The study undertook flood frequency analysis (FFA) at the Yarrawonga gauge (GS 409025 downstream of the weir) and Tocumwal gauge (GS 409202). Of particular interest to the current study is the analysis of gauge data at the Yarrawonga gauge, including a comparison of estimates of peak flows for significant flood events between 1905 and 1979 to determine a reliable annual maximum series. The FFA considered peak flows (including censoring of the 1867 and 1870 events) and flood volumes (over a 14 day, 21 day and 28 day period). A 1% AEP peak flow estimate at the Yarrawonga gauge ranged from 269,000 ML/d to 445,000 ML/d depending on the period assessed, censoring of events and probability distribution

Corowa, Howlong and Mulwala Flood Study

adopted. The study adopted a 1% AEP peak design flow rate of 387,000 ML/d at Yarrawonga. The peak flow to peak volume ratio was used to inform the selection of historic flood hydrographs for design flood events. These design flow hydrographs were used in a MIKEFLOOD hydraulic model that was calibrated using the October 1975 and October 1993 flood events. Design flood inundation maps were produced considering no levee failure and levee failure scenarios. Various structural and non-structural mitigation measures were also assessed with recommendations provided.

3.1.7.Albury City to Greater Hume Murray River Flood Study (GHD 2012)

The study, herein referred to as the ‘Albury Flood Study’ (GHD 2012) defined flood behaviour for a 34 km stretch of the Murray River floodplain using flood frequency analysis (FFA) and a TUFLOW hydraulic model. Design flood events ranging from a 5 year ARI to 500 year ARI were defined for the study area, extending from Hume Dam to approximately 20 km downstream of Albury. Stream gauge data was used to conduct an FFA that considered hydrology both pre- and post-construction of Hume and Dartmouth Dams. Steady state design flows were input into a TUFLOW model which adopted a 10 m grid and was calibrated to the 1975 flood event and validated against the 1917 flood event. Design flood behaviour was used to produce flood hazard maps, hydraulic category maps and flood profile plans. The flood study did not model the PMF event, as the MDBA was undertaking a study in relation to dam break scenarios at the time, which would provide estimates of the PMF event at Albury It is understood that Albury City Council is currently updating this study for the South Albury Levee Upgrade

3.1.8.Corowa Shire Local Flood Plan (NSW SES 2015)

The NSW State Emergency Service (SES) is the legislated combat agency for flood response. The Corowa Shire Local Flood Plan (NSW SES 2015) covers preparedness measures, the conduct of response operations and the coordination of immediate recovery measures from flooding within the Corowa Shire LGA (now part of Federation Council). Relevant information from this document is primarily contained within Volume 2, which defines the hazard and risk of floods in the area. It describes the Murray River system, the characteristics of flooding, flood history, extreme flooding (including consideration of Hume Dam failure) and flood consequences. It estimated that Murray River flooding up to and including the 1% Annual Exceedance Probability (AEP) event will cause minimal damage within the towns. The adopted estimated design gauge levels in the plan are provided in Table 8 It is also observed that historic flood levels, apparently adopted from Gutteridge Haskins Davey (1986), incorrectly attribute the 1917 flood level to the 1870 event, and vice versa. The document incorrectly states that the 1870 flood was the flood of record, when this should be the 1917 flood.

Table 8: Design Flood Levels at Gauges from SES (NSW SES 2015)

3.1.9.Yarrawonga Weir Flood Incident Management Plan (MDBA 2016)

The MDBA provided WMAwater with extracts from the Yarrawonga Weir Flood Incident Management Plan (MDBA 2016). The document provides rules to be followed in order to safely route a flood through Yarrawonga Weir in the case that communications with MDBA are unavailable. The operation of the weir is required to pass floods without damaging the structure, with requirements for flows less than 68,000 ML/d to be passed through the southern structure only. The gate is to be operated incrementally to pass a flood event, with various requirements (such as minimum gate openings, maximum differential gate openings and gate opening order) to ensure the structural integrity of the weir.

3.1.10. Albury Floodplain Risk Management Study and Plan (WMAwater 2016)

This study (WMAwater 2016) investigated floodplain risk management measures for the town of Albury, assessing the impacts of flooding from the Murray River and local catchment runoff. Flooding from the Murray River was estimated using modelling from GHD (2012), including the adoption of 250,000 ML/d as the 1% AEP flow rate downstream of Hume Dam. The only update made to this modelling was the inclusion of the PMF event, which was estimated to be 14,900 m3/s in lieu of the Hume Dambreak study from MDBA being finalised. The MDBA confirmed that the estimated PMF from this study compared well with their draft PMF results at the time.

3.1.11. South Corowa Stormwater Drainage Strategy (Spiire 2018)

This drainage strategy for South Corowa (Spiire 2018) covers the area south of Spring Drive. The report investigated local drainage issues and identified measures to upgrade the drainage network to achieve a 1 in 5 year ARI design standard. This consisted of new and upgraded pits and pipes, regrading of table drains and maintenance of the system.

3.1.12. Hume to Yarrawonga Hydrodynamic Model (MDBA 2019)

This report (MDBA 2019) outlines the MIKE 1D/2D hydrodynamic model that was developed to assess the 2016 Hume to Yarrawonga Constraints Measure business case that outlined the feasibility and environmental outcomes of increasing operational flows at Doctors Point from 25,000 ML/day to 40,000 ML/day. The business case utilised an existing MIKE11 model developed in 2006. This model was updated and refined to be a 1D/2D linked MIKE FLOOD model based on 2001 LiDAR and 2016/2017 cross section data. While limitations in the terrain data were

Corowa, Howlong and Mulwala Flood Study

noted, the model was calibrated to two events with the aim of producing a good fit at the 40,000 ML/day flow rate, which was the primary purpose of the model (simulate a 40,000 ML/day flow). The model was not available to WMAwater, however the cross section data was provided (Appendix A of the report). The report also examined gauge data (including gaugings to evaluate reliability), with some useful commentary provided.

3.1.13. Stormwater Options Assessment – John Street, Corowa (GHD 2019a)

This stormwater options assessment (GHD 2019a) investigated the issues with the existing drainage network for the catchment including John Street in Corowa, and investigated options for improvement. The study noted that a storm event in October 2017 produced flooding at numbers 6, 8 and 10 Gordon Street, with one property experiencing above floor flooding. The October 2017 storm occurred on the 19th October, with reports that the majority of the 47.6 mm of daily rainfall recorded at the Corowa Airport rainfall station fell within one hour. The March 2012 event was also noted as a significant rainfall event. The drainage system was assessed to have very limited capacity (typically less than a 20% AEP). A number of options were recommended to help alleviate flood problems, particularly at the Gordon Street low point.

3.1.14. Mulwala Drainage Improvement Project (GHD 2019b)

Stage 1 of the Mulwala Drainage Improvement Project (GHD 2019b) involved a scoping study to identify existing data and issues relevant to the preparation of a Stormwater Drainage Improvement Plan. Two areas were assessed in this study, in the vicinity of Romney Street and Savernake Road / North Street. The report noted the extremely flat terrain and the need to pump water from detention basins. The report also noted some operational issues with the pump stations, including a power failure during the March 2013 storm event that resulted in basins overflowing. The study recognises that flood impacts are generally limited to ‘nuisance flooding’, although drainage upgrades and new drainage infrastructure would be required to provide the desired level of service.

3.1.15. Mulwala Stormwater Stage 2 Options Study (GHD 2019c)

Stage 2 of the Mulwala Drainage Improvement Project (GHD 2019c) built upon the Stage 1 study and investigated a number of drainage upgrade options. The study analysed the capacity of the existing system and options to improve it through the provision of basins, gravity pipes and rising mains. The report also details information on the existing stormwater network and channels in Mulwala.

3.1.16. Yarrawonga Framework Plan: Stormwater Drainage Strategy (Alluvium 2019)

The study (Alluvium 2019) investigated the assets required to manage minor flows within pipes, manage major flows within roadways and waterways and treat stormwater to meet water quality requirements within the Victorian town of Yarrawonga. While most of the report contains local

Corowa, Howlong and Mulwala Flood Study

details specifically for Yarrawonga (not within the current study area), the report does mention a major rainfall event in 2012 that caused local flooding. It was reported that 277 mm of rain fell in less than 7 days. It is likely that this storm event also caused local flooding issues in Mulwala.

3.2. Terrain Survey

3.2.1.Ground Survey

Topographic ground survey was provided by Council for several sites across the three towns. The following surveys were provided as electronic CAD files:

• Majors Creek at Howlong-Burrumbuttock Road, Howlong (Walpole Surveyors, April 2012)

• Lowe Square Recreation Reserve, Howlong (Spiire, June 2021)

• Corowa Aerodrome Drainage Channels, Corowa (Spiire, November 2019)

• Bullecourt Road, Corowa (12P Consulting, August 2020)

• Ball Park Caravan Park, Corowa (Spiire, May 2021)

• Lucan Street / Bayly Street Drainage Channel, Mulwala (Spiire, March 2020)

Most of these surveys covered a substantial ground area and the data consisted of survey point data as well as a triangular irregular network (TIN) of the ground surface. This data was used to verify the LiDAR data (see Section 3.2.2). The Bullecourt Road survey consisted of 0.2 m ground contours and a digital elevation model (DEM), however this was not in a format that could be readily used for LiDAR verification. Surveys that consisted of drainage channels were also used to supplement the LiDAR data in the hydraulic model.

3.2.2.Aerial Survey

3.2.2.1.

LiDAR Datasets

Aerial survey is typically undertaken using a Light Detection and Ranging (LiDAR) device mounted to an aircraft. LiDAR datasets provide a high quality survey over large areas. The NSW Government (Spatial Services) holds this data and it is publicly available through the Elevation Information System (ELVIS, https://elevation.fsdf.org.au/). The accuracy of these datasets is typically 0.3 m in the vertical (95% confidence interval) and 0.8 m in the horizontal (95% confidence interval). The ground digital elevation models (DEM), which were developed from filtered ground return points, downloaded at the highest resolution possible. A number of datasets were available across the study area. These are listed below, with their extents shown in Figure 5:

• 5 m DEM available across the northern portion of the Murray River consisting of the following tiled datasets:

o Berrigan, January 2015

o Buraja, February 2015

o Walbundri, February 2014

• 1 m DEM available downstream of Yarrawonga Weir consisting of the Wakool tiled dataset from 2015

• 1 m DEM available for Albury, upstream of the study area, consisting of the following tiled datasets:

o Albury, February 2017

o Tallangatta, February 2017

Corowa, Howlong and Mulwala Flood Study

• 2 m DEM available between Albury and Howlong, consisting of the following tiled datasets:

o Albury, February 2017

o Walbundrie, February 2017

• 2 m DEM available between Corowa and Mulwala, consisting of the following tiled datasets:

o Dookie, February 2017

o Wangaratta, February 2017

• 1 m DEM available from Albury to Mulwala, consisting of the following tiled datasets:

o Albury, November 2020

o Tallangatta, November 2020

o Walbundrie, November 2020

o Buraja, November 2020

o Wangaratta, November 2020

o Berrigan, November 2020

The last dataset in the list is the most recent, highest resolution and has the largest extent, covering the entire study area (referred to as the 2020 LiDAR data herein). This will be the primary terrain dataset utilised for this study, supplemented by the other datasets where required. Datasets captured later than 2019 were projected using the GDA2020 datum. For the purpose of consistency with existing GIS data, these DEMs were reprojected in GDA94.

3.2.2.2. LiDAR Verification

A verification exercise was undertaken for the 2020 LiDAR data. Verification was undertaken against reliable survey points from the NSW Survey Control Information Management System (SCIMS). This data is publicly available through NSW Spatial Information Exchange (SIX, https://six.nsw.gov.au/). SCIMS points were collected across the entire study area, from Howlong to Mulwala. These are typically located within NSW and within the urban areas of the towns, with some points located on the Riverina Highway and Spring Drive and at other locations. The following filtering of points was applied in order to obtain a reliable dataset:

• Removal of points marked as “Destroyed”, “Uncertain” or “Not Found” for status

• Removal of points marked as “U” for Vt class (unknown/unreliable survey)

• If the difference between the point and the LiDAR dataset was greater than 1 m

This yielded a total of 239 reliable points. A histogram of the difference in level between the 2020 LiDAR and these SCIMS points is shown in Diagram 1. The histogram shows that the differences are primarily within 0.3 m, which is considered a reasonable match. There is a slight skew, indicating a bias for the LiDAR data to be slightly higher than the survey marks. The average difference is approximately 0.1 m. This demonstrates a high quality dataset that is considered reliable for the purposes of flood modelling.

Diagram 1: Histogram of the difference between the 2020 LiDAR dataset and the SCIMS points

Verification was also undertaken against the four survey datasets provided by Council (Section 3.2.1). These survey datasets contained TIN triangles that were used to generate a ground TIN and then DEM that was directly compared with the 2020 LiDAR data. The results are contained in Figure 6. The difference in levels is typically within 0.1 m, which is considered a good match and reinforces the validation with the SCIMS points. A brief discussion is provided for each dataset below.

• Lowe Square Recreation Reserve, Howlong (Spiire, June 2021). This dataset is very similar to the LiDAR, typically being within 0.1 m, with some isolated areas showing a greater difference. The main area is a BMX track with shaped dirt jumps that have changed over time.

• Majors Creek at Howlong-Burrumbuttock Road (Walpole Surveyors, April 2012). The LiDAR data is typically 0.1 to 0.2 m higher than the survey dataset on the floodplain, although the opposite occurs in the channelised sections. Howlong-Burrumbuttock Road is within 0.1 m, indicating that the aerial survey of hard surfaces is accurate. The differences seen here could be attributed to the nature of the fields at the time of survey. It is interesting to note that the parcel of land at the very northern extent of the ground survey shows almost no difference in level between the two surveys, while the fields of interest south of this show changes most likely due to agricultural activity (such as levelling, slashing, ploughing, etc). The ground surface across the floodplain is expected to exhibit minor changes depending on the season and use of the land.

• Ball Park Caravan Park, Corowa (Spiire, May 2021). This dataset is very similar to the LiDAR, typically being within 0.1 m.

• Lucan Street / Bayly Street Drainage Channel (Spiire, March 2020). At Bayly Street and to the north, the aerial survey is very similar to the ground survey. In the channel, however, the LiDAR data is typically higher than the survey data and the shape is not as defined, which is to be expected the case for such a narrow and deep channel. In this case the

Howlong and Mulwala Flood Study

ground survey would be used in the model to accurately represent this channel. As the channel becomes wider and shallower towards the Murray River floodplain, the LiDAR becomes more accurate.

Overall, the LiDAR dataset is considered to be of very high quality and reliable for flood modelling, noting some of the limitations in some areas such as drainage channels

3.2.3.Bathymetric Survey

Bathymetric survey of the Murray River was provided by the Department of Planning and Environment (DPE, now DCCEEW) This contained a number of surveys have been undertaken for various reaches of the Murray River in the past. For the reach of interest, the best available data was from MDBA (2019). The data was in the form of a spreadsheet that contained the coordinates of cross section and a profile of the river bed. The cross sections are spaced approximately 5 km to 10 km apart. Cross sections from survey conducted in 1997, 2006, 2012 and 2016 were provided. The 2016 cross sections were used as the basis of the Hume to Yarrawonga MIKE hydraulic model (MDBA 2019), and it is assumed that this is the most recent and best quality data, and hence was also adopted for this study. A total of 21 cross sections were available within the reach of interest, from upstream of Howlong to just upstream of the confluence with the Ovens River, on the Murray River and its anabranches It is noted, however, that these spaced cross sections provide very limited information of river bathymetry over the whole length of interest. The cross sections only provide a very localised snapshot of the bathymetry which was used to estimate bathymetry for the entire reach. High quality hydrographic survey using multibeam sounding was available downstream of Yarrawonga Weir. It is understood that this survey was conducted between 2017 and 2020. A DEM can easily be formed due to the density of points surveyed.

Bathymetric Survey of Lake Mulwala was provided by Goulburn-Murray Water. A short report was produced for the development of the bathymetry model (Australian Water Technologies 1999). A depth sounder was used to take 19 transects in March 1999. The floodplain immediately upstream of Lake Mulwala (known as the Everglades) could not be surveyed due to restricted boat access. A 3D model of the lake bathymetry was derived from the transects. It was noted that the Murray River channel was not accurately represented in the model as the width of the channel was less than the distance between transects. Limitations of the data were outlined in the report. The data provided by Goulburn-Murray Water consisted of bed level contours generated from the 3D model at 0.5 m (as shown in Figure 2 of Australian Water Technologies 1999)

3.3. Buildings

A buildings layer was obtained from Microsoft that contains over 11 million buildings Australiawide (https://github.com/microsoft/AustraliaBuildingFootprints). The data is derived from aerial and satellite imagery from 2013 to 2018 for Microsoft’s Bing product and is freely available to download and use under the Open Data Commons Open Database License. The building extents are automatically delineated based on learning algorithms.

3.4.

Stormwater Infrastructure

3.4.1.

Pit and Pipe Network

Howlong and Mulwala Flood Study

Council supplied WMAwater with GIS files of Council’s pits and pipes. A summary of the number of pits and pipes within each town, as well as the percentage with useful data attached is shown in Table 9. There are approximately 2,400 pits across the towns, with 80%to 90% having elevation data in the form of an invert level or a depth to invert from the surface. There are also approximately 2,400 conduits (pipes and box culverts) in the database, with sizes attributed to most of them, although invert information is limited.

Table 9: Summary of stormwater network data provided

This database only contains the ‘pit and pipe’ stormwater network and does not contain information about the large cross drainage culverts.

3.4.2.Culverts and Bridges

Council did not hold any specific asset information related to culverts and bridges in the towns. Data for culverts and bridges was sourced from design or work-as-executed (WAE) drawings, or from field measurements. These are discussed in the following sections.

3.4.2.1.

Council Drawings

Council provided the following design drawings:

• Sturt Street and River Road (MR197) reconstruction design drawings, 1990 and 1992, including bridges over the Murray River floodplain (at Howlong)

• Federation Way (MR86) design drawings, 2004, including new bridges over the Murray River floodplain

These drawings were used to estimate hydraulic parameters for the representation of bridges crossing the Murray River floodplain in the hydraulic model.

3.4.2.2.

Transport for NSW Drawings

Transport for NSW (TfNSW) was contacted to obtain information related to the bridge crossings of the Murray River floodplain (TfNSW assets). TfNSW provided the following drawings:

• River Road crossing at Howlong (including three approach bridges and the John Conway Bridge over the Murray River, 1999 design drawings)

• Federation Way crossing at Corowa (Tim Fischer Bridge on approach to Murray River crossing, 2003 design drawings)

Corowa, Howlong and Mulwala Flood Study

• Bridge Road crossing at Corowa (John Foord Bridge over the Murray River, 1890 design drawings)

• Melbourne Street crossing at Mulwala (Murray River crossing, design drawings 1921)

• Melbourne Street extension to Yarrawonga (viaduct crossing of Lake Mulwala, design drawings 1928?)

• Yarrawonga to Oaklands Railway crossing at Mulwala (crossing of Lake Mulwala, design drawings 1928)

3.4.2.3. Field Survey

WMAwater identified a number of cross drainage culverts that were not contained in Council’s asset database. Council staff undertook a field survey an collected the following details:

• Culvert type

• Culvert dimensions

• Estimated depth of cover

• Photograph

• Additional observations from the field

The survey consisted of 8 culverts in Corowa, 12 culverts in Howlong and 9 culverts in Mulwala.

3.5. Yarrawonga Weir

Yarrawonga Weir is operated by Goulburn-Murray Water on behalf of the MDBA. The weir was completed in 1939 and impounds Lake Mulwala, which has a capacity of approximately 117,500 ML (Goulburn-Murray Water 2021). From Lake Mulwala, water can be discharged to the Yarrawonga Main Channel (to service the Murray Valley Irrigation Area of Victoria with a capacity of 3,100 ML/d) and Mulwala Canal (to service the Berriquin, Denimein, Deniboota and Wakool irrigation districts of NSW, with a capacity of 10,000 ML/d). Water is also discharged downstream via 8 vertical gates on a southern structure, with an additional 2 gates on a northern structure that are used during flood events greater than 60,000 ML/d. The maximum operating level for Lake Mulwala is 125.15 mAHD, and the FSL is 124.9 mAHD.

Goulburn-Murray Water provided details of the gate dimensions and operating procedures. Original design drawings circa 1937 were provided in addition to details of the datum conversion to AHD. Two operational documents were also provided – the Yarrawonga Weir Review of Flood Security (Australian Dams Alliance 1999, relevant sections only) and the Yarrawonga Weir Flood Incident Management Plan (MDBA 2016, section 4 only). The flood security report details the safety of the weir structure in significant flood events, including the estimated discharge rates and structure performance. This document includes an assessment of historic floods, a flood frequency analysis and estimation of extreme flood flows (up to the PMF).

The flood management plan outlines the operation of the gates during floods in the event that communications with MDBA are unavailable. While there are no prescriptive operations, it is understood that the intent is to gradually open the gates at the beginning of a flood event to allow the weir pool level to be maintained at approximately FSL. After the flood peak has passed, the gates are gradually lowered to maintain the weir pool at FSL. This essentially passes the incoming

Corowa, Howlong and Mulwala Flood Study

flood hydrograph through the weir. Generally, prior to the flood peak, the water level in the weir pool may be drawn down to a level in the range of 124.3 mAHD and 124.7 mAHD. It is noted that the operation of the gates during a flood event may be complicated, with gates operating independently of each other and openings based on anticipated inflows, lake levels, rainfall, weir outflows (gauged and estimated) and visual inspections of the weir and flow behaviour.

Historic daily gate operations were provided for October 2016, October 2022 and November 2022.

3.6. Aerial Imagery

Aerial imagery for the towns was available through the following platforms:

• Google Maps (www.maps.google.com.au), latest available April 2019 (Howlong), July 2021 (Corowa) and October 2021 (Mulwala)

• Nearmap (www.nearmap.com), latest available January 2010 (Mulwala) and October 2021 (Corowa and Howlong)

• SIX Maps (six.maps.nsw.gov.au), latest available December 2014 (all towns)

3.7. Gauge Data

Both rainfall and stream gauges are available for the study area, as shown in Figure 7 and described in the following sections.

3.7.1.Daily Rainfall

There are 27 daily rainfall gauges in the vicinity of the study area from the Bureau of Meteorology (BoM). Stations with any reasonable amount of data were collected, and the availability of data from 1900 is shown in Diagram 2 The data for each station is typically 95-100% complete. 11 of these stations are currently open and surround the study area.

2: Available daily rainfall gauge records for the study area

Corowa, Howlong and Mulwala Flood

Diagram

3.7.2.Sub-Daily Rainfall

Corowa, Howlong and Mulwala Flood Study

There are four BoM sub-daily rainfall stations with data available within approximately 50 km of Corowa. These are Automatic Weather Stations (AWS) provide rainfall data at a minute resolution. These stations are summarised in Table 10 and are currently open. Traditional pluviograph data was also available for several stations, however these were typically closed prior to 2016.

Table 10: Summary of AWS Rainfall Data

Station Number

072160

082039

081124

082138

Station Name

Date Start

Albury Airport September 2008

Rutherglen Research July 2010

Yarrawonga July 2002

Wangaratta Aero May 2010

In addition to the official BoM rainfall stations, private rainfall data was also obtained. In particular, for the recent January 2022 event, this rainfall data is valuable to understand the nature of the storm event, which was highly localised to the Corowa area. This rainfall data was sourced from Weather Underground (www.wunderground.com). A summary of the rainfall stations available in this area is provided in Table 11.

Table 11: Summary of Private Rainfall Stations around Corowa

Station Name Location

ICOROW4

Cotton Street, Corowa

ICOROW6 Adam Street, Corowa

ICOROW7

IWAHGU1

Walker Street, Corowa

Main Street, Wahgunyah

Sub-daily rainfall data was also available at several stream gauge sites, as outlined in Table 12 Further details about these gauge stations can be found in Section 3.7.3

Table 12: Summary of Rainfall Data at Stream Gauges

Gauge Number

401027

403246

409216

3.7.3.Stream Gauges

Station Name

Date Start

Murray River at Hume Dam –Storage Gauge No. 2 July 2005 (continuous recording start)

Gullivers Creek @ Lilliput (Rutherglen Research Stn) November 1991 (closed Feb 1995)

Murray River @ Yarrawonga Weir (Head Gauge) July 1992

Stream gauges are available on the Murray River within the study area. Stream gauges are operated by WaterNSW. There are also gauges on major tributaries operated by Victorian Department of Environment, Land, Water and Planning (DELWP), since they are on the southern side of the Murray River. A summary of the available gauge data is provided in Table 13

Table 13: Summary of Stream Gauge Data Available

Gauge Number

409025

Station Name

Yarrawonga Weir (Head Gauge)

Murray River Downstream Yarrawonga Weir WaterNSW

402205 Kiewa River @ Bandiana

403248

403246

Indigo Creek @ D/S Creamery Bridge

Gullivers Creek @ Lilliput (Rutherglen Research Stn)

409600 Lake Moodemere @ Rutherglen

403247

Black Dog Creek Upstream of Dugays Bridge

403241 Ovens River @ Peechelba

409722 Yarrawonga Main Channel 5km U/S 409700

1. WL = Water Level, Q = Flow, RF = Rainfall

Corowa, Howlong and Mulwala Flood Study

Dec 2009

Dec 2009 –current 15 min

Jan 1938 –Dec 1960 Monthly Max3 WL, Q

Dec 1960 –Apr 2012 Subdaily2 Apr 2012 –current 15 min

Oct 1965 –Jan 2010 Subdaily2 WL, Q Jan 2010 –current 15 min

Jun 1999 –Jun 2017 Subdaily2 WL, Q Jun 2017 –current 15 min

Nov 1991 –Jan 1995 Subdaily2 WL, Q, RF

Jul 2010 –current 135 field readings WL

Aug 1998 –Jun 2017 Subdaily2 WL, Q Jun 2017 –current 15 min

Feb 1990 –Mar 2011 Subdaily2 WL, Q Mar 2011 –current 15 min

Jul 2014 –current 15 min WL, Q

2. Recording resolution varies, but is generally a reasonably high resolution

3. There is limited flow data at this gauge, only from 1908 to 1952, and currently only records water level

4. Only data from 1960 is available through the WaterNSW online portal. Monthly maximum discharge data was obtained from Pinneena 10.2.

3.8. Flood Marks and Flood Data

Council did not hold any information about historic flood marks due to Murray River flooding, except for a flood mark on a metal plaque located on a concrete pole outside the Council Chambers of Corowa (Photo 1) During the inception meeting with the Floodplain Management

Committee (FMC), a member who previously worked for Council, indicated that there was a black marker line lower on the pole that indicated the height of the 2016 flood event. Levels were not available for these markers, however the black marker was measured to be 0.65 m above the ground and the 1975 flood level is 0.94 m above that.

Corowa, Howlong and Mulwala Flood Study

Photo 1: Flood Marks on concrete pole outside Council Chambers at Corowa

2016 Flood Level

Additional flood marks were obtained from the Victoria Flood Database (https://discover.data.vic.gov.au/dataset/victoria-flood-database). This database contained historic flood extents for several events (based on flood information and flood aerial photography) as well as point information of flood levels.

During the early stages of the project, a significant rainfall event occurred over Corowa. The rainfall, which fell on the night of the 29th January, 2022, caused overland flooding that affected a number of properties. Information was provided by Council from the NSW State Emergency Service (SES) that indicated 9 properties had above floor flooding (up to 300 mm deep) of the 16 properties assessed by the SES. Council received 38 service requests following the event from residents. Council, in their surveillance and investigation of the town following the event, estimated that between 30 and 40 properties were likely to have had above floor flooding. Council also provided a CAD file with 92 surveyed flood marks following the event. These marks are focussed around South Corowa (around Brocklesby Street), Central Corowa (in the vicinity of Sophia Close) and the Corowa industrial area (near the Aerodrome). Photographs of the flood marks were also taken by Council staff and provided to WMAwater. The SES properties, Council service requests and Council flood marks are shown in Figure 8

Drone photography captured approximately 36 hours after the event by 12P Consulting show the extent of areas still inundated from the storm event, particularly around the industrial area near Corowa Aerodrome.

3.9. Media

A web search revealed that there are a number of reports in the media and in photos and videos uploaded by the community of flooding that may be useful for model calibration. These include:

• October 2016 Flood Stories: https://www.bordermail.com.au/story/4177315/park-engulfed-by-the-murray/ https://www.abc.net.au/news/2016-10-04/ball-park-caravan-moves-cabins-amid-risingmurray-river/7899922

• October 2016 Flood Updates by Corowa Caravan Park on their Facebook page: https://www.facebook.com/Corowa-Caravan-Park-487626527972329/

• October 2016 Flood Photography taken by Tony Reeckman (photos used with permission in this document) https://www.tonyreeckmanphotography.com/corowa-floods-2016.html https://www.youtube.com/watch?v=ADYXeDpiZXk

• October 2016 Floods from drone footage taken by Paul Upton https://www.youtube.com/watch?v=FViDsXEZzKE

• January 2022 Flood Stories: https://www.corowafreepress.com.au/news/border-rocked-by-wild-weather/ https://www.bordermail.com.au/story/7601423/water-was-waist-deep-in-corowa-streetnew-homes-impacted/

3.10. Site Visit

Corowa, Howlong and Mulwala Flood Study

A site visit was conducted on 9th and 10th November, 2021 and was attended by WMAwater and Council staff. The townsof Howlong, Corowa and Mulwala were visited, including the Murray River waterfront and key local drainage features of each town. The Victorian towns of Wahgunyah and Yarrawonga were also briefly inspected.

4. COMMUNITY CONSULTATION

4.1. Newsletter and Questionnaire

A community newsletter and survey were developed for the study and are shown in Appendix B These were uploaded to a dedicated webpage on Council’s website. The webpage contained general information about the project, the newsletter and provided the ability for the questionnaire to be completed and submitted online. The community was informed of the webpage in Council’s Snippets advertisement, published in the Corowa Free Press and Yarrawonga Chronicle, and through Council’s Facebook page The webpage was visited almost 300 times. The purpose of this community consultation phase was to inform residents of the study’s commencement and invite them to contribute their knowledge and experience of flooding.

4.2. Questionnaire Responses

A total of 36 responses were submitted. 34 were submitted online and two were handed in. 35 of the respondents resided in Corowa, while one was from Howlong. A map showing the location of the residents and a summary of flood information provided is shown in Figure 9. A graphical summary of the responses to each question is contained in Appendix B, with responses also and discussed below.

Most respondents provided their address and contact details, stating they could be contacted for further information. Respondents had lived or worked at their current address for between 1 and 69 years, with the average being 19 years. Approximately three quarters of the properties were residential, with the remaining being commercial or rural in nature. The majority of respondents were aware that parts of the towns were prone to flooding, with approximately 80% affected by overland flooding only. The level of flood affectation ranged from approximately 50% who had been affected by above floor flooding to approximately 70% who had been flooded in their yard. The source of this water was primarily from the road, neighbouring properties and stormwater pipes, with only 2 respondents being affected by the Murray River or local creek flooding. Almost 80% of respondents noted that there was damage to clean up and almost 30% had access routes cut off. The majority of respondents stated that the water took days to drain away. A total of 23 respondents stated they had additional information such as photographs, and 17 people attached photographs of flooding to their submission. A compilation of the photos received is contained in Appendix C

While most respondents referred specifically to the January 2022 flood event, it is assumed that other respondents whomade specific overland flood observations were alsoreferring tothis event. The responses indicate the primary issue within Corowa is local overland flooding.

4.3. Community Drop-In Sessions

A community drop-in session was held in each of the towns to provide an opportunity for residents to speak to Council staff and/or WMAwater staff to find out more information about the project and provide information about their experiences of flooding in the area. The following sessions were conducted:

Corowa, Howlong and Mulwala Flood Study

• Corowa: Monday 21 February 2022 5:30pm – 8:30pm at Council Offices

• Mulwala: Tuesday 22 February 2022 9:00am – 11:00am at Mulwala Library

• Howlong: Tuesday 22 February 2022 12:30pm – 2:30pm at Howlong Library

There were approximately 30 people in attendance at the Corowa session, which in part was due to the recent extreme rainfall event that had occurred. While some informal discussions were had relating to the project and general flood concepts, there were 13 responses that were recorded by Council and WMAwater staff for 15 properties. The location of these properties is shown in Figure 10. There were a range of concerns from residents, which are summarised as follows:

• Shallow overland flooding of property and floor levels in the January 2022 event and also in other less severe rainfall events in the past. This was primarily attributed to inadequate street drainage and overflows from the road into property.

• Recent developments causing increased runoff and/or altered flow paths. Water quality issues were also raised in regard to industrial development.

• Capacity issues were raised with regard to the industrial development near the Aerodrome, and the channels and culverts that divert water around the Aerodrome site to the Murray River. Inundation of roads and paddocks around Redlands Road and Skehans Lane were observed for several days following the January 2022 event.

There was one attendee at the Mulwala session and four attendees at the Howlong session. These attendees were generally interested in finding out about the study and discussing issues related to development in Howlong.

4.4. Public Exhibition

TO BE COMPLETED AT A LATER DATE

5. HYDROLOGIC MODEL

5.1.

Introduction

A hydrologic model is a tool for estimating the amount of runoff that flows from a catchment for a given amount of rainfall, and the timing of this runoff flow. Stream gauges (which measure water level in a stream) are a way of directly measuring this information, but they are expensive to setup and maintain. They also require a long record (several decades) to be of most use for flood estimation. Most of the smaller creeks in NSW are not gauged, and there are no suitable stream gauges within the overland flow areas of the towns (including Majors Creek). Only the Murray River has streamflow data that is suitable for Flood Frequency Analysis, and this is documented in Section 8 In the case of local runoff in the towns, using a computer-based hydrologic model is the best practice method for determining how much flow occurs from rainfall information (which is more widely available from rain gauges). This type of hydrologic model is referred to as a runoffrouting model.

A range of runoff-routing hydrologic models is available as described in Australian Rainfall and Runoff (ARR) 2019 (Ball et al. 2019). These models allow the rainfall to vary in both space and time over the catchment and will calculate the runoff generated by each sub-catchment. The generated flow hydrographs then serve as inputs at the boundaries of the hydraulic model, which provides details about flood levels and velocities.

The WBNM hydrologic runoff-routing model was used to determine flows from each subcatchment. WBNM is widely used throughout Australia to estimaterunoff from both rural and urban areas. The WBNM model has a relatively simple but well supported method, where the routing behaviour of the catchment is primarily assumed to be correlated with the catchment area. Where flow data is available at a stream gauge, the WBNM model can be calibrated to this data through adjustment of various model parameters including the stream lag factor, storage lag factor, and/or rainfall losses. Further details regarding the WBNM software can be found in the WBNM User Guide (Boyd et al. 2012).

A hydrological model for each of the towns was developed and used to calculate the flows for each individual sub-catchment for inclusion in the local TUFLOW hydraulic models. The hydraulic model is discussed in Section 6

5.2. Sub-catchment Delineation

The sub-catchments were delineated using the best available LiDAR data for each town (see Section 3.2.2.1). For Majors Creek (in the Howlong catchment), a portion of the catchment was not covered by any LiDAR dataset, and hence the Shuttle Radar Topography Mission (SRTM) data was used. This dataset has approximately a 30 m grid resolution and is of a much lower quality than LiDAR datasets. The data, however, was considered accurate enough for the purpose of delineating the broader Majors Creek catchment.

Subcatchments were delineated using stormwater infrastructure that was to be included in the TUFLOW model (see Section 6), hydraulic controls (such as embankments and bridges) and

Corowa, Howlong and Mulwala Flood Study

consideration of what would constitute a ‘major overland flow path’. In most cases, only subcatchments that contributed to major overland flow paths within the study area of each town were delineated, and fringe areas draining directly to the Murray River were not included. Subcatchments within urban areas are typically smaller to capture the drainage catchment to the stormwater system, while sub-catchments within rural areas are typically larger. A summary of the subcatchments delineated for Howlong, Corowa and Mulwala is presented in Table 14, and shown in Figure 11, Figure 12 and Figure 13, respectively.

Table 14: Summary of subcatchment delineation for each town

1 Howlong Town defined as west of Kywong-Howlong Road (including north of Drew Lane), while the Majors Creek catchment is defined as east of Kywong-Howlong Road, for the purposes of reporting catchment delineation characteristics. There is a small area to the west of Kywong-Howlong Road, just north of Jude Street, that does drain across Kywong-Howlong Road into Majors Creek.

For Mulwala, it was difficult to determine subcatchments due to the flat nature of the town. In areas outside the town, particularly to the north west, the terrain is undulating and results in numerous small depressions and trapped areas of ponding. Subcatchments for this area that is either a trapped low point or drains to the north (toward the Mulwala Canal) were not included

5.3. Hydrologic Model Parameters

The model input parameters to represent each sub-catchment are:

• A lag factor (termed ‘C’), which can be used to accelerate or delay the runoff response to rainfall;

• A stream flow routing factor, which can accelerate or decelerate in-channel flows occurring through each sub-catchment;

• An impervious area lag factor;

• Catchment area; and

• The percentage of catchment area with a pervious/impervious surface

The ‘C’ lag factor of 1.6 was adopted. This is the recommended default value for an ungauged catchment in NSW. There was not sufficient information (including calibration data) to warrant deviating from this value. Likewise, a stream routing factor of 1.0 was adopted, representing a natural channel. The default impervious lag factor of 0.1 was adopted. Catchment areas were calculated based on sub-catchment boundaries in a GIS program. The impervious fractions within the catchment are discussed in Section 5.4 below.

5.4. Impervious Fraction

Runoff from connected impervious surfaces such as roads, gutters, roofs or concrete surfaces occurs significantly faster than from vegetated surfaces. This results in a faster concentration of flow within the downstream area of the catchment, and increased peak flow in some situations. ARR 2019 (Ball et al. 2019) identifies three types of areas for the purpose of estimating urban storm losses:

• Directly Connected Impervious Areas (DCIA) which are impervious areas directly connected to the drainage system;

• Pervious Areas consisting of parks and bushland areas; and

• Indirectly Connected Areas (ICA) which consist of impervious areas which are not directly connected to the drainage system and the pervious areas which interact with indirectly connected impervious areas (ICIA)

To account for this, ARR 2019 recommends the use of ‘Effective Impervious Area’ (EIA), which is a concept identifying the amount of impervious area that acts as directly connected for total runoff purposes, including consideration of both the DCIA and ICIA. This is typically calculated as a percentage of the Total Impervious Area (TIA = DCIA + ICIA). Using the literature from Australian studies in ARR 2019, the ratio of EIA/TIA is typically in the range from 60% to 80%. Given the reasonably large blocks of land and low-density development within these towns, a lower ratio of 60% has been adopted. It is estimated that for a typical urban area within Mulwala and Corowa, the TIA is approximately 50% (i.e. 50% of the urban area is impervious). This yields an overall EIA of approximately 30% (60% x 50%). A lower EIA was adopted for the urban areas of Howlong, due to the typically larger, elongated blocks where buildings and paved surfaces take up a smaller fraction of the lot. Commercial and industrial areas were assumed to have an EIA of approximately 70%, while road corridors were assumed to be 60% impervious. A summary of the adopted EIA percentages for each land use type is shown in Table 15 An overall EIA percentage was assigned to each subcatchment based on the land use within the subcatchment. For the urban areas, the overall EIA is typically 30% to 40% in Corowa and Mulwala, and 20% to 30% in Howlong.

Corowa, Howlong and Mulwala Flood Study

Table 15: Land use categories and percent impervious fraction for the WBNM model

Suburban residential 30 or 20

Grass / open areas / vegetated areas 0

5.5. Rainfall Losses

corridor includes asphalt plus grassed verges

Nominal 5% EIA for buildings, sheds, etc.

EIA of 30% adopted for Mulwala and Corowa urban areas. An EIA of 20% was adopted for the Howlong urban area, given the typically larger and elongated blocks present.

This was also adopted for Majors Creek catchments upstream of HowlongBurrumbuttock Road.

Methods for modelling the proportion of rainfall that is “lost” to infiltration are outlined in ARR 2019 (Ball et al. 2019). The methods are of varying degrees of complexity, with the more complex options only suitable if sufficient data is available. The method most typically used for design flood estimation is to apply an initial and continuing loss to the rainfall, in accordance with ARR 2019 (Ball et al. 2019). The initial loss represents the wetting of the catchment prior to runoff starting to occur, including interception, infiltration and the filling of localised depressions. The continuing loss represents the ongoing infiltration of water into the saturated soils while rainfall continues. The initial/continuing loss method was adopted for this study.

6. HYDRAULIC MODEL

6.1. Introduction

Hydraulic modelling is the simulation of how floodwaters move over the terrain. A hydraulic model can estimate the flood levels, depths, velocities and extents across the floodplain. It also provides information about how the flooding changes over time. The hydraulic model can simulate floodwater both within the creek banks, and when it breaks out and flows overland, including flows through structures (such as bridges and culverts), over roads and around buildings.

Two-dimensional (2D) hydraulic modelling is currently the best practice standard for flood modelling. It requires high resolution information about the topography, which is available for this study from the LiDAR aerial survey (Section 3.2.2.1). Various 2D software packages are available (SOBEK, TUFLOW, RMA-2, MIKE FLOOD). The TUFLOW package was adopted as it meets requirements for best practice, and is currently the most widely used model of this type in Australia for riverine flood modelling.

The TUFLOW modelling package includes a finite difference or finite volume numerical model for the solution of the depth averaged shallow water equations in two dimensions. The TUFLOW software has been widely used for a range of similar floodplain projects both internationally and within Australia and is capable of dynamically simulating complex overland flow regimes. The TUFLOW model version used in this study was 2020-10-AC-w64 (using the finite volume HPC solver). The TUFLOW Heavily Parallelised Compute (HPC) solver has the ability to run on a Graphics Processor Unit (GPU). The new HPC GPU models are significantly faster than TUFLOW Classic models, which rely on a Central Processing Unit (CPU). Further details regarding TUFLOW software can be found in the User Manual (BMT WBM 2018).

In TUFLOW the ground topography is represented as a uniform grid with ground elevations and Mannings ‘n’ roughness value assigned to each grid cell. The size of grid is determined as a balance between the model result definition required and the computer processing time needed to run the simulations. The greater the definition (i.e. the smaller the grid size) the greater the processing time need to run the simulation.

6.1. TUFLOW Hydraulic Model Approach

For this project, four separate hydraulic models were developed – one for the Murray River and a localised model for each of the towns. Flooding due to the Murray River for all towns was simulated using the Murray River model. Local overland flooding was simulated in each of the local town models. The TUFLOW model domains are shown in Figure 14

6.2. TUFLOW Hydraulic Model Extent and Resolution

The Murray River TUFLOW model 2D domain covers the Murray River floodplain from approximately 9 km upstream of Howlong (approximately 2.5 km upstream of the Federation Council boundary) to approximately 8 km downstream of Yarrawonga Weir (approximately 1.5 km downstream of the Federation Council boundary). This is shown in Figure 14 The model covers

Corowa, Howlong and Mulwala Flood Study

an area of approximately 570 km2. A model grid size of 20 m was adopted, with sub-grid sampling (SGS) enabled. SGS enables terrain features present in the high-resolution LiDAR data to be accounted for even with a large grid size. SGS can more accurately represent the storage and conveyance characteristics of each cell, without the need to reduce the cell size (which increases run time).

The local TUFLOW models cover the urban areas of the towns and have a 2 m grid resolution. The Howlong local model extends from Drew Lane and Howlong-Burrumbuttock Road in the north to the Murray River in the south. The model extends west of Howlong-Goombargana Road (encompassing Belmore Street and Martin Street) and covers the Majors Creek floodplain on the eastern side, to Whittaker Lane. The Corowa local model extends from the intersection of the Riverina Highway and Honour Avenue in the north-east, to the Corowa Golf Club in the south. The eastern boundary is the Murray River and extends just west of Skehans Lane. The Mulwala local model extends fromLambruck Lane in the north to the Yarrawonga Mulwala Golf Club Resort in the south. The model is bounded by Lake Mulwala to the east and covers the residential areas to the western extent of Bayly Street and Pimpala Crescent. A large portion of the Department of Defence land is not included (those areas consisting of trapped low points and areas generally draining to the north).

6.3. Terrain

The 2D terrain for each of the TUFLOW models was based on the available LiDAR data (Section 3.2.2.1). The Murray River model relies on the 1 m 2020 LiDAR data, although this is supplemented by other LiDAR datasets where the 2020 LiDAR data does not quite cover the model domain. This is primarily the 1 m Wakool dataset from 2015 for the floodplain downstream of Yarrawonga Weir. The Murray River model, although utilising a 20 m grid cell size, applied SGS at 1 m resolution to make full use of the LiDAR.

The in-bank bathymetry of the Murray River is not represented in the LiDAR data, and as such the available bathymetry data (Section 3.2.3) was used to generate DEMs of the Murray River bathymetry. Upstream of Lake Mulwala, an interpolated river bad was developed based on the available cross sections, following the river alignment and tying into the LiDAR data at the bank. A DEM of the available bathymetry data downstream of Yarrawonga Weir was also developed.

For Lake Mulwala, the available contour data was used to generate a DEM. This bathymetry dataset did not cover the floodplain immediately upstream of Lake Mulwala, known as the Everglades (Australian Water Technologies 1999). For this region, areas inundated with water were lowered to a level between 124.5 mAHD and 123.5 mAHD It was noted that the bathymetry data did not sufficiently define the Murray River channel through the lake (Australian Water Technologies 1999) or Everglades, and hence a 60 m wide channel was carved through the Everglades and Lake Mulwala, along the main channel through the Everglades and roughly connecting low points in the DEM, linearly interpolated between the upstream and downstream Murray River bathymetry datasets (approximately 117.8 mAHD to 113 mAHD). The total volume of Lake Mulwala in the model, to the FSL of 124.9 mAHD was calculated to be approximately 114,100 ML. This is approximately 3% less than the reported volume of 117,500 ML (GoulburnMurray Water 2021) and considered a reasonable approximation. The terrain for the Murray River

model can be seen in Figure 15

Corowa, Howlong and Mulwala Flood Study

The local overland models are covered entirely by the 1 m 2020 LiDAR dataset. The terrain for Howlong, Corowa and Mulwala can be seen in Figure 16, Figure 17 and Figure 18, respectively.

There were several modifications made to this terrain to ensure topographic features were represented correctly. These modifications are discussed in the following sections and also shown in the figures listed above

6.3.1. Embankments

There are a number of embankments throughout the model domains that were included as breaklines. These breaklines ensure that the crest level (overtopping level) of the embankment is accurately represented in the TUFLOW model where these form an obstruction to flow or where flow paths cross over the embankment. This consisted of levees on the Murray River floodplain, road embankments and the railway embankment. In some locations where there are large culverts or bridge structures, the LiDAR typically shows an interpolated ground surface under the road. Depending on the way each structure is modelled, these can require modification to represent the road correctly in the 2D domain (where the structure is modelled in 1D), or to represent the opening correctly (where the structure is modelled in 2D).

6.3.2. Drainage Channels

Breaklines were also included for many of the drainage channels in the local model areas, to ensure that the channel invert is sufficiently represented in the TUFLOW model and conveyance of water between the TUFLOW grid cells is adequate. Breaklines were sampled from the LiDAR DEM, taking the minimum value of nearby cells to represent the invert.

There are two significant drainage channels that convey flows through the study area – the drainage channels around the Corowa Aerodrome and the Bayly Street drainage channel in Mulwala. The Corowa Aerodrome drainage channels convey water from the Corowa industrial area (off Redlands Road), around either side of the aerodrome to the Murray River. Flows originate from the western portion of the Corowa town, including overflows from the Whitehead Street Wetlands. The Bayly Street drainage channel conveys flows from the intersection of Bayly Street and Lucan Street to the Murray River. Flows originate from the Mulwala town area west of the railway line. For these drainage channels, detailed topographic survey was available. A DEM was created from the survey data and a breakline was also created to enforce the channel invert.

6.4. Surface Roughness

Surface roughness values have been defined across the TUFLOW 2D domains, represented by the Mannings ‘n’ coefficient. The roughness is based upon the land use, which was visually inspected using the available aerial imagery (Section 3.6) The cadastre and Local Environmental Plan (LEP) zoning was also used as a guide. Each land use category was assigned a Mannings ‘n’ value, as outlined in Table 16. The spatial distribution of these categories is shown in Figure 19 for the Murray River, and in Figure 20, Figure 21 and Figure 22 for the local models of Howlong,

Corowa and Mulwala, respectively

Table 16: Mannings ‘n’ values used in the TUFLOW model

Corowa, Howlong and Mulwala Flood Study

1. Note that two different values for the floodplain were calibrated. This is explained in Section 7.13

These values are within typical values and were adjusted during the calibration stage. The values adopted for the Murray River model typically align with those adopted for the Hume to Yarrawonga MIKE Flood Model (MDBA 2019), with the exception of floodplain vegetation, which was higher than the MIKE Flood Model value of 0.071. The values for the Murray River model development areas (residential, commercial and industrial areas) are higher than for the local models as individual buildings are not represented in the Murray River model, whereas they are represented in the local models (see Section 6.5).

6.5. Buildings

Buildings were represented in the local overland models as solid obstructions to flow by blocking them out of the TUFLOW grid. The buildings layer obtained from Microsoft was used (see Section 3.3). The building footprints were checked against the latest aerial imagery, with some minor adjustments made for recent developments. These buildings are shown in Figure 20, Figure 21 and Figure 22 for Howlong, Corowa and Mulwala, respectively A higher Manning’s ‘n’ value was adopted for areas of development in the Murray River model to account for buildings rather than modelling individual buildings, due to the larger cell size (see Section 6.4).

6.6. Culverts and Bridges

Large floodplain culverts and bridge structures were included in the Murray River model. Culverts were modelled as 1D elements and bridge structures were modelled as flow constrictions in the 2D domain. This consists of the Murray River crossings at Howlong (River Road), Corowa

Corowa, Howlong and Mulwala Flood Study

(Federation Way and Bridge Road) and Muwala (Melbourne Street and the railway crossing). Details of these structures were obtained from design drawings (Section 3.4.2.1 and Section 3.4.2.2). Large box culverts on Bridge Road and Federation Way were modelled as well as the large bridge structures. The bridge soffit and deck levels were included in the TUFLOW model, with an estimate of the hydraulic obstruction and form losses due to the piers and deck. These loss coefficients were estimated using the relevant literature (Austroads 2018). These culverts and bridges are shown in Figure 23

Culverts with a size of 525 mm width/diameter and greater were included in the local town TUFLOW models as 1D elements, except for trapped low points where flows would accumulate and would only be drained by pipes smaller than 525 mm (where these pipes may determine ponding levels). Details of these culverts were primarily sourced from a field survey (Section 3.4.2.3), with some culverts being contained in Council’s stormwater database (Section 3.4.1) or surveyed as part of the Corowa Aerodrome channel survey and Mulwala Bayly Street channel survey (Section 3.2.1). These culverts are shown in Figure 24, Figure 25 and Figure 26 for the Howlong, Corowa and Mulwala models, respectively.

6.7. Pit and Pipe Network

The pit and pipe network was included in the local town TUFLOW models as a 1D network dynamically linked to the 2D domain. The pits enable the transfer of flows from the 2D domain to the 1D pipes below the ground. The pipes carry flows to the outlet where it discharges to the 2D domain or has a boundary condition. The pit and pipe data received from Council (Section 3.4.1) was used in the TUFLOW models, with the following processing undertaken:

• Pipes smaller than 525 mm in diameter were removed (including pits connected to these pipes), except for trapped low points where flows would accumulate and would only be drained by pipes smaller than 525 mm.

• Pipes were checked and adjusted to ensure consistent pipe sizes (downstream pipes were the same size or larger than upstream pipes), including assigning reasonable pipe sizes to those where data was missing

• TUFLOW requires the polylines representing the pipes to be digitised from upstream to downstream, so these were modified where this was not the case

• Pipe lengths were based on the provided GIS line lengths

• Invert levels of outlets (where they were not provided) were obtained from the LiDAR data.

• Invert levels of pipes were assumed to be, in order of preference:

o Invert level specified in Council’s database

o Surface elevation from the LiDAR data minus the depth to invert specified in Council’s database

o Surface elevation from the LiDAR data minus the pipe diameter (or box culvert height), minus 0.3 m (assumed cover)

• Pipes were assigned a Mannings ‘n’ of 0.012

• Inlet pits were assumed to be kerb inlets 0.9 m wide and 0.15 m high (an approximate size observed in each of the towns)

Corowa, Howlong and Mulwala Flood Study

A summary of the pit and pipe network included in each local TUFLOW model is presented in Table 17

Table 17: Summary of stormwater elements in the local TUFLOW models

The pit and pipe networks are shown in Figure 24, Figure 25 and Figure 26 for Howlong, Corowa and Mulwala, respectively.

6.8. Yarrawonga Weir

In the Murray River model, Yarrawonga Weir is required to be modelled. Details of the weir structure were provided (see Section 3.5) and included in the model as an operational gated spillway, with the following configuration:

• 8 gates in the southern bank and 2 gates in the northern bank

• Gate width of 12.192 m

• Gate height of 8.2 m

• Invert (gate seat) level of 118.8 mAHD

• A 1D gated spillway structure was assumed in TUFLOW

• Default discharge coefficients for gated spillways applied

Gate operations were also required to be modelled, given the sluice gate type of structure (ie. the gates could not be assumed to just be fully open at the start of the event, as water could potentially drain to a very low level). The following observations were made based on the gauge information:

• Minimum gauged water level in the lake is approximately 119.1 mAHD. It is periodically drawn down for management of Egeria densa and weir maintenance.

• Average water level in the lake is approximately 124.75 mAHD

• Maximum water level in the lake is approximately 125.2 mAHD

• In flood events, water appears to be passed such that there is very little change to the lake level during the event

The provided information on gate operations (see Section 3.5) was utilised in addition to the above observations to derive gate operations for the TUFLOW model. It is noted that the gate operations in reality are complex in nature and rely on a range of data sources and human interpretation and judgement. In the model, there are simplifications to this process, the primary ones being:

• An assumption that all gates operate in a synchronous nature.

• Gates operate in an automated way (i.e. if certain criteria are met, the gates are opened, closed or remain in their current state).

• Reliance on the current Lake Mulwala water level only to determine gate openings (i.e. no ‘forecasting’ of inflows or reliance on measured flows of any kind).

The gate operations adopted in the TUFLOW model are shown in Diagram 3, with the following

additional considerations:

Corowa, Howlong and Mulwala Flood Study

• Gate speed of 1 cm/min assumed (eg. it takes 10 minutes to open 10 cm or 0.1 m).

• Wait time of 30 minutes from when a gate opening change is implemented to when the water level in the lake is assessed and gate opening can be changed again.

• Both northern and southern gates operate in tandem, although it is understood that the northern gates are only operated in flow conditions above 60,000 ML/day.

• The irrigation offtakes are not included in the model, although it is understood that the Mulwala Canal has a capacity of 10,000 ML/d and Yarrawonga Main Channel has a capacity of 3,100 ML/d. It is also understood that up to 13,000 ML/day is able to be passed through the hydro power station.

It is likely that these considerations result in ‘quicker’ gate operations than what happens in reality. This is needed, however, due to the reliance on Lake Mulwala water levels to operate the gates – the flows to be passed have essentially arrived at the lake and raised the water level. In reality, there is the ability to predict inflows and operate the gates accordingly.

The Mulwala Canal and Yarrawonga Main Channel are included in the Murray River model terrain, however, the gates to these irrigation channels were modelled to be closed for the duration of flood events. This is so that elevated water levels in Lake Mulwala are not able to enter the channels and potentially overtop and inundate low lying terrain. It is only if the channel is overtopped from overland flows that the channels would convey flow.

In the Mulwala local overland model, the Mulwala Canal is included, with a static water level in the channel consistent with that adopted for Lake Mulwala (see Section 6.9.2).

Gate

Operation

Open the gate by 0.1 m

125.5 mAHD

Open the gate by 0.05 m

125.15 mAHD

Corowa, Howlong and Mulwala Flood Study

Lake Mulwala

If water level is rising, open the gate by 0.05 m

124.9 mAHD

If water level is rising, open the gate by 0.05 m

If water level is falling, close the gate by 0.05 m

124.7 mAHD

If water level is falling, close the gate by 0.05 m

124.5 mAHD Initial water level

Close the gate by 0.1 m

Outflow

118.8 mAHD Gate invert level

Note: Not to scale

Diagram 3: Assumed Yarrawonga Weir Gate Operations in the TUFLOW model

6.9. Model Boundaries

6.9.1.

Inflows

Inflows into the Murray River model were applied at two locations – the Murray River, at the upstream extent of the model, and the Ovens River, at the model boundary (just upstream of the Murray Valley Highway). These inflow locations are shown in Figure 23 Flows were sourced from gauged flows on both rivers for calibration events and from FFA for design events. For recent calibration events (2016 and 2022), two local tributary inflows were also included (Indigo Creek and Black Dog Creek) based on gauge records. Specific inflows for each calibration event are discussed in Section 7

Inflows into the local town TUFLOW models were sourced from the WBNM simulated runoff hydrographs for each subcatchment. Where subcatchments drain to urban pits, the flow is applied to 2D cells at the pit inlet. Elsewhere, the subcatchment flow is applied to a group of 2D cells

Howlong and Mulwala Flood Study

located at the subcatchment outlet (the lowest point of the terrain). For Majors Creek, the total flows simulated by the WBNM model for several flow paths were applied at the relevant subcatchment outlets, located upstream of Howlong-Burrumbuttock Road, with local subcatchment flows applied downstream of these points.

6.9.2. Downstream Boundary

In the Murray River model, a single stage-discharge boundary was applied at the downstream end of the model. The stage discharge relationship is automatically calculated by TUFLOW, given the 2D cross section of each river and a channel slope (1% was adopted, based on the bathymetry data). This allows water to exit the model at a rate proportional to the calculated water level. This produces a ‘normal flow depth’ at the boundary. A number of other overland flow boundaries are also provided, particularly where breakout flows occur through Mulwala in rare events and at other locations in extreme events. A lower slope factor (0.1%) was adopted for those breakout flow boundaries, due to the relatively flat nature of the land The outflow boundaries can be seen in Figure 23

For each of the local overland models, stage-discharge boundaries were applied at all locations where water could exit the model domain. These are primarily discharges to the Murray River, but also includes some locations where water can flow to the north and west. This is typically in the order of 5% for discharges to the Murray River (generally steep terrain) and 0.1% for overland flows to the north and west. A constant water level was assumed for Lake Mulwala in the local Mulwala model of 124.75 mAHD, representing an average lake level based on gauged data. The overland flow boundaries can be seen in Figure 24, Figure 25 and Figure 26 for Howlong, Corowa and Mulwala, respectively. For those culverts discharging to the Murray River, a 1D boundary was applied with a constant water level at the obvert of the culvert outlet.

7. MODEL CALIBRATION

The aim of the calibration process is to ensure the modelling system can replicate historical flood behaviour. There are assumptions in the modelling inputs, such as the effect of vegetation on flow and the amount of infiltration into the soil, which can be adjusted to improve the match between observed and modelled flood levels. A good match to historical flood behaviour provides confidence that the modelling methodology and schematisation can accurately represent the important flood processes in the catchment.

As discussed in Section 2.2, there are two different flood mechanisms that affect the study area. Flooding from the Murray River is simulated in the Murray River model while overland inundation is simulated in the local overland models. Due to the availability of stream gauge data, there is more information available regarding large flood events for the Murray River than for the local overland events. These events are discussed below.

7.1. Murray River Flood Events

Five Murray River flood events were selected to be modelled as part of this flood study. These events are summarised in Table 18. All of the events selected were within the top 10 events at all of the relevant Murray River gauges.

Table 18: Summary of Murray River Flood Events Modelled

1. Gauge level in metres using gauge datum

2. Peak flow in megalitres per day, to the nearest 10

3. Event/year rank based on all gauge level data

The following events were not selected for calibration:

• 1917 – although this is the largest event post 1900, there was very little data to calibrate the model to, including lack of gauge data at Doctors Point, Howlong and Yarrawonga, and conflicting records at Corowa (low gauge level recorded in the WaterNSW dataset). The age of this event also makes it difficult to determine floodplain conditions.

• 1931 – although this is the fourth largest event recorded at the Corowa gauge, there was very little data to calibrate the model to, including lack of gauge data at Howlong and Yarrawonga. The age of this event also makes it difficult to determine floodplain conditions.

• 1970 – this was a reasonably large event, being ranked 4th at Howlong and 5th at Corowa, however two events from 1970 were already modelled that were larger than this.

• 1993 – there was some flood data available from the Victoria flood database (see Section 3.8), however, an analysis of the gauge data indicated this was actually an Ovens River event (Murray River flow is approximately one third of the Ovens River flow), and there was very little value in modelling this event.

• 2012 - there was some flood data available from the Victoria flood database (see Section 3.8), however, an analysis of the gauge data indicated this was actually a Victorian event – with substantial flows in Indigo Creek, Black Dog Creek and the Ovens River event (Murray River flow contributes approximately 30%-40% of the total flow), and there was very little value in modelling this event.

7.2. Local Overland Flood Events

7.2.1.March 2012 Flood Event

There is little known about the impacts of the March 2012 storm event. The March 2012 event was noted as a significant rainfall event at both Corowa (GHD 2019a) and Yarrawonga (Alluvium 2019). There was substantial rainfall in the preceding 3 days to the storm event (over 100 mm at Mulwala and almost 40 mm at Howlong).

An analysis of the rainfall was undertaken using the available rainfall data (see Section 3.7) for Howlong. The results are presented in Figure 27 and Figure 28. The cumulative rainfall plots (Figure 27) indicate the storm was fairly widespread, with approximately 100 mm being recorded between 3 pm on the 29th February and 7 pm on the 1st March, 2012 at Yarrawonga, Wangaratta and Rutherglen. A higher rainfall total was recorded at Albury (approximately 140 mm). The Howlong Post Office daily rainfall station (074054) recorded 116 mm on the 1st March and 2nd March 2012, which sits between the recorded rainfalls at Rutherglen and Albury, being consistent with its location between these two towns. The rainfall recorded on the 1st March 2012 (94 mm) was the 3rd highest daily total at the Howlong Post Office. All the temporal distributions display a very similar pattern. As such, the most reliable estimation of rainfall over Howlong was assumed to be the Howlong Post Office (074054) rainfall total, distributed using the Rutherglen gauge (082039) temporal pattern. This is shown in the figures as ‘Scaled Rutherlgen’. A comparison with the intensity-frequency-duration (IFD) design rainfall data for Howlong is shown in Figure 28. For the scaled Rutherglen gauge, the rainfall is between a 1% AEP and 0.5% AEP event for durations between 12 and 24 hours, and between a 50% AEP and 2% AEP for durations between 2 hours and 6 hours.

An analysis of the rainfall was undertaken using the available sub-daily rainfall data (see Section 3.7.2) for Mulwala. The results are presented in Figure 29 and Figure 30 The Yarrawonga gauge (081124) gauge was assumed to be the most reliable representation of rainfall over Mulwala. The cumulative rainfall plot (Figure 29) indicates that approximately 100 mm being recorded between 4 pm on the 29th February and 7 pm on the 1st March, 2012. This is also consistent with nearby sub-daily gauges (discussed for Howlong above). A comparison with the IFD design rainfall data for Mulwala is shown in Figure 30 The rainfall is estimated to be between a 5% AEP and 2% AEP event for durations longer than 10 hours, and between a 50% AEP and 10% AEP for durations shorter than 10 hours

Given the size of the rainfall event and the information that has been reported, the March 2012 flood event is estimated to be one of the most significant recent events for Mulwala and Howlong. Despite having no valuable information for the purposes of model calibration, this event was modelled for Howlong and Mulwala, to demonstrate a recent large rainfall event.

7.2.2.January 2022 Flood Event

In the initial stages of this project, a severe storm event hit the town of Corowa. In the early hours of 30th January 2022, an intense storm occurred over Corowa. The storm only lasted approximately 4 hours (11 pm on the 29th to 3 am on the 30th), but records indicate a significant

Corowa, Howlong and Mulwala Flood Study

amount of rain fell. A resident of Corowa on the Floodplain Management Committee (FMC) reported 125 mm on their private rainfall gauge. The Corowa Airport BoM daily rainfall station apparently recorded 182 mm of rainfall at 9am on the 30th, however the official BoM records do not have an entry for this day, presumably due to the abnormally high rainfall it did not pass quality control measures. As a comparison, the highest daily rainfall total recorded at the Corowa Airport gauge was 184 mm on 7th January 1934, with the next highest being 124 mm. It is also unlikely that these high rainfall totals occurred over the short timespan that the January 2022 event did.

The storm appeared to be a localised storm cell on the BoM rain radar from Yarrawonga, and it is likely such an intense and localised storm could produce considerably different rainfall across the town. A screenshot of the radar as it passed over Corowa is shown in Diagram 4. The storm appears to move in a south-easterly direction over Corowa, and toward Rutherglen, where storm damage and flooding was also reported (Corowa Free Press 2022a, Corowa Free Press 2022b). There was no rain in the 3 weeks preceding the storm.

Corowa, Howlong and Mulwala Flood Study

Diagram 4: Radar image of the storm over Corowa at 12:49am on 30th January 2022 (Source: theweatherchaser.com)

An analysis of the rainfall was undertaken using the available sub-daily rainfall data (see Section 3.7.2). The results are presented in Figure 31 and Figure 32. The cumulative rainfall plots (Figure 31) confirm that the storm was localised, with gauges at Yarrawonga, Wangaratta and Albury all recording less than 50 mm of rainfall. A local resident’s gauge at Howlong also recorded just 40 mm. It is also noted that one local gauge in Corowa also recorded a low rainfall total (ICOROW4, less than 30 mm) and another local gauge appears to have stopped working early in the storm event (ICOROW7, data ceases at midnight).

The BoM Rutherglen Research Station gauge (082039) only recorded 41 mm, but has a temporal pattern that is very consistent with a number of other local gauges (ICOROW4, ICOROW6, and IWAHGU1), although slightly lagged behind the gauges in Corowa (indicative of the direction the storm was travelling). Since this is the only official BoM station with data close to Corowa, this is

likely to be the most reliable temporal pattern.

Corowa, Howlong and Mulwala Flood Study

The rainfall total for Corowa is estimated to be between 125 mm and 182 mm, based on reliable daily rainfall estimates. This only aligns with one gauge, IWAHGU1, which recorded 142 mm. One gauge, ICOROW6 recorded 226 mm (which appears to be an outlier), while the remaining gauges recorded less than 50 mm. As such, the Rutherglen gauge was scaled to 153.5 mm (an average of 125 mm and 182 mm) for comparison purposes and plotted on the figures as well.

A comparison with the IFD design rainfall data for Corowa is shown in Figure 32. For those gauges considered to be reliable (IWAHGU1 and the scaled Rutherglen gauge), the rainfall sits well above the 1 in 2000 (AEP) event for all durations from 30 minutes to 9 hours. This indicates the extreme nature of the storm event. This event was modelled for Corowa, given the amount of information to compare the modelled and observed flooding for the purpose of model calibration.

7.3. Methodology

7.3.1. Murray River Model Calibration

The Murray River TUFLOW model calibration was undertaken for five large historic events. Recorded flows were applied at the Murray River upstream boundary based on recorded data at Doctors Point (409017). Recorded flows were also applied for the Ovens River, based on gauge data at Peechelba (403241). As the TUFLOW model boundary does not extend to either of these gauges, the flow hydrographs were lagged to account for the travel time between the gauge and the model boundary This was considered sufficient based on a review of the flow hydrographs at both Doctors Point (409017) and Corowa (409002). An example is shown in Diagram 5 for the 2016 event. The peak flow at Corowa is within 200 ML/d of the peak flow at Doctors Point, exhibiting no attenuation. There is also some change in the hydrograph shape, however given that the model covers approximately two thirds of the flood routing from Doctors Point to Corowa, this is considered to be sufficiently covered in the model.

Corowa, Howlong and Mulwala Flood Study

Diagram 5: 2016 event recorded flow hydrographs at Doctors Point and Corowa

For a number of events, flows for the Ovens River were not available. For these events, the flow was estimated based on the difference between the flow recorded at Corowa (409002) and the flow recorded at Yarrawonga (409025), accounting for travel time from the gauges to Yarrawonga.

The modelled flood results were then compared to the observed data. This primarily consisted of the stream gauges at Howlong, Corowa and Yarrawonga, although this was supplemented with additional flood data available for each event. The model parameters were adjusted until a reasonable fit was obtained to the observed data.This primarily consisted of altering the Mannings ‘n’ values. It is noted that the model was not altered for historic conditions as the conditions were largely unknown (for example floodplain terrain/vegetation in the 1970’s). Two substantial changes to hydraulic structures in recent history was the construction of the Federation Way crossing at Corowa (completed in 2005) and works completed on Bridge Road (completed in 2008), on the lower terrace at Corowa. For historic events prior to 2005, Federation Way was removed from the DEM and hydraulic structures were removed. Information about the previous road elevation and timber bridges along Bridge Road were not available, and hence no modifications were made to Bridge Road for historic events.

7.3.2.Local Overland Model Calibration

The approach to calibration for the local overland models was a joint calibration process of both the WBNM hydrologic models and TUFLOW hydraulic models. Rainfall loss parameters in WBNM and the Mannings ‘n’ roughness values in TUFLOW were adjusted until a reasonable match to the known flood level marks was achieved. The only valuable flood mark information was for Corowa from the January 2022 storm event. There were no flood observations available for

Howlong or Mulwala.

7.4. Murray River 1974 Event Calibration

Corowa, Howlong and Mulwala Flood Study

The Murray River 1974 event was simulated using the recorded flows at Doctors Point (409017) for the Murray River, delayed by 10 hours to account for the travel time between the gauge and the model boundary. There were no recorded flows for the Ovens River, and hence a comparison of flows at Corowa (409002) and Yarrawonga (409025) was undertaken to derive an indicative hydrograph. While only the main flood peak was simulated, with flows above 40,000 ML/d in the Murray River, a warmup period of 48 hours was included to account for flows prior to the main peak. The simulation started on the 3rd October 1974 at approximately 10 am and was run for approximately 39 days. The inflow hydrographs are shown in Diagram 6

The results of the TUFLOW model calibration to the gauge data is shown in Figure D1. There is a very good match to the Howlong gauge water level, with the modelled peak water level being almost identical to the recorded level. The shape of the hydrograph and timing is also considered to be a very good match. At the Corowa gauge, the modelled peak water level is only slightly higher than the recorded level, but still within 0.1 m. The modelled peak flow is also slightly higher, but still within 5% of the recorded flow at the gauge. The modelled hydrograph tends to rise and fall quicker than the recorded hydrograph, however, there is still a reasonable match. There were no recorded water levels in Lake Mulwala for this event. Downstream of Yarrawonga Weir, at the Yarrawonga gauge, the modelled peak water level is higher than the recorded level by approximately 0.08 m, with the modelled peak flow being approximately 6% higher. However, given the uncertainty in assumptions regarding theOvens River inflows andthe operation of gates,

Diagram 6: Inflow hydrographs for the 1974 flood event

Corowa, Howlong and Mulwala Flood Study

this is considered to be a good match to the overall hydrograph shape and magnitude of flows released. A summary of the calibration to peak gauge levels and flows is shown in Table 19

Table 19: Comparison of peak levels and flows at the gauges for the Murray River 1974 event

Note: Levels rounded to the nearest centimetre and flows rounded to the nearest 100 ML/d

There was some flood data available from the Victorian Flood Database (see Section 3.8) for the 1974 event. A map of the 1974 flood event, including the flood marks and extent comparison can be found in Figure D2. The available flood data and comparison to the model results are summarised as follows:

• One flood mark of ‘medium’ reliability, recorded at the Piper Street crossing of the Yarrawonga Main Channel. It is thought that this water level is representative of water levels in the canal. The operations of the canal are unknown, and for the purposes of flood modelling, the canals are assumed to not have water flowing in them. It is not the intention of this model to simulate flows in the irrigation canals and as such no water level has been modelled in the canal.

• One flood mark of ‘high’ reliability, recorded immediately downstream of Yarrawonga Weir. The modelled level was 0.04 m higher than the recorded level, indicating a good match. This indicates a better match than to the Yarrawonga Gauge, located approximately 500 m downstream of this flood mark.

• Estimated flood extents of ‘low’ to ‘medium’ reliability, covering the majority of the Murray River study area. The modelled extents are very close to the observed extents in the upstream portion of the model, from the model boundary to Lake Mulwala. There are only a few areas where the modelled extent is slightly larger than the observed. The modelled extents within Lake Mulwala match reasonably well, although the modelled extent downstream of Yarrawonga Weir (on the Victorian side) is less than that observed. The match to the extents, considering their reliability, is considered to be good.

The calibration to the 1974 event was considered to be very good.

7.5. Murray River 1975 Event Calibration

The Murray River 1975 event was simulated using the recorded flows at Doctors Point (409017) for the Murray River, delayed by 10 hours to account for the travel time between the gauge and the model boundary. There were no recorded flows for the Ovens River, and hence a comparison

Corowa, Howlong and Mulwala Flood Study

of flows at Corowa (409002) and Yarrawonga (409025) was undertaken to derive an indicative hydrograph. While only the main flood peak was simulated, with flows above 50,000 ML/d in the Murray River, a warmup period of 48 hours was included to account for flows prior to the main peak. The simulation started on the 21st October 1975 at approximately 10 am and was run for approximately 12 days. The inflow hydrographs are shown in Diagram 7.

7: Inflow hydrographs for the 1975 flood event

The results of the TUFLOW model calibration to the gauge data is shown in Figure D3. There is a good match to the Howlong gauge water level, with the modelled peak water level being within 0.1 m of the recorded level. The shape of the hydrograph and timing is also considered to be a good match, although the rising limb is slightly overestimated. At the Corowa gauge, the modelled peak water level is also within 0.1 m of the recorded level and matches very closely. The modelled peak flow is very close to the recorded flow at the gauge (within 1%). The modelled hydrograph matches the shape of the main hydrograph peak and falling limb well. The start of the rising limb is not well matched simply due to the period of the event modelled and the warmup time of the model. There were no recorded water levels in Lake Mulwala for this event. Downstream of Yarrawonga Weir, atthe Yarrawonga gauge,themodelled peak water level is approximately 0.1 m higher than the recorded level, with the modelled peak flow being within 3% of the recorded flow. The timing of the model is earlier than the recorded event, however, given the uncertainty in assumptions regarding the Ovens River inflows and the operation of gates, this is considered to be a good match. A summary of the calibration to peak gauge levels and flows is shown in Table 20

Diagram

Corowa, Howlong and Mulwala Flood Study

Table 20: Comparison of peak levels and flows at the gauges for the Murray River 1975 event

Note: Levels rounded to the nearest centimetre and flows rounded to the nearest 100 ML/d

Council has a recorded flood mark for the 1975 event on a pole outside the Corowa Civic Centre (see Section 3.8). The level of this flood mark was estimated by using the nearby LiDAR ground points and measured height of the mark above the ground. The level was estimated to be 134.94 mAHD. The modelled level at this location was 134.92 mAHD, just 0.02 m below the estimated level, which is considered to be a good match.

There was some flood data available from the Victorian Flood Database (see Section 3.8) for the 1975 event. A map of the 1975 flood event, including the flood marks can be found in Figure D4. The available flood mark data and comparison to the model results is summarised as follows:

• 16 ‘modelled’ levels. These are presumably from a previous flood model that was used to simulate the 1975 flood event. The reliability of this data is unknown. The differences between the current modelled and previous modelled levels are in the range of -0.5 m to +0.8 m. The average difference, however, is 0.2 m, with most levels being within 0.1 m to 0.3 m. These points are located from the upstream model boundary to just downstream of Corowa.

• 3 flood marks of ‘low’ reliability, located in the vicinity of Yarrawonga Weir. There are two points on the bank, both upstream (Lake Mulwala) and downstream (Murray River) of the weir on the Yarrawonga side. The difference between the modelled and observed level at both of these points is approximately 0.2 m, indicating a reasonablematch. This difference is greater than the difference at the Yarrawonga Gauge, located further downstream of these points. The third flood mark is located at the weir itself, on the downstream side. The observed level at this point was approximately 0.1 m lower than the corresponding downstream point, which brings into question its reliability. At this location, the modelled level is approximately 0.4 m higher than the observed level. Given its location at the weir itself and the turbulent nature of the outflows from the weir, this is not considered to be a good calibration point.

The calibration to the 1975 event was considered to be very good.

7.6. Murray River 1992 Event Calibration

The Murray River 1992 event was simulated using the recorded flows at Doctors Point (409017) for the Murray River, delayed by 10 hours to account for the travel time between the gauge and the model boundary. There were no recorded flows for the Ovens River, and hence a comparison of flows at Corowa (409002) and Yarrawonga (409025) was undertaken to derive an indicative hydrograph. While only the main flood peak was simulated, with flows above 40,000 ML/d in the Murray River, a warmup period of 48 hours was included to account for flows prior to the main peak. The simulation started on the 7th October 1992 at approximately 5 am and was run for approximately 32 days. The inflow hydrographs are shown in Diagram 8

The results of the TUFLOW model calibration to the gauge data is shown in Figure D5. There is a good match to the Howlong gauge water level, with the modelled peak water level being within 0.1 m of the recorded level. The shape of the hydrograph and timing is also considered to be a good match, although two smaller peaks either side of the main peak are overestimated. At the Corowa gauge, the modelled peak water level is also within 0.1 m of the recorded level and matches very closely. The modelled peak flow is almost identical to the recorded flow at the gauge (within 2%). The modelled hydrograph matches the shape of the main hydrograph peak and falling limb well. The first peak, however, is modelled to be slightly earlier than the recorded peak. The water level in Lake Mulwala is typically within 0.1 m of the observed water level, although the shape of the time series is not well matched, presumably due to specific gate operations. Downstream of Yarrawonga Weir, at the Yarrawonga gauge, the modelled peak water level is reasonably close to the recorded level, being approximately 0.1 m higher, with the modelled peak flow being within 7% of the recorded flow. The timing of the model matches well with the recorded

Diagram 8: Inflow hydrographs for the 1992 flood event

Corowa, Howlong and Mulwala Flood Study

peak. There are minor fluctuations in the modelled flows due to the gate operations being triggered at different water levels, but the overall shape is considered reasonable given the uncertainty in assumptions regarding the Ovens River inflows and the operation of gates. A summary of the calibration to peak gauge levels and flows is shown in Table 21.

Table 21: Comparison of peak levels and flows at the gauges for the Murray River 1992 event

Note: Levels rounded to the nearest centimetre and flows rounded to the nearest 100 ML/d

There were no other flood marks for the 1992 event. There was some flood extent data available from the Victorian Flood Database (see Section 3.8) for the 1992 event. A map of the 1992 flood event, including the observed flood extents can be found in Figure D6. The available flood extent data was of ‘low’ reliability and located around Lake Mulwala and immediately downstream of Yarrawonga Weir. The modelled extent of Lake Mulwala on the Victoria side matches reasonably well with the estimated observed extent. The modelled extent downstream of Yarrawonga Weir is typically slightly less than that observed. The overall calibration to the 1992 event was considered to be very good.

7.7. Murray River 1996 Event Calibration

The Murray River 1996 event was simulated using the recorded flows at Doctors Point (409017) for the Murray River, delayed by 13 hours to account for the travel time between the gauge and the model boundary. There were no recorded flows for the Ovens River, and hence a comparison of flows at Corowa (409002) and Yarrawonga (409025) was undertaken to derive an indicative hydrograph. While only the main flood peak was simulated, with flows above 20,000 ML/d in the Murray River, a warmup period of 48 hours was included to account for flows prior to the main peak. The simulation started on the 28th September 1996 at approximately 7 am and was run for approximately 37 days. The inflow hydrographs are shown in Diagram 9

Diagram 9: Inflow hydrographs for the 1996 flood event

The results of the TUFLOW model calibration to the gauge data is shown in Figure D7. There is a good match to the Howlong gauge water level, with the modelled peak water level being within 0.1 m of the recorded level. The shape of the hydrograph and timing is also considered to be a good match, although the modelled water level is consistently slightly higher than the recorded level. At the Corowa gauge, the opposite is true, with the water level being consistently slightly lower than the recorded water level. The modelled peak flow is also slightly lower, but still within 2% of the recorded peak. The modelled hydrograph matches the shape of the recorded hydrograph well, although the initial peak is modelled to arrive earlier than the recorded peak. The water level in Lake Mulwala is typically within 0.2 m of the observed water level, although the shape of the time series is not well matched, presumably due to specific gate operations. Downstream of Yarrawonga Weir, at the Yarrawonga gauge, the modelled peak water level and flow is almost identical to that recorded. The timing of the model matches very well with the recorded peak. There are minor fluctuations in the modelled flows due to the gate operations being triggered at different water levels, but the overall shape is considered very good given the uncertainty in assumptions regarding the Ovens River inflows and the operation of gates. A summary of the calibration to peak gauge levels and flows is shown in Table 22

Corowa, Howlong and Mulwala Flood Study

Table 22: Comparison of peak levels and flows at the gauges for the Murray River 1996 event

Note: Levels rounded to the nearest centimetre and flows rounded to the nearest 100 ML/d

There were no other flood marks or observations for the 1996 event. The flood behaviour can be seen in Figure D8. The calibration to the 1996 event was considered to be good.

7.8. Murray River 2016 Event Calibration

The Murray River 2016 event was simulated using the recorded flows at Doctors Point (409017) for the Murray River, delayed by 15 hours to account for the travel time between the gauge and the model boundary. Recorded flows were available for the Ovens River at Peechelba (403241) and these flows were delayed by 8 hours to account for the travel time between the gauge and the model boundary. Recorded flows were also available for two smaller tributaries – Indigo Creek which enters the Murray River approximately 5 km upstream of Howlong (187 km2 catchment area), and Black Dog Creek, which enters approximately 8 km downstream of Corowa (635 km2 catchment area). Both of these creeks traverse the Victorian side of the Murray River. Flows are available from Indigo Creek at D/S Creamery Bridge (403248) and Black Dog Creek Upstream of Dugays Bridge (403247). Due to the proximity of the gauges to the Murray River, no delay was applied to these flows and they were injected directly into the Murray River at the location of the tributary discharge point. Peak flows from these tributaries represent less than 5% of the peak Murray River flow upstream of the Ovens River in the 2016 event.

Initially, only the main flood peak was simulated, however, a longer lead time was included to test the assumption of modelling only the peak. It was found that there was negligible change to the simulation of the main peak, although a better representation of the early stages of the rising limb was obtained, particularly for locations further downstream. Flows above 20,000 ML/d in the Murray River were simulated, until they dropped below 40,000 ML/d. A warmup period of 48 hours was included to account for flows prior to the period simulated. The simulation started on the 27th October 2016 at approximately 3 am and was run for approximately 44 days. The inflow hydrographs are shown in Diagram 10

Diagram 10: Inflow hydrographs for the 2016 flood event

The results of the TUFLOW model calibration to the gauge data is shown in Figure D9. There is a good match to the Howlong gauge water level, with the modelled peak water level being approximately 0.05 m lower than the recorded level. The recorded hydrograph has some ‘noise’ in it at the peak (it is understood that a loose component on the gauge caused this fluctuation, M. Dixon, personal communication 8/2/23), such that if this was removed, the modelled peak water level would be just 0.03 m lower than the recorded water level. The shape of the hydrograph and timing is also considered to be a good match, although the rising limb shows a slightly different shape. At the Corowa gauge, the modelled water level is almost identical at the peak of the flood, with a larger volume of water modelled prior to the peak. The modelled peak flows are also very similar to the recorded peak. The water level in Lake Mulwala is up to 0.11 m higher than the observed water level, and exhibits a different shape, presumably due to specific gate operations. Note that these peak water levels (both in the model and recorded at the gauge) occur early in the event (in September). When the flood peak occurs (in October, as shown in Figure D9), the modelled water level in Lake Mulwala is a maximum of 124.82 mAHD, 0.16 m above the maximum recorded water level of 124.66 mAHD during this period. Downstream of Yarrawonga Weir, at the Yarrawonga gauge, the modelled peak water level is approximately 0.06 m higher than the recorded level, with flows being within 2% The timing of the model matches very well with the gauge. A summary of the calibration to peak gauge levels and flows is shown in Table 23

Corowa, Howlong and Mulwala Flood Study

Table 23: Comparison of peak levels and flows at the gauges for the Murray River 2016 event

Note: Levels rounded to the nearest centimetre and flows rounded to the nearest 100 ML/d

* There is substantial ‘noise’ in the data near the peak and the recorded water level appears to oscillate. The recorded peak was 6.83 m, but a more representative peak level of 6.77 m, assuming a ‘smooth’ hydrograph, was adopted

A validation of the gate operations was also undertaken for the 2016 event. While the true indication of gate operations in the model would be the simulation of flows downstream of the weir (calibration to the Yarrawonga gauge flow), it is still useful to compare the modelled and actual gate operations. Daily gate operations were provided for the 2016 event (see Section 3.5). The gate openings, provided as an area per gate, were converted to a representative opening height across all gates for direct comparison to the model. The comparison is shown in Diagram 11. The calculated discharge (based on gate operations, as provided) matches the simulated discharge through the gates well. This is to be expected given the good match to the downstream Yarrawonga gauge. The gate openings in the model, however, are slightly lower than the actual gate openings, although the timing is correct. This can be attributed to the fact that the modelled water level in Lake Mulwala is higher by approximately 0.1 m to 0.2 m throughout the simulation – giving a higher discharge for the same gate opening due to a higher upstream head. As the simulated Lake Mulwala level gets closer to the actual lake level (towards the end of the simulation on the 15th October 2016), it can be seen that the simulated gate opening more closely matches the actual gate opening.

Diagram 11: Comparison actual and simulate Yarrawonga Weir gate operations for the October 2016 event

Given the simplification of gate operations in the model (reliance on Lake Mulwala water level, automated gate opening functions, simultaneous operation of all gates, etc) and the difference in simulated and actual water levels in Lake Mulwala, this is considered to be a reasonable match.

Council has a recorded flood mark for the 2016 event on a pole outside the Corowa Civic Centre (see Section 3.8). The level of this flood mark was estimated by using the nearby LiDAR ground points and measured height of the mark above the ground. The level was estimated to be 134.0 mAHD. The modelled level at this location was also 134 0 mAHD This is considered to be an excellent result.

Flood photography and observations were also available for the 2016 event (see Section 3.9)

The Corowa Caravan Park, located off Federation Avenue was inundated in the 2016 event. The Border Mail (https://www.bordermail.com.au/story/4177315/park-engulfed-by-the-murray/) reported that approximately 90% of the Corowa Caravan Park was inundated when water entered on the 19th September, 2016. Photos indicated a relative shallow depth (< 0.5 m). However, floodwater continued to rise, and photos from 7th October 2016 were posted on the Corowa Caravan Park Facebook page (https://www.facebook.com/profile.php?id=100047007085811), at the peak of the flood, which indicated the flood depth may be more in the order of 2 m. The modelled flood depth across the site is in the range of 1.3 m to 2.1 m. A comparison of the photos and the modelled flood depths are shown in Figure D11.

Flooding also affected the lower terrace of Corowa, in the vicinity of Bridge Street. This includes the Ball Park Caravan Park and open areas around the Corowa Rowing Club and Corowa Rutherglen Football Club. Photos published by the ABC (https://www.abc.net.au/news/2016-10-

Corowa, Howlong and Mulwala Flood Study

04/ball-park-caravan-moves-cabins-amid-rising-murray-river/7899922) on October 4, prior to the flood peak showed the inundation around the Corowa Rowing Club. Photographer Tony Reeckman took a number of photos with a drone on the 5th October around 11 am and 6th October around 2 pm. While this was a day prior to the peak of the flood, the extents would be very close to the peak extent. A comparison was made between the modelled flood extent and these photographs, as shown in Figure D12 The modelled flood extents match very well to the photographs taken. The model shows flooding in all locations where flooding was observed.

The Corowa Bindaree Holiday Park, located off Honour Avenue, was also inundated in the 2016 event. Tony Reeckman also photographed the holiday park using a drone. A comparison was made between the modelled flood extent and these photographs, as shown in Figure D13. Again, the modelled flood extent aligns very well with the observed flood extents.

7.9. Murray River 2022 Event Calibration

Following the completion of an initial calibration, the November 2022 Murray River event occurred. The event was simulated using the recorded flows at Doctors Point (409017) for the Murray River, delayed by 15 hours to account for the travel time between the gauge and the model boundary. Recorded flows were available for the Ovens River at Peechelba (403241) and these flows were delayed by 8 hours to account for the travel time between the gauge and the model boundary. Recorded flows were also available for two smaller tributaries – Indigo Creek at D/S Creamery Bridge (403248) and Black Dog Creek Upstream of Dugays Bridge (403247). Due to the proximity of the gauges to the Murray River, no delay was applied to these flows and they were injected directly into the Murray River at the location of the tributary discharge point. Peak flows from these tributaries represent less than 10% of the peak Murray River flow upstream of the Ovens River in the 2022 event. When this event was simulated, it became clear that therewere additional tributary inflows that were not accounted for that directly contributed to the peak flow recorded at Corowa. This can be seen in the direct comparison between the flow hydrographs at Doctors Point and Corowa (Diagram 12). The peak flow at Corowa is approximately 27,000 ML/d higher than at Doctors Point.

Diagram 12: Comparison of recorded flow hydrographs on the Murray River and its tributaries, in additional to daily rainfall for the November 2022 event

In addition to this, local rainfall gauges recorded between 57.6 mm (74004 Balldale Post Office) and 77.6 mm (82021 Indigo) in the two days prior to the Corowa peak occurring. Rainfall recorded at the Rutherglen Research Station (69.2 mm, station 82039) is shown on Diagram 12. The impact of this rainfall on runoff can be seen in the hydrographs recorded at the Indigo Creek at D/S Creamery Bridge and Black Dog Creek Upstream of Dugays Bridge gauges. It can be seen, however, that the peak at Corowa lies in between the peaks recorded on Indigo Creek (upstream Howlong) and Black Dog Creek (downstream Corowa), indicating that additional inflows in between Howlong and Corowa are likely to have contributed to the recorded peak at Corowa. Based upon this analysis, an additional tributary inflow was applied upstream of Corowa to account for this local tributary inflow, with a peak of 26,000 ML/d.

While only the main flood peak was simulated, with flows above 45,000 ML/d in the Murray River, a warmup period of 48 hours was included to account for flows prior to the period simulated. The simulation started on the 27th October 2022 at approximately 3 pm and was run for approximately 33 days. The inflow hydrographs are shown in Diagram 10

Diagram 13: Inflow hydrographs for the 2022 flood event

The results of the TUFLOW model calibration to the gauge data is shown in Figure D14. There is generally a good match to the Howlong gauge water level, with the majority of the hydrograph being slightly higher than the recorded level, although the peak is slightly lower by 0.1 m. It is thought that some of the additional ungauged tributary inflows may have contributed to Murray River flows upstream of Howlong that were not modelled. At the Corowa gauge, the modelled water level is very close to the recorded water level along the full hydrograph (with the exception of the very start due to the model warm up period). The peak flood level was matched by the model, with the peak flows being approximately 5% lower. The water level in Lake Mulwala is up to 0.2 m higher than the observed water level, and exhibits a different shape, presumably due to specific gate operations. Downstream of Yarrawonga Weir, at the Yarrawonga gauge, the modelled peak water level is approximately 0.07 m higher than the recorded level, with peak flows being within 4%. The timing of the model matches very well with the gauge. A summary of the calibration to peak gauge levels and flows is shown in Table 24

Corowa, Howlong and Mulwala Flood Study

Table 24: Comparison of peak levels and flows at the gauges for the Murray River 2022 event

Note: Levels rounded to the nearest centimetre and flows rounded to the nearest 100 ML/d

Considering the largely unknown contribution of local tributary inflows, the calibration to the 2022 event is considered to be very good.

A validation of the gate operations was also undertaken for the 2022 event. Daily gate operations were provided for the 2022 event (see Section 3.5). The gate openings, provided as an area per gate, were converted to a representative opening height across all gates for direct comparison to the model. The comparison is shown in Diagram 14. The calculated discharge (based on gate operations, as provided) matches the simulated discharge through the gates well. This is to be expected given the good match to the downstream Yarrawonga gauge. The gate openings in the model, however, are slightly lower than the actual gate openings, although the timing is correct. This can be attributed to the fact that the modelled water level in Lake Mulwala is higher by approximately 0.1 m to 0.2 m throughout the simulation – giving a higher discharge for the same gate opening due to a higher upstream head.

Howlong and Mulwala Flood Study

Diagram 14: Comparison actual and simulate Yarrawonga Weir gate operations for the November 2022 event

Given the simplification of gate operations in the model (reliance on Lake Mulwala water level, automated gate opening functions, simultaneous operation of all gates, etc) and the difference in simulated and actual water levels in Lake Mulwala, this is considered to be a good match.

7.10. Corowa January 2022 Event Calibration

The January 2022 storm event over Corowa was modelled in the WBNM hydrologic model using the IWAHGU1 private rainfall gauge. The resulting runoff was used as inflows into the TUFLOW hydraulic model to simulate flood behaviour. It was assumed that no pumped stormwater systems were operational, which aligns with reports from the community. The resulting flood depths, levels, velocities and extents were compared to the available flood data (Section 3.8, 4.2 and 4.3). There were a number of sources of flood information and the comparison of modelled flood behaviour with this information is contained in the following sections.

7.10.1.

Calibration to Flood Marks

Surveyed flood marks were available for the January 2022 flood event in Corowa (see Section 3.8). A comparison of the recorded flood levels and depths is provided in Table 25. The flood marks have been grouped according to their location, as shown in Figure D16. The difference between the modelled flood level and recorded flood level (either a surveyed flood level, or calculated flood level based on a surveyed ground level and measured flood depth) is provided as well as the difference between the modelled flood depth and the measured flood depth, where this was available.

Corowa, Howlong and Mulwala Flood Study

Table 25: Corowa Overland Flood Model Calibration to Flood Marks

Corowa Industrial Area

Central Corowa

The results indicate a reasonable match. The modelled flood level at the airport (FM01) is 0.1

Corowa, Howlong and Mulwala Flood Study

higher than the surveyed level. In the industrial area, the flood levels are typically within ±0.1 m, with the exception of FM06 and FM07, which are approximately 0.2 m higher. The modelled flood levels in central Corowa are on average 0.1 m to 0.2 m higher. FM08 is the only point that is lower, by approximately 0.1 m. The match generally improves the further south the flood mark is located. The flood level is overestimated by approximately 0.2 m in the vicinity of Malbec Circuit (FM09), approximately 0.1 m in the vicinity of Sophia Close (FM14), and is almost identical in the vicinity of Nixon Street (FM21). The three points located south of Nixon Street (FM22, FM23 and FM24) are all within 0.01 m of the surveyed flood level. In South Corowa, the match is within 0.05 m for three of the points (FM25, FM27 and FM28), although two points show a larger difference, up to 0.3 m. These points are all located within 50 m of each other, and the two higher points are considered to be less reliable (for example water may have pushed debris up a solid fence and hence the debris line may not be representative of the true flood level). Overall, the calibration to the flood marks is slightly over in some locations, but is considered to be good.

7.10.2. Calibration to Community Questionnaire Data

The modelled flood behaviour was compared with community responses to the questionnaire that was made available at the commencement of the project (see Section 4.1 and 4.2). A summary of the submission and the comparison to the modelled flood results can be seen in Table 26. The location of these points is also shown in Figure D17

Table 26: Corowa Overland Flood Model Calibration to Community Questionnaire Responses ID Summary of Questionnaire Response Comparison with Modelled Flooding

Q01 Due to sewer backwatering

Q02 No specific comment

Q03 Excess water in backyard.

Q04 Jan 2022 event - 5cm water in yard.

Q05

Q06

Q07

Jan 2022 event - 6in water at front porch, unit inundated.

Water flows from street to property; 30mm depth; Shed flooded; Martin street floods regularly.

6.2m maximum level (Corowa gauge?) due to Hume Dam releases.

Not applicable

Not applicable

Water modelled in backyard.

Local drainage runoff not modelled.

Local drainage runoff not modelled.

Water modelled to flow from street through property, approximately 0.05 to 0.2m deep.

Not applicable

Q08

Q09

Q10

Jan 2022 event - horses standing in 50cm of flood water in yards. 20cm of flood water through stables, tack shed, & hay sheds; Water was from Eastern farms.

Water through the shed was 15cm high, 20cm in yards; End of Vera and McDonald Ave flooded. Neighbours across road flooded above floor.

Jan 2022 event - Pinot Crescent inundated. Water ponds and almost came into house.

Q11 Flood water almost up to knees.

Q12 Jan 2022 event - water reached the back

Modelled flood depth of 0.3-0.4m in fields, 0.1m-0.3m around buildings.

Local drainage runoff not modelled.

Modelled water in Pinot Crescent, with ponding depths of 0.3-0.4m across property.

Modelled flood depth of up to 0.3m in the street and 0.2m on the property.

Water modelled to reach the southern side of

Howlong and Mulwala Flood Study

ID Summary of Questionnaire Response Comparison with Modelled Flooding step of our house. the house (assumed to be the rear) with 0.1m depth.

Q13

Q14

Q15

Q16

Q17

Q18

Q19

Q20

Q21

Semi-frequent inundation on property. Overland flow from Sanger St, covers River & Riesling St and backs up.

Approx 400mm deep, water from Birdwood Ln.

Jan 2022 event - 20mm through house, 400mm around property, 50mm through shed.

Church Ln floods, water enters rear shed and sits in neighbour's front yard for days.

Entire property flooded; no access to back gate and land behind on Dominion road.

Jan 2022 event - access to the buildings was made by wading through water.

Jan 2022 event - knee deep water with lowest point of the park being ~1m deep. Road into park cut.

Water from road into property, flooded garage, 100mm deep around entire yard.

Jan 2022 event - Norseman Road and Hammersley Road completely under water over a foot deep.

Local drainage runoff not modelled.

Water modelled to be 0.2-0.3m deep in Birdwood Street, up to 0.1m deep on property.

Modelled depth of 0.4m around property, 0.3m deep at house and shed.

Flooding modelled on Church Lane, with water ponding in yard and neighbour's yard.

Flooding modelled all around property up to 0.2m deep.

Water modelled to be 0.1m deep at front of property.

Water modelled to be typically 0.3-0.5m deep across park, up to 1m deep.

Modelled flood depths typically up to 0.2m across yard.

Norseman Road modelled to be inundated typically up to 0.3m, Hammersley Road inundated up to 0.5m.

Q22 Above floor flooding, above ankle deep. Local drainage runoff not modelled.

Q23 No specific comment Not applicable to Corowa - located in Howlong.

Jan 2022 event - water in the street was knee high; above floor flooding from rear of house.

Q24

Q25

Q26

Q27

Q28

Q29

Water modelled to be up to 0.15m deep in Birdwood Street, up to 0.2m deep at rear of house.

Water flooded into garage, 10 cm deep. No house number provided. Church Street flooded in several locations in model.

Jan 2022 event - front and backyard was inundated with water up to 32cm. Water in the carport was almost a foot deep. Water from street and neighbour's property.

Access to property cut off.

Water modelled to be up to 0.3m deep in front yard, 0.4m deep in back yard.

Modelled flood depths up to 0.4m in Boral Court would restrict access to property.

Jan 2022 event - roads cut over night. A number of roads modelled to be inundated to a depth that is not trafficable.

Floodwater from Federation Ave, Parade Pl & Gray St flooded 4, 6 and 8 Gray St with 5cm water flowing to rear of properties. Water 10cm above Gray St kerb.

Water modelled to flow from Federation Ave, across Gray Street and through numbers 2, 4 and 8. Water modelled to be approximately 0.1-0.15m deep in Gray Street gutter.

Q30 Above floor flooding. Local drainage runoff not modelled.

Q31

Jan 2022 event - above floor flooding, shallow depth; 30-40cm in shed.

Water modelled to be 0.1-0.3m deep across property.

Corowa, Howlong and Mulwala Flood Study

ID Summary of Questionnaire Response Comparison with Modelled Flooding

Q32

Q33

Q34

Q35

Q36

Jan 2022 event - above floor flooding 100mm deep, whole yard + Pioneer Cemetery under 30+cm of water.

Jan 2022 event - water 10cm deep from neighbour's property lapped floor level.

Fergusons Road, Larcombes Road and Skehans Lane were all under water and closed for a number of days.

Jan 2022 event - water came from rear lane 2ft deep all over property and caused above floor flooding.

Jan 2022 event - 50mm water through machinery shed.

Modelled flood depths of 0.3-0.4m across property, extends across Federation Ave into Pioneer cemetery.

Local drainage runoff not modelled.

Water modelled to pond on Skehans Lane with depths in the range of 01-0.3m. Fergusons Road and Larcombes Road outside model extent.

Water modelled on property from Hume Lane, 0.3-0.7m deep, typically 0.5m deep.

Water modelled to be up to 0.1m deep around shed.

The results indicate a good match to observed flood behaviour, with modelled depths aligning well with community estimates and general observations (such as flooded roads or source of floodwater) matching the model.

7.10.3. Calibration to Community Drop-in Session Data

The modelled flood behaviour was compared with community input at the drop-in session that was held at the commencement of the project (see Section 4.3). A summary of the descriptions from the residents and the comparison to the modelled flood results can be seen in Table 27. The location of these points is also shown in Figure D18.

Table 27: Corowa Overland Flood Model Calibration to Community Drop-in Session Data

ID Summary of Resident Observations Comparison with Modelled Flooding

D01

Water from John Street enters property, approx 0.3m deep.

D02 Road floods from 16 Martin St to Nixon St, inadequate drainage.

Only flows in the channel adjacent to John St modelled.

Flows in Martin St modelled from number 20 to Nixon Street.

D03 New estate has increased flows and drainage channel not maintained. Not applicable.

D04 Sewer backflows. Not applicable.

D05 Out buildings impacted, Federation Ave wetland spillway did not function correctly.

D06 Park flooded, amenities block 0.3m deep, access cut, levee may restrict overland flows draining.

D07 Flooding of Redlands Rd, Skehans Ln.

Increased runoff from development. Scour, blockage and water quality issues.

D08

Yard and garage flooded.

D09 Shallow above floor flooding.

Inundation modelled on property, with shallow flows from Federation Ave impacting buildings.

Modelled flooding in park, up to 1m deep.

Modelled inundation of Redlands Rd and Skehans Ln.

Local drainage runoff not modelled.

Modelled depths of up to 0.15m in Sanger St outside property, flood extent abuts building.

Corowa, Howlong and Mulwala Flood Study

ID Summary of Resident Observations Comparison with Modelled Flooding

D10 House not flooded above floor.

D11

Drainage issues in industrial estate, Skehans Ln drain undersized and Spring Dr culverts inadequate.

D12 House flooded above floor by 2in. Neighbours also flooded above floor. Issues with stormwater pump station nearby.

Inundation modelled opposite house, but local flows down Chivell St not modelled.

Inundation modelled in industrial area, inundation of Skehans Ln and Spring Dr just overtopped.

Modelled flood depth of 0.3-0.5m around dwelling.

D13 Flows from road down driveway toward north. Flooding only modelled from the end of Armour Street. Local drainage runoff not modelled.

D14

Drain on eastern side of Skehans Lane is undersized and floods across western side of road on property.

D15 Drain on eastern side of Skehans Lane is undersized and floods across western side of road on property. Flood levels just below floor level in Jan 2022 event.

Drainage channel capacity modelled to be exceeded, with water flowing to the west through property.

Drainage channel capacity modelled to be exceeded, with water flowing to the west through property. Modelled flood depths up to 0.2m around dwelling.

Overall, the modelled flood behaviour shows a reasonable match to the flood observations. A number of properties indicated above floor flooding, and a comparison with modelled flood depths outside the dwelling indicated this was possible, although would only be confirmed through a comparison with floor levels.

7.10.4. Calibration to SES Assessments

Following the January 2022 storm event, the SES conducted assessments of a number of impacted properties (see Section 3.8). These assessments typically note the inundation depth above floor level and the source of the water. The depths are typically given in large range brackets of just above floor level (up to 5 mm), 5 mm to 0.3 m and 0.3 m to 1.5 m. The modelled flood behaviour was compared with the data from these assessments. The flood depth at the property is provided above the surrounding ground level. Based on a visual inspection of properties around Corowa, a typical floor level would be approximately 0.2 m above the surrounding ground level. A summary of the descriptions from the assessments and the comparison to the modelled flood results can be seen in Table 28. The location of these points is also shown in Figure D19

Table 28: Corowa Overland Flood Model Calibration to SES Damage Assessment Data

ID Summary of SES Damage Assessment Comparison with Modelled Flooding

S01 Water below floor level

S02 Water up to 0.3m above floor level

S03 Water below floor level

S04 Water up to 5mm above floor level

S05 Water below floor level

Storm damage. Local drainage runoff not modelled.

Modelled flood depths up to 0.3m on property, abuts dwelling.

Local drainage runoff not modelled.

Modelled flood depths on property in the range of 0.4-0.6m.

Modelled flood depths in the range of 0.3-

ID Summary of SES Damage Assessment

S06 Water 0.3m - 1.5m above floor level

S07 Water up to 5mm above floor level

S08 Water up to 5mm above floor level

S09 Water below floor level

S10 Water below floor level

S11 Water up to 0.3m above floor level

S12 Water below floor level

S13 Water below floor level

S14 Water up to 5mm above floor level

S15 Water up to 0.3m above floor level

S16 Water below floor level

Corowa, Howlong and Mulwala Flood Study

Comparison with Modelled Flooding 0.4m on property.

Modelled flood depths in the range of 0.10.2m on property.

Modelled flood depths in the range of 0.40.5m on property.

Modelled flood depths in the range of 0.10.3m on property.

Storm damage. Local drainage runoff not modelled.

Storm damage. Local drainage runoff not modelled.

Modelled flood depths in the range of 0.10.3m on property.

Storm damage. Local drainage runoff not modelled.

Modelled flood depths in the range of 0.10.4m on property.

Storm damage. Local drainage runoff not modelled.

Ingress from groundwater. Outside model extent.

Modelled flood depths less than 0.2m on property.

Overall .the modelled flood behaviour shows a reasonable match to the SES assessments. In a number of cases, the damage was storm damage – from rainfall entering the dwelling (eg. leaking roof) rather than flooding. For those properties inundated by overland flows, a comparison with modelled flood depths outside the dwelling indicated this was possible, although would only be confirmed through a comparison with floor levels.

7.10.5. Calibration to Council Service Requests

Following the January 2022 storm event, a number of service requests were placed with Council. While these pertain to general complaints and damage of Council infrastructure, they can provide information regarding the flood behaviour. The modelled flood behaviour was compared with the data from these service requests. A summary of the descriptions from the assessments and the comparison to the modelled flood results can be seen in Table 29. The location of these points is also shown in Figure D20.

Table 29: Corowa Overland Flood Model Calibration to Council Service Requests

ID Summary of Council Service Request Comparison with Modelled Flooding

C01 Sewer backflowing. Not applicable.

C02 Drain blocked and water lapping at 88 Adams St door.

C04 100mm of water flowing down Birdwood St with limited stormwater pits.

Modelled flood depth in the range of 0.2-0.5m on property, abuts dwelling with typical flood depth of 0.3m.

Modelled flood depths up to 0.15m on Birdwood St.

ID Summary of Council Service Request

C05 Debris on nature strip.

C07 Flood damage and water flowing down from Sophia Close.

C08

C09

Flooding on property and potential sewer issues.

Water in excess of channels floods property, sewer issues.

C10 Flood damage. Water 1ft deep in Martin St and water in yards along Federation Ave. Sewer backwatering.

C11 Gallipoli Ln flooded and entered property.

C12 Hume Ln flooded and inundated property, sewer backflowing.

C13 Water from school crossed road and washed out driveway.

C14 Flood damage to Hermitage Dr.

Corowa, Howlong and Mulwala Flood Study

Comparison with Modelled Flooding

Flowpath modelled to cross Hume Street, would most likely deposit debris.

Water modelled to flow onto property from Sohpia Close.

Floodwater modelled on property.

Water modelled to spill from channel and flow through property.

Water modelled to be up to 0.5m deep in Martin St, flows through property and ponds at Federation Ave, to a depth of 1m.

Modelled inundation of Gallipoli Ln to depths of 0.2m, flows through property to Gallipoli St.

Modelled inundation of Hume Ln to depths of 0.5m, with inundation of property.

Local drainage runoff not modelled.

Modelled flood depths up to 0.2m on Hermitage Dr, relatively fast flowing down to Nixon St.

C15 River Ln flooded and entered property. Shallow inundation of River Ln modelled (<0.1m) that enters property.

C16 Culvert blocked with flood debris. Local drainage runoff not modelled.

C17 Stormwater drain/sewer backflowing in yard. Local drainage runoff not modelled.

C18 Flood damage to road. Local drainage runoff not modelled.

C19 Water in front garden.

C20 Water in excess of channel capacity inundated driveway by 1m, with water flowing over Nixon Rd to sewer ponds.

Modelled flood depths of 0.1-0.2m in front garden.

Driveway modelled to be flooded to a depth of approximately 0.6m, with flood depths up to 1m in front yard.

C21 Water flows onto property and causes flooding. Located in Howlong.

C22 Water from road entered garage, flood damage on road.

C23 Flood damage on driveway.

Flood depths of 0.1m modelled on Whitehead St, but does not enter property.

Flood depths of 0.1m modelled on Whitehead St, but does not enter property.

C24 Crushed rock from bike track and road on driveway. Local drainage runoff not modelled.

C25 4-6in of water on road, with flood damage to road and driveway.

C26 Wetlands on Federation Ave overflowed into park.

Local drainage runoff not modelled.

Wetlands modelled to overflow into park, but also a number of other sources of flooding for the site.

C27 Flood debris from Guy Rd in Birdwood Ln. Shallow inundation (<0.1m) modelled on property, from both Guy St and Birdwood Ln.

C28 Flood damage to road.

Modelled flood depth up to 0.8m on Sauvignon Dr.

ID Summary of Council Service Request

C29 Property flooded from water from View St.

C30 Flood damage at Tower St and Chisnall St roundabout

C31 Levee bank opposite property breached and flooded property.

C32 Drainage channel at rear of property does not flow, causes flooding on property.

C33 Drains required cleaning.

C34 Blocked drains and water ponding on Hermitage Drive and Nixon Street.

Corowa, Howlong and Mulwala Flood Study

Comparison with Modelled Flooding

Local drainage runoff not modelled.

Local drainage runoff not modelled.

Water modelled to spill from channel and flow through property.

Overland flow modelled on property up to 0.2m deep.

Flooding modelled on Cobar Ct up to 0.5m deep.

Water modelled to pond on Hermitage Dr and Nixon St up to 0.8m deep.

C01 Sewer backflowing. Not applicable.

Overall, the modelled flood behaviour shows a reasonable match to the Council Service requests. A number of residents noted flood damage in areas that have not been included in the flood model but are subject to local drainage runoff. It is likely in these areas that shallow but fast-moving overland flow was experienced given the intensity of the storm and this would cause damage. A number of other residents noted that there were issues with the sewer system that is not investigated as part of this flood study.

7.10.6. Calibration to Drone Photography

Drone photography captured approximately 36 hours after the event by 12P Consulting show the extent of areas still inundated from the storm event, particularly around the industrial area near Corowa Aerodrome. A comparison was made between the modelled flood extent and these images. Allowing for the recession of floodwaters following the flood peak, the modelled extent was compared at the end of the model run, 12 hours after the commencement of the storm. Comparisons are shown in Figure D21, Figure D22 and Figure D23 for the industrial area, Corowa Aerodrome and Skehans Lane, respectively. The results indicate a reasonable match.

7.11. Howlong March 2012 Event Modelling

The March 2012 storm event over Howlong was modelled in the WBNM hydrologic model using the rainfall total from the Howlong Post Office daily rainfall gauge, distributed in time using the Rutherglen Research Station gauge. The resulting runoff was used as inflows into the TUFLOW hydraulic model to simulate flood behaviour. There was no flood data available to verify the flood behaviour, except for one community questionnaire response and one Council service request.

The questionnaire response was from Jude Street which indicated that the property was not flood affected. The modelled flood depth for the 2012 event at this property was in the order of 0.1 m. It appears that this property, however, was developed in approximately 2016 (based on aerial imagery over time), and hence would not have been impacted by the 2012 event. It is likely that in smaller events that have been experienced since 2016, that the property would not have been impacted. The parameters adopted for the calibrated Corowa overland flood model were adopted for the Howlong overland flood model, which is considered to be the best practice approach given the limited data available for calibration.

Howlong and Mulwala Flood Study

The Council service request was for a property on Hammer Street (see C21 in Table 29). The resident noted that after the 2022 event (approximately 40 mm in Howlong), water entered the back yard. The modelled flood depth on the property is in the range of 0.1 m to 0.8 m in the 2012 event, with water entering from the back yard and discharging onto Hammer Street. The simulated flood behaviour can be seen in Figure D24

7.12. Mulwala March 2012 Event Modelling

The March 2012 storm event over Mulwala was modelled in the WBNM hydrologic model using the rainfall from the Yarrawonga gauge. The resulting runoff was used as inflows into the TUFLOW hydraulic model to simulate flood behaviour. There was no flood data available to verify the flood behaviour. The parameters adopted for the calibrated Corowa overland flood model were adopted for the Howlong overland flood model, which is considered to be the best practice approach given the limited data available for calibration. The simulated flood behaviour can be seen in Figure D25

7.13. Discussion of Calibrated Parameters for the Murray River Model

The primary calibration parameter for the Murray River model was the floodplain roughness (Mannings ‘n’). The Mannings ‘n’ values were adjusted, with consideration of typical recommended values (Babister and Barton 2012) and calibrated values in previous hydraulic models and models for adjacent reaches of the Murray River. The calibrated values (see Table 16) produced a good match to the gauge data. However, a significantly different value was adopted for the 2016 and 2022 events compared to events prior to this. The 1974, 1975, 1992 and 1996 events adopted a Mannings ‘n’ of 0.045 for floodplain grass, 0.075 for floodplain vegetation and 0.038 for the Murray River channel. For the 2016 and 2022 events, the Mannings ‘n’ of floodplain grass was increased to 0.05, floodplain vegetation was increased to 0.09 and the Murray River channel was increased to 0.048 This represents a 10% to 25% increase in roughness. This was required as when the 2016 event was initially modelled with flows from the Doctors Point gauge (noting that flows are derived from recorded gauge levels using a ‘rating curve’), the modelled water levels at the Howlong and Corowa gauges were much lower than those recorded.

There are two potential reasons for this:

1. The estimated flows are incorrect. If the rating curve for a gauge is incorrect, then the estimate of the flow will be incorrect for a particular gauge level. If the flow estimates for the 2016 event are too low, then this would result in lower water levels being simulated at Howlong and Corowa.

2. There has been a change in the conditions of the floodplain such that water levels are higher now than they have been in the past for the same flow conditions. This would mean that the estimated flow from the gauge rating curve is correct, but the model needs to be adjusted to represent the higher water levels that have been observed.

To investigate this, the historic rating curves at Doctors Point and Corowa were analysed to understand any changes that have occurred that may have affected the recent events It became evident that there was a significant shift in the rating curves at both Doctors Point and Corowa.

Corowa, Howlong and Mulwala Flood Study

The rating curves from the 1996 event and 2016 events were compared, as shown in Diagram 15 and Diagram 16 for the Doctors Point and Corowa gauges, respectively.

Diagram 15: Comparison of the 1996 and 2016 rating curves at Doctors Point gauge

Diagram 16: Comparison of the 1996 and 2016 rating curves at Corowa gauge

This shift in rating curve is most prominent at approximately 100,000 ML/d, which is the peak of

Corowa, Howlong and Mulwala Flood Study

the 2016 flood event. Using the recorded water levels at Doctors Point for the 2016 event, adopting the 1996 and 2016 rating curves give significantly different results, as shown in Diagram 17 The estimated flows using the 2016 rating curves matches the official flow record for the 2016 event at both gauges. Adopting the 1996 rating curve, however, results in peak flows approximately 20,000 ML/d higher. Adopting the 1996 rating curve for the 2016 event would result in a higher peak flow by approximately 20% that would be input into the TUFLOW model.

Diagram 17: Estimated 2016 flow hydrographs at Doctors Point using both the 1996 and 2016 rating curves

In discussions with WaterNSW, the owner and operator of the gauges on the Murray River, it was understood that the rating curves for both the Doctors Point and Corowa gauges, including the recent changes, are considered to be accurate (M. Dixon, personal communication 8/2/23). It is understood that the rating curve at Doctors Point is fairly complicated above approximately 55,000 ML/d, with a number of different flow paths being activated above this, leading to a complex rating curve. The newer rating curve, however, is supported by two gaugings from both the 2016 and 2022 events, as shown in Diagram 18 The current rating curve is shown in Diagram 18, which is almost identical to the 2016 rating curve for the gauge heights shown. The rating curve at Corowa is simpler given the confined nature of the floodplain at the gauge location. The current rating curve is shown in Diagram 19, which is almost identical to the 2016 rating curve in the area of interest (gauge height of approximately 6.5 m to 7.8 m). The newer rating curve at Corowa is also supported by six gaugings from the 2016 and 2022 events, as shown in Diagram 19 There are an additional 6 gaugings between gauge heights of 5 m and 6.5 m which also indicate this shift. It is also noted that there is one gauging from the 1996 flood event which lies closer to the current rating curve than the 1996 rating curve. This point indicates that the shift in rating curves may have already been in progress in the mid 1990’s.

Corowa, Howlong and Mulwala Flood Study

In the early 2000’s and 2010’s, gauging methodology generally transitioned to acoustic doppler current profiler instrumentation, which is considered to provide more accurate information for estimating flow across a stream. This change in instrumentation, however, was not considered to

Diagram 18: Doctors Point Gauge Rating Curves and Gaugings

Diagram 19: Corowa Gauge Rating Curves and Gaugings

be the source of the change in rating curves. Both the historic and current rating curves are considered by Water NSW to be representative (as far as instrumentation will allow). This leads to the conclusion that the estimated flows are correct and that changes in floodplain conditions are responsible for this change in the flood behaviour that results in higher water levels for a given flow. This is also supported by an analysis of travel times along the Murray River. As an example, when estimating the hydrograph shift to account for travel time between the Doctors Point gauge and the TUFLOW model boundary, the travel time of the flood peak between the Doctors Point gauge and Howlong gauge was calculated. The travel time of the 1974, 1975 and 1992 flood events was calculated to be approximately 16 hours. The travel time of the 1996 flood event was calculated to be longer, at 21 hours and the 2016 and 2022 events longer again at 24 hours. This aligns with the changes in rating curves and gaugings over time. This provides further confirmation of a change in floodplain conditions.

There may be a number of contributing factors to this change in floodplain conditions, however, two of the most prominent factors are:

1. Re-snagging of the Murray River. Snags, also known as large woody debris, are trees, branches and root masses that are found in rivers (MDBA, no date). Snags provide an important habitat for aquatic life. Prior to the 1990’s, it is understood that large-scale desnagging operations regularly took place on the Murray River. The Living Murray Initiative funded a plan in 2004 to identify areas for re-snagging, or the placement ofsnags, between Hume Dam and Yarrawonga Weir (MDBA, no date). Three priority areas were initially identified and approximately 4,000 snags were reinstated commencing in August 2006 (MDBA, no date). A monitoring program commenced in 2007 to assess the benefits to aquatic life (MDBA, no date).

2. Densification of floodplain vegetation. There is thought to be less clearing of the floodplain in recent times than what has occurred historically. This can lead to more vegetation on the floodplain such as shrubs and trees, in addition to more dense vegetation in areas of existing vegetation. In addition to this, it has been observed that following the breaking of the millennium drought around 2010, that the floodplain vegetation flourished (M. Dixon, personal communication 8/2/23), with more shrubs and undergrowth. An analysis of aerial imagery was undertaken over time to see if these changes were evident. Aerial imagery was available from 2010 to 2022 from Nearmap and from 2005 to 2022 from Google Earth (see Section 3.6). A comparison of imagery over time at Howlong and Corowa are shown in Photo 2 and Photo 3, respectively. The photos indicate a densification of floodplain vegetation, both at Howlong and Corowa. This was also evident in aerial imagery available from Google Earth.

Both of these factors were noted by WaterNSW staff and members of the FMC as occurring. This is considered to be sufficient justification for the increase in Mannings ‘n’ roughness values for the 2016 and 2022 events, as outlined in Table 16

The Murray River model overall calibration is considered to be good. The model results fit well with the observed data, including gauge levels and flows across all six events. There is confidence that the TUFLOW hydraulic model can represent conveyance and routing along the Murray River, including flood levels and velocities across the floodplain and at hydraulic structures. The model is considered to be suitable for simulation of design flood events.

Corowa, Howlong and Mulwala Flood Study

Photo 2: Howlong Aerial Imagery in 2010 (top) and 2018 (bottom) (Source: Nearmap)

Corowa, Howlong and Mulwala Flood Study

7.14. Discussion of Calibrated Parameters for the Overland Models

There is very little ‘calibration’ of hydraulic model parameters (such as Mannings ‘n’), due to the nature of flooding and the uncertainties regarding the rainfall itself. The most important features are the terrain (in which high quality LiDAR data is being used) and hydraulic structures (which have been included in the model). As such, ‘calibration’ typically involves consideration of the WBNM hydrologic model and representation of the storm.

A number of different ‘storms’ were simulated over Corowa for the January 2022 event, including changing the total rainfall depth (varying between 125 mm and 182 mm) and changing the temporal pattern (using Rutherglen and IWAHGU1 rainfall stations). It was found that in the key areas of flooding within Corowa where calibration data was available (such as Sophia Close), the change in peak flood level across the full range of variations was less than 0.1 m. Initial and

Photo 3: Corowa Aerial Imagery in 2010 (top) and 2018 (bottom) (Source: Nearmap)

Corowa, Howlong and Mulwala Flood Study

continuing losses were also varied, with a similar result. The peak flood levels are relatively insensitive to changes in both initial and continuing losses. The main areas of flooding appear to be more storage-driven rather than conveyance driven. This means that the total rainfall volume is the most important consideration. Given the storm was high-intensity and lasted only a short period, the continuing loss and temporal pattern do not make any substantial difference to the flood levels. The initial loss is also relatively insensitive, as there is enough rainfall excess in any case to fill the flood storage areas. Initial losses up to 90 mm were tested, which was considered reasonable given the lack of rainfall preceding the event. The adopted initial loss was 30 mm, which was considered a reasonable estimation. Design probability neutral burst initial losses range from 7 mm to 26 mm across the suite of AEPs and durations. The adopted continuing loss was 1.8 mm/h, aligning with the design continuing loss The adopted initial and continuing loss values do not necessarily represent a ‘calibrated’ value, but rather a typical value that is justifiable.

For the 2012 event in Howlong, the rainfall depth was derived from the Howlong Post Office daily rainfall gauge, and distributed temporally using the Rutherglen gauge. An initial loss of 10 mm was adopted (considering the rainfall preceding the storm) and a continuing loss of 1.8 mm/h (design continuing loss).

For the 2012 event in Mulwala, the rainfall depth and temporal pattern was adopted from the Yarrawonga station. An initial loss of 10 mm was adopted (considering the rainfall preceding the storm) and a continuing loss of 1.0 mm/h (design continuing loss).

A summary of the rainfall parameters adopted for each model is shown in Table 30.

While there is no available calibration data for Howlong and Mulwala, there is considerable calibration data available for Corowa. The Corowa Overland model calibration is considered to be reasonable. The model results generally fit with the observed flood behaviour, although there is a slight overestimation of flood levels compared to surveyed levels for the January 2022 event. There is confidence in the WBNM model to simulate rainfall runoff and in the TUFLOW hydraulic model to represent flood levels and velocities throughout Corowa. Since a similar approach has been adopted for Howlong and Mulwala, all the overland flood models are considered to be suitable for the simulation of design flood events.

Table 30: Overland Model Rainfall Calibration Parameters

8. FLOOD FREQUENCY ANALYSIS

A Flood Frequency Analysis (FFA) refers to statistical procedures that use recorded and related flood data to identify an underlying probability model of flood peaks. This model can then be used to estimate the magnitude of flood peaks for a range of probabilities (for example, estimating the 1% AEP peak flow). It can be undertaken graphically or using a probability distribution, and is advantageous as it does not require assumptions regarding the relationship between rainfall and runoff – all factors affecting flood magnitude are derived from the historical record. However, the reliability of the flood frequency approach depends largely upon the length and quality of the observed record and accuracy of the rating curve (as discussed in Section 7.13)

The FFA for this study was undertaken for the Murray River and Ovens River, where gauge data exists. The FFA process involved two stages: The first stage was to establish a homogenous flood record using gauged data and information about events that occurred prior to the gauged record. From this data, an Annual Maximum Series (AMS) was developed, that is, a list of peak flows for each available year. The second stage involved fitting a probability distribution to the data and using the resulting curve to estimate the peak design flows. This process was undertaken in accordance with ARR19 (Ball et al. 2019) and the results were used to understand the relative magnitude of historic flood events and to derive an estimate of peak flows in the Murray River for a range of AEPs.

The following sections outlined the FFA process and results for each of the gauge sites relevant to the study area.

8.1. Doctors Point Flood Frequency Analysis

8.1.1.

Data

The stream gauge at Doctors Point (409017) commenced toward the end of 1929 and is currently operational. It is located downstream of the Murray River and Kiewa River confluence, essentially recording flows from the Hume Dam and Kiewa River (see Figure 7). There is another gauge located at Union Bridge (409001), which commenced in 1865 (GHD 2012). This is located downstream of Doctors Point at the upstream side of the Lincoln Causeway Union Bridge site at Albury. Early data includes peak flood heights for the 1867 and 1870 floods, with continuous annual maximum stage heights available from 1885 onwards (GHD 2012). Daily data from WaterNSW was available from 1908, including discharge data to 1952, when the ratings were discontinued at the station. Based on flood data at both of these stations, an AMS can be generated for Murray River flows at Albury (location referred to as Doctors Point) from 1887 to 2022 (last complete year available when the FFA was undertaken).

Hume Dam, located approximately 8 km east of the Doctors Point gauge on the Murray River, was constructed between 1919 and 1936. It is understood that the dam started storing inflows in 1929 (GHD 2012). The Hume Dam affects flooding downstream, as it can store floodwaters arriving at the dam. Dartmouth Dam, located upstream of Hume Dam, can also affect flooding at Albury, however, to a lesser extent due to its proximity from Albury and its catchment area (approximately 24% of the Hume Dam catchment). The attenuation of the flood peak depends on

Corowa, Howlong and Mulwala Flood Study

the available airspace, the volume of water in the incoming flood and the peak flow rate. In general, the dams have a greater ability to attenuate smaller flood events than larger flood events. The larger flood events typically occur in wet years when the dams may be close to full supply level (FSL) and the flood may be passed downstream with little attenuation. An example of this is the 1975 flood at Hume Dam – the largest flood since the dam’s construction. In their 1975-76 annual report, the River Murray Commission indicated that from July to December 1975, Hume Dam remained full (River Murray Commission 1977). There were several flood events in this period, as shown in Table 31. The dam was able to attenuate some of the earlier floods, however, by October the capacity of the dam to attenuate flood peaks was significantly less. The last flood event in October 1975 had an estimated peak inflow of 181,000 ML/d and a peak release of 172,000 ML/d. This represents just a 5% reduction in peak flow.

Table 31: Floods at Hume Reservoir 1975-76 (Source: River Murray Commission 1977)

This conclusion about the dam providing less attenuation for larger flood events was also supported by the FFA investigations undertaken for the pre and post-dam records as part of the Albury Flood Study (GHD 2012).

8.1.2.Rating Curve

The Doctors Point gauge records water level data only, which is then converted into a flow rate using a rating curve. Rating curves define a relationship of height to flow at the gauge location. The relationship is defined by a series of instantaneous flow measurements at known heights (called ‘gaugings’). The Doctors Point gauge is managed by WaterNSW and the rating curve was investigated and discussed in Section 7.13. The highest gauging that has been undertaken at the gauge is 7.05 m above gauge height datum, recorded on the 27th October 1975. This is close to the highest recorded level during the 1975 event. The flow estimates up to approximately 200,000 ML/d would be considered reliable. Flow estimates above this, however, would require extrapolation of the rating curve, which involves some uncertainty. It is also noted that some of the early flows in the record were derived from the Union Bridge gauge, which has a discontinued rating curve (as of 1952). The reliability of the Union Bridge rating curve is unknown.

8.1.3.Annual Maximum Series (AMS)

ARR (Ball et al 2019) recommends that FFA should be applied to peak flows or discharges. In frequency analysis of flows, the fitting of a particular distribution is typically carried out analytically, by fitting a probability distribution. The data may consist of an annual series, where the largest

Corowa, Howlong and Mulwala Flood Study

peak in each year is used, or a partial series, where all flows above a selected base value are used. The relative merits of each method are discussed in detail in ARR. In general, an annual series approach is preferable as there are more methods and experience available, in addition using a series of annual maximums lowers the risk of two successive peaks being dependent. An AMS was adopted for this study.

A homogeneous gauge record is required to undertake an FFA. This means that the variations in the record should only be due to natural variations in factors affecting flooding (such as rainfall and antecedent moisture conditions). This means removal of other factors such as gauge rating curve changes (that are unrelated to physical cross section changes), change in gauge station location, construction of storages (large storages or growth in farm dams in the catchment) or changes in land use (such as land clearing or urbanisation) (Ball et al 2019). Since the Hume Dam influences flood behaviour downstream, a homogenous record would not cover both the pre-dam and post-dam period, without accounting for the effects of the dam. It was considered reasonable in this situation to derive an AMS for the post-dam period, which would include the effects of Hume Dam on the Murray River flows. This was the approach adopted for the Albury Flood Study (GHD 2012). An AMS for the period of 1929-2022, inclusive (94 years) was developed based on the available information, as outlined in Table 32.

Table 32: Doctors Point AMS Sources Period Gauge1

1929-1932

1933-1936

1937-1952

Both Doctors Point and Union Bridge daily flows available. Take the maximum of both gauges.

Doctors Point gauge has no data available. Take the Union Bridge daily flow.

Daily flows available at Doctors Point and Union Bridge. Although there are missing days in the record, annual peaks appear to be captured.

2012

GHD AMS is generally similar to the gauge record, except for 1931, which is approximately 30% higher with a peak of 161,000 ML/d.

GHD AMS is generally similar to the gauge record.

GHD AMS is generally similar to the gauge record.

2012

2012

1952-1960

1961-2022

Daily flows available at Doctors Point only. Although there are missing days in the record, annual peaks appear to be captured, except for 1954, where the flood peak occurred in the second half of the year where there is no data available.

Automatic gauge at Doctors Point with sub-daily recordings. The gauge record is considered to be reliable.

GHD AMS is generally slightly higher (approximately 5%) than the gauge record. It has a reasonable estimate of the 1954 event (compared to the flow at Corowa).

GHD 2012

2012

GHD AMS is generally similar or slightly lower (within 5%) of the gauge record. GHD AMS only extends to 2010. Gauge

1. Based on gauge data from WaterNSW. See Section 3.7.3

GHD

GHD

GHD

A comparison of the AMS from each source and the adopted AMS is contained in Table E1 of Appendix E

8.1.4.Low Flow Censoring

The AMS from 1929 to 2022 was adopted to undertake the FFA at Doctors Point. It is recognised that an AMS may contain discharges which are not associated with a true “flood” event (Ball et al 2019) These low peaks may not represent the physical processes driving large floods and may unduly influence the fit of the probability distribution. It is the larger events that are typically of interest when undertaking an FFA, and hence it is desirable to fit the larger events rather than smaller events. The identification and removal of Potentially Influential Low Flows (PILFs) is considered an important step in an FFA (Ball et al 2019).

ARR (Ball et al) recommends a multiple Grubbs-Beck test to identify a threshold below which flows can be ‘censored’. This test provides an objective way of identifying PILFs. A multiple Grubbs-Beck test did not identify any PILFs in the Doctors Point dataset. However, ARR (Ball et al 2019) recommends that a visual assessment of the fitted frequency curve be undertaken in unison with this test. A visual inspection of the curve identified that low flows were influencing the curve, with the distribution ‘overshooting’ the high flows. Various censoring levels were tested and a low flow censor of 30,000 ML/d was considered appropriate for Doctors Point. The influence of this low flow censor can be seen by comparing the two graphs in Figure 33 The fit to rarer flood events is improved, with a tighter confidence interval when the low flow censor is adopted. It is understood that this threshold is just above the maximum regulated release for irrigation purposes (the approximate channel capacity at Albury being 25,000 ML/d, NSW SES 2015, MDBA 2021b) and below the minor flood level at Albury (approximately 44,000 ML/d, MDBA 2021b) This results in censoring of 43 years.

8.1.5.Historic Event (High Flow) Censoring

The AMS from 1929 to 2022 was adopted to undertake the FFA at Doctors Point. The three largest events on record, however, are the 1867, 1870 and 1917 events (see Section 2.3), which occurred prior to the construction of the Hume Dam. These flows are therefore not contained in the AMS. Flow estimates are available for these events, as shown in Table 33

Table 33: Doctors Point Historic Event Flows (ML/d)

1. Based on gauge data from WaterNSW. See Section 3.7.3.

The incorporation of these events into the FFA is considered important, as these large events significantly affect the fit of the curve at the upper end. It is uncertain, however, the affect that Hume Dam would have on these flows. Converting these flows to a ‘post-dam’ flow is problematic, as it would heavily depend on the initial water level in the dam and the operation of the dam. As

Corowa, Howlong and Mulwala Flood Study

an indicator, it was previously discussed that the next highest event on record, the 1975 event with a peak flow of approximately 181,000 ML/d, was only attenuated approximately 5% to a flow of 172,000 ML/d. A 5% reduction in peak flow for the historic events to account for Hume Dam would estimate the minimum peak flow rate to be 214,000 ML/d (1867 event using the GHD 2012 flow). A censoring threshold equal to the 1975 event (200,600 ML/d) was adopted for the 1867, 1870 and 1917 events, as these floods are considered to be larger than the 1975 event. The effect of including these events raises the 1% AEP flow estimate from approximately 200,000 ML/d to 250,000 ML/d. This is shown in Figure 34 It was assumed that all other years between 1867 and 1929 were below this threshold.

Testing of this threshold indicated that it has a substantial impact on the upper end of the curve, as shown in Table 34. Flows from 150,000 ML/d (assuming significant attenuation in Hume Dam) to 225,000 ML/d (the lowest estimate of the historic event pre-dam flows) were tested. The resulting 1% AEP flow estimated range from 200,000 ML/d to 290,000 ML/d.

Table 34: Doctors Point 1% AEP Flow Estimate Sensitivity to High Flow Censoring Threshold (ML/d)

Threshold1 1% AEP Flow Estimate2 150,000

1. Threshold for the 3 historic events of 1867, 1870 and 1917.

2. Adopting Log Pearson Type III distribution including censoring of low flows < 30,000 ML/d

8.1.6.Probability Distribution

The FFA undertaken aims to fit a probability distribution to AMS, taking into account the censoring described above. Recent research has suggested that the fitting method is as important as the adopted distribution. The traditional fitting method has generally been based on moments and this makes the fit very sensitive to the highest and lowest values. Recent research has shown that Lmoment and Bayesian likelihood approaches are much more robust than traditional moment fitting and are now the recommended methods (Ball et al 2019)

For this analysis a Bayesian maximum likelihood approach was adopted. The Bayesian method allows for the inclusion of censored events described above. The TUFLOW FLIKE software (version 5.0.251.0) developed by Kuczera (1999) was utilised in this study, as recommended in ARR 2019 (Ball et al 2019) Two probability distributions were tested, Log Pearson Type III (LPIII), which is commonly used in FFA; and the Generalised Extreme Value (GEV) distribution, which is a more recently developed family of distributions that combine Gumbel, Frechet and Weibull families of distributions. These two distributions are recommended in ARR 2019 (Ball et al 2019) and resulted in very similar results, as shown in Figure 35 and summarised in Table 35

Corowa, Howlong and Mulwala Flood Study

Table 35: Doctors Point Design Peak Flows (ML/d) with GEV and LPIII Distributions

Note: Flows rounded to the nearest 100 ML/d

8.1.7.Results

The LPIII distribution was adopted for this study, as it appeared to produce a slightly better fit upon visual inspection. The final FFA results are shown in Figure 36, and adopted flows are provided in summarised in Table 44 below

Table 36: Adopted Doctors Point Design Peak Flows (ML/d)

Based on the FFA, the top 5 post-dam events, plus the calibration events, were assigned an annual exceedance probability (based on interpolation of the probability distribution). The results are shown in Table 37

Table 37: Doctors Point Annual Exceedance Probability of Historic and Calibration Events

8.1.8.Discussion of Results

As discussed in Section 8.1.5, the censor threshold for the high flow events has a marked impact on the FFA results. The inclusion of the three historic events censored at the 1975 peak flow rate is considered to be a reasonable approach. This results in a 1% AEP peak flow of approximately

Corowa, Howlong and Mulwala Flood Study

250,000 ML/d. A comparison of this study’s flows with previous estimates is provided in Table 38

Table 38: Doctors Point Comparison of FFA Flows (ML/d)

Note: Flows rounded to the nearest 100 ML/d

1. As reported in the Albury Flood Study (GHD 2012)

2. The South Albury Levee Upgrade – Murray River Flood Study (Water Technology 2024) completed an independent FFA at Albury resulting in a 1% AEP flow rate of between 154,000 ML/d and 315,000 ML/d, depending on the adopted AMS, probability distribution and censoring. The study ultimately adopted the GHD 2012 FFA results.

The results are generally consistent with the FFAs previously undertaken. In particular, the adopted 1% AEP design flow rate remains the same as that adopted at Albury since the 1980’s. The adopted FFA is considered a good representation of the expected flow rates for various probabilities based on the data available.

8.2. Corowa Flood Frequency Analysis

8.2.1.

Data

The stream gauge at Corowa (409002) commenced mid-way through 1909 and is currently operational. It is located upstream of the John Foord Bridge on the Murray River (see Figure 7) Daily data from WaterNSW was available from 1909 to 1967, with sub-daily data available after this period. Continuous 15 minute recordings are available from 2012. An AMS can be generated for Murray River flows at Corowa from 1909 to 2022 (last complete year available when the FFA was undertaken).

As discussed for Doctors Point, Hume Dam affects flooding downstream of the dam, as it can store floodwaters arriving at the dam. This has a direct impact on flows arriving at Corowa. It was previously discussed that there is generally little change in flood flows between Albury and Corowa (see Section 7.3.1) and as such, the same principles adopted for the Doctors Point FFA were applied to the Corowa FFA and are discussed below.

8.2.2.Rating Curve

The Corowa gauge records water level data only, which is then converted into a flow rate using a rating curve. The Corowa gauge is managed by WaterNSW and the rating curve was investigated and discussed in Section 7.13. The highest gauging that has been undertaken at the gauge is 8.18 m above gauge height datum, recorded on the 20th October 1974. This is close to the highest recorded level during the 1974 event. As such, the flow estimates up to approximately 155,000 ML/d would be considered reliable. Flow estimates above this, however, would require extrapolation of the rating curve, which involves some uncertainty. It is noted that there is one gauging from 1917 with a level of 7.94 m but a flow of 220,000 ML/d. The current rating curve for the gauge does not follow this gauging, which may represent floodplain conditions of 1917.

8.2.3.Annual Maximum Series (AMS)

ARR (Ball et al 2019) recommends that FFA should be applied to peak flows or discharges and an AMS approach was adopted for this study. A homogeneous gauge record is required to undertake an FFA. Since the Hume Dam influences flood behaviour downstream, a homogenous record would not cover both the pre-dam and post-dam period, without accounting for the effects of the dam. It was considered reasonable in this situation to derive an AMS for the post-dam period, which would include the effects of Hume Dam on the Murray River flows. This was the approach adopted for the Doctors Point FFA (Section 8.1.3). An AMS for the period of 1929-2022, inclusive (94 years) was developed based on the available gauge records from WaterNSW (Section 3.7.3). This comprises daily flow data from 1929 to 1966 and sub-daily flow data from 1967 to 2022. This data was analysed in conjunction with the Doctors Point gauge to determine if any annual flood peaks were missing. The gauge record appeared to contain all the annual maximum peaks at Corowa. The adopted AMS is contained in Table E2 of Appendix E

8.2.4.Low Flow Censoring

The AMS from 1929 to 2022 was adopted to undertake the FFA at Corowa. In line with the findings from the Doctors Point FFA, low flows were also censored at Corowa (see Section 8.1.4). The same threshold of 30,000 ML/d was adopted. This results in censoring of 47 years.

8.2.5.Historic Event (High Flow) Censoring

The AMS from 1929 to 2022 was adopted to undertake the FFA at Corowa. The three largest events on record, however, are the 1867, 1870 and 1917 events (see Section 2.3), which occurred prior to the construction of the Hume Dam. These flows are therefore not contained in the AMS. Flow estimates are available for these events, as shown in Table 39 Since there are some discrepancies, gauge heights are also provided in Table 40.

Table 39: Corowa Historic Event Flows (ML/d)

Corowa, Howlong and Mulwala Flood Study

1. Based on gauge data from WaterNSW. See Section 3.7.3

2. According to Table 4.1 of the Detailed Report

3. According to Appendix C3 of the Detailed Report. It was noted that official records for the 1917 event have a flow of 306,000 ML/d, which was revised upon discussions with gauge officers.

Table 40: Corowa Historic Event Gauge Heights (m)

1. Based on gauge data from WaterNSW. See Section 3.7.3

The flow provided for the 1917 event from the gauge is anomalously low, considering the flow recorded at Albury was 240,000 ML/d. This may be attributed to errors in the rating curve at the time (see Section 8.2.2). Discussions with WaterNSW suggested that the daily flows recorded may have just missed the peak. The recorded gauge height from WaterNSW was 7.9 m, while other sources indicate the 1917 event reached 8.8 m (Gutteridge Haskins Davey 1986). Discussions with WaterNSW indicated that they do not have any other information regarding the 1917 flood other than the gauge record (M. Dixon, personal communication 9/2/2023). Based on the flow at Doctors Point being 240,000 ML/d, it would seem that a flow of 235,000 ML/d at Corowa is likely, as provided in Table 4.1 of Gutteridge Haskins Davey (1986).

For the 1867 and 1870 events, these appear to be of a similar magnitude, although smaller than the 1917 event, even though there are discrepancies in the peak flows reported in Gutteridge Haskins Davey (1986). It would seem that the 1870 event reached approximately 8.7 at the Corowa gauge, while the 1917 event reached approximately 8.8 m. It was previously discussed that the Corowa Local Flood Plan (NSW SES 2015) appears to have swapped these two events (see Section 3.1.8).

Despite the discrepancies, the incorporation of these events into the FFA is considered important, as these large events significantly affect the fit of the curve at the upper end. This was demonstrated at the Doctors Point gauge (see Section 8.1.5) In accordance with the approach adopted for Doctors Point, a censoring threshold equal to the 1975 event (198,200 ML/d) was adopted for the 1867, 1870 and 1917 events, as these floods are considered to be larger than the 1975 event.

8.2.6.Probability Distribution

Corowa, Howlong and Mulwala Flood Study

For this analysis a Bayesian maximum likelihood approach was adopted, allowing for the inclusion of censored events described above. The TUFLOW FLIKE software (version 5.0.251.0) developed by Kuczera (1999) was utilised in this study, as recommended in ARR 2019 (Ball et al 2019). Two probability distributions were tested, LPIII and GEV, as recommended in ARR 2019 (Ball et al 2019). The two distributions produced very similar results, as shown in Table 41

Table 41: Corowa Design Peak Flows (ML/d) with GEV and LPIII Distributions

Note: Flows rounded to the nearest 100 ML/d

8.2.7.Results

The LPIII distribution was adopted for this study, as it appeared to produce a slightly better fit upon visual inspection. The final FFA results are shown in Figure 37, and adopted flows are provided in summarised in Table 42 below

Table 42: Adopted Corowa Design Peak Flows (ML/d)

Based on the FFA, the top 5 post-dam events, plus the calibration events, were assigned an annual exceedance probability (based on interpolation of the probability distribution). The results are shown in Table 43.

Corowa, Howlong and Mulwala Flood Study

Table 43: Corowa Annual Exceedance Probability of Historic and Calibration Events

8.2.8.Discussion of Results

The FFA results at Corowa are comparable to those obtained for Doctors Point, as shown in Table 44. The FFA indicates that there is some attenuation of flow between Doctors Point and Corowa for more frequent events, while rarer events may have higher flows. This could be driven by the 2022 event, which recorded a flow approximately 25% higher at Corowa than Doctors Point. It is recognised, however, that this is not typically the case for the majority of flood events on the Murray River. Generally, flow rates at Corowa at similar to, or slightly less than that at Doctors Point for flood events. Neglecting the 2022 event, the flow at Corowa has not been more than 10% higher than Doctors Point (for annual maximum events exceeding 30,000 ML/d). A comparison of this study’s flows with previous estimates is provided in Table 44

Note: Flows rounded to the nearest 100 ML/d

The results are generally consistent with the FFA previously undertaken in Gutteridge Haskins Davey (1986) The adopted 5% AEP and 2% AEP events are approximately 3% lower, while the 1% AEP event is approximately 10% lower. The higher flows obtained by Gutteridge Haskins Davey (1986) is likely to be attributed tothe inclusion of the 1917 event with a flow of 272,000 ML/d (approximately equal to the 1% AEP flow rate in the FFA). It is unclear where the flow estimates came from in the Corowa Shire Local Flood Plan (NSW SES 2015), however, these are thought to be based on the Hume Dam Dambreak Flood Analysis prepared by the NSW Department of Water Resources in 1992. The 5% AEP and 2% AEP events are generally consistent, although the 1% AEP flow is lower than that adopted in the current study.

Table 44: Corowa Comparison of FFA Flows (ML/d) and Levels (m)

Corowa, Howlong and Mulwala Flood Study

As gauge heights were previously discussed (see Section 8.2.5), design levels have also been provided in Table 45 While the Gutteridge Haskins Davey (1986) and NSW SES (2015) levels are similar, the URS (2009) 1% AEP level is approximately 0.5 m above these estimates. The URS (2009) study does not provide any flow rates for comparison.

Table 45: Corowa Gauge Design Flood Levels (m)

1. Derived from the corresponding levels for the FFA flows

2. Stated to be based on Gutteridge Haskins Davey (1986), except for the 5 year ARI (recorded here as the 20% AEP) which came from the Report on Delineation of Flood Frequency Zones in the River Murray Flood Plains, and the 0.5% and 0.2% AEP events were extrapolated from the more frequent event levels.

3. It is unclear where these levels have come from, but they are thought to be from the Hume Dam Dambreak Flood Analysis prepared by the NSW Department of Water Resources in 1992

8.3. Peechelba Flood Frequency Analysis

8.3.1.Data

The stream gauge at Peechelba (403241) commenced near the start of 1990 and is currently operational. It is located on the Ovens River, approximately 15 km south east of the confluence with the Murray River (see Figure 7). As such, it provides information about flows from the Ovens River entering the Murray River. This is the main contributor of inflows between Corowa and Yarrawonga Weir. Sub-daily data was available from the Victoria Department of Environment, Land Water and Planning. There are, however, a substantial number of missing days in the early record, such that only the 1993 annual maximum flood peak can be relied upon between 1990 and 1997. In all other years, the annual maximum flood peak was not recorded. This analysis was undertaken by comparing with the Ovens River gauge located approximately 20 km upstream at Wangaratta (403200). This gauge commenced mid 1885 and is operational today. Based on the available flood data an AMS could only be generated for 1998 to 2022 (last complete year available when the FFA was undertaken). This results in only 25 years of data.

8.3.2.Record Extension

Since there is a long gauge record at Wangaratta, and on the same river, it was considered reasonable to extend the Peechelba gauge record based on recorded flows at Wangaratta. There are several key watercourses which converge at, or just downstream of, the Wangaratta township. Apart from the Ovens River, Reedy Creek, which runs parallel to the Ovens River at Wangaratta is the most significant. There is also a gauge located on Reedy Creek at Wangaratta (403209),

Howlong and Mulwala Flood Study

which has been operational since mid-1940. In flood events, Reedy Creek typically conveys more flow than the Ovens River, although these watercourses converge upstream of Peechelba.

To extend the Peechelba gauge, a correlation between the flows recorded at Wangaratta and flows recorded at Peechelba is required. A number of tests were undertaken to determine this correlation, including:

• Adopting different flow estimations at Wangaratta, including the Ovens River gauge only, Reedy Creek gauge only and combined Ovens River and Reedy Creek gauges. It was found that including the Reedy Creek gauge did not improve the correlation and would require adoption of two separate data sets and correlations (since the Ovens River gauge contained data back to 1885, while the Reedy Creek gauge only contains data back to 1940). The Ovens River at Wangaratta gauge was adopted for the correlation of flows to Peechelba.

• Adopting different flow series, such as annual peak flows, monthly peak flows and peaks over a certain threshold. It was found that a correlation between annual peaks produced the best correlation. Adding in more data points did not improve the correlation, but added ‘noise’ to it.

• Adopting different fits, including linear and polynomial equations. It was found that a simple linear relationship fitted the data best up to approximately 30,000 ML/d (at Wangaratta), and a quadratic fitted the data best for higher flows.

The adopted correlation is shown in Diagram 20. It is noted that the highest gauge flow captured at both gauges occurred in 1993, and this is the largest Ovens River event in recorded history (Water Technology 2017). As such flows for all other years would be interpolated and there would be no extrapolation.

Diagram 20: Adopted Ovens River flow correlation between Wangaratta and Peechelba

The correlated flows were compared with the Reedy Creek gauge at Wangaratta (403209), and an indicative combined flow series at Wangaratta, consisting of both the Ovens River and Reedy Creek. The correlated flows estimated at Peechelba were generally larger than the flows recorded for Reedy Creek, and were very similar to the indicative combined flows at Wangaratta. In particular, the three largest events available at the Reedy Creek gauge (between 1940 and 1993) were the 1974, 1975 and 1981 events. The correlated flows at Peechelba were between 10% and 30% higher than the indicative combined flows at Wangaratta. This is comparable to the 1993 event, in which the peak flow in the Ovens River at Wangaratta was approximately 66,000 ML/d and the peak flow in Reedy Creek at Wangaratta was approximately 122,000 ML/d, giving an indicative combined flow of 187,000 ML/d. The peak flow at Peechelba was approximately 230,000 ML/d, 20% higher than the indicative combined flow at Wangaratta. The results indicate that there is a high correlation between the Ovens River at Wangaratta, Reedy Creek at Wangaratta and Ovens River at Peechelba and the correlated flows generated are considered to be a reasonable estimation of peak flows at Peechelba.

8.3.3.Annual Maximum Series (AMS)

ARR (Ball et al 2019) recommends that FFA should be applied to peak flows or discharges and an AMS approach was adopted for this study. The recorded gauge flows at Peechelba were used, where available. The record was extended using a correlation with the Wangaratta gauge, as described in Section 8.3.2. An AMS for the period of 1886-2022, inclusive (137 years) was developed. The adopted AMS is contained in Table E3 of Appendix E.

8.3.4.Low Flow Censoring

The AMS from 1886 to 2022 was adopted to undertake the FFA at Peechelba. In line with the findings from the Doctors Point FFA, low flows were also censored at Peechelba (see Section 8.1.4). A multiple Grubbs-Beck test was undertaken, which indicated only one outlier with a threshold of 3,600 ML/d. A higher threshold of 20,000 ML/d was adopted, as this was found to produce a more reasonable fit to the data. This results in censoring of 41 years.

8.3.5.Historic Event (High Flow) Censoring

The largest flood on record for the Ovens River is the October 1993 event, which recorded a peak flow of approximately 230,000 ML/d at Peechelba. This event is included in the AMS. There is anecdotal evidence to suggest that the 1870 and 1917 floods may have been larger than the 1993 event, however, there is limited information available regarding these events (Water Technology 2017). As such, no high flow censoring was undertaken.

8.3.6.Probability Distribution

For this analysis a Bayesian maximum likelihood approach was adopted, allowing for the inclusion of censored events described above. The TUFLOW FLIKE software (version 5.0.251.0) developed by Kuczera (1999) was utilised in this study, as recommended in ARR 2019 (Ball et al 2019). Two probability distributions were tested, LPIII and GEV, as recommended in ARR 2019 (Ball et al 2019). The two distributions produced very similar results, as shown in Table 46

Table 46: Peechelba Design Peak Flows (ML/d) with GEV and LPIII Distributions

Note: Flows rounded to the nearest 100 ML/d

8.3.7.Results

The LPIII distribution was adopted for this study, as it appeared to produce a slightly better fit upon visual inspection. The final FFA results are shown in Figure 38, and adopted flows are provided in summarised in Table 47 below.

Table 47: Adopted Peechelba Design Peak Flows (ML/d)

Howlong and Mulwala Flood Study

8.3.8.Discussion of Results

The FFA results at Peechelba are comparable to those reported in GHD 2012, for the combined flows (assumed Ovens River and Reedy Creek) at Wangaratta. The Peechelba flows compared to the combined flows at Wangaratta estimated by Water Technology (2017) with an FFA are approximately 20% lower for more frequent events and 10% lower for rarer events. However, the hydraulic model results provided by Water Technology (2017), indicate a similar flow magnitude to the Peechelba FFA. A summary of the previous FFA results is provided in Table 48 The results are considered to be reasonably consistent with previous estimates. The Ovens River is not the focus of the current study, and the FFA at Peechelba was only undertaken to obtain coincident design inflows for the Murray River hydraulic model. This is discussed further in Section 9.1

Table 48: Peechelba and Wangaratta Comparison of FFA Flows (ML/d)

AEP This studyPeechelba

Ovens River at Wangaratta1 (GHD 2012) Ovens River at Wangaratta2 (Water Technology 2017) Reedy Creek at Wangaratta2 (Water Technology 2017) Combined Flows at Wangaratta2 (Water Technology 2017) Combined Flows at Wangaratta3 (Water Technology 2017)

Note: Flows rounded to the nearest 100 ML/d

1. Flows reported in GHD 2012 are from the Wangaratta Flood Mitigation Study Stage 1 Report (SKM 1996). Although the reported catchment area aligns with the catchment of the Ovens River to Wangaratta, it is assumed that these flows are for the combined Ovens River and Reedy Creek catchments at Wangaratta, since the 1% AEP flow is much higher than the maximum recorded flow of 66,000 ML/d for the 1993 event.

2. FFA results as provided in Tables 4-1, 4-2 and 4-3 of the Wangaratta Urban Waterways Flood Investigation Report (Water Technology 2017).

3. Hydraulic model results as provided in Table 5-1 of the Wangaratta Urban Waterways Flood Investigation Report (Water Technology 2017).

8.4. Yarrawonga Flood Frequency Analysis

8.4.1.

Data

The stream gauge at Yarrawonga (409025) commenced in 1938 and is currently operational. It is located downstream of Yarrawonga Weir and records Murray River flows discharged from the weir (see Figure 7) Monthly maximum data was extracted from Pinneena (1938-1960) and sub-daily data from WaterNSW was available from 1960. In addition to this, the ‘Murray River Regional Flood Study Dicks/Seppelts levees to downstream of the Ulupna Creek confluence’ Report (Water Technology 2011) investigated flooding downstream of Yarrawonga Weir, and obtained additional flood records for Yarrawonga from the Victorian State Rivers and Water Supply Commission (SR&WSC). Two datasets were obtained, labelled SR&WSC-A and SR&WSC-B. These were observed to be ‘working files’ from the 1970’s and 1980’s and contained data which enabled the development of an AMS from 1905 to 1979. Based on these sources, an AMS can be generated for Murray River flows at Yarrawonga from 1905 to 2022 (last complete year available when the FFA was undertaken).

As discussed for Doctors Point, Hume Dam affects flooding downstream of the dam, as it can store floodwaters arriving at the dam. This has some impact on flows arriving at Yarrawonga, however, this is not as prominent at Yarrawonga due to the distance between Hume Dam and the Yarrawonga gauge, and the additional catchment area that contributes to flows at Yarrawonga. In particular, the Ovens River can generate floods at Yarrawonga independently of the Murray River

Yarrawonga Weir itself has minimal impact on flood flows on the Murray River, with the weir essentially passing flows directly downstream with little attenuation. The is due to the relatively small size of the weir pool (compared to Murray River flows), and the operation of the gates to maintain FSL during a flood event (see Section 3.5).

8.4.2.Rating Curve

The Yarrawonga gauge records water level data only, which is then converted into a flow rate using a rating curve. The Yarrawonga gauge is managed by WaterNSW The highest gauging that has been undertaken at the gauge is 8.04 m above gauge height datum, recorded on the 6th October 1993. This is close to the highest recorded level during the 1993 event. As such, the flow estimates up to approximately 200,000 ML/d would be considered reliable. Flow estimates above this, however, would require extrapolation of the rating curve, which involves some uncertainty. It is assumed that the rating curve has been validated against theoretical discharges from Yarrawonga Weir given a lake level and gate opening configuration, as Goulburn-Murray Water, the weir operator, utilises information at the gauge and forecasts discharges from the weir.

8.4.3.Annual Maximum Series (AMS)

ARR (Ball et al 2019) recommends that FFA should be applied to peak flows or discharges and an AMS approach was adopted for this study. A homogeneous gauge record is required to undertake an FFA, and the influence of storages upstream of the gauge are not considered to be a significant influence for large flood events (as noted in Section 8.4.1). An AMS for the period of

Corowa, Howlong and Mulwala Flood Study

1905-2022, inclusive (118 years) was developed based on the available gauge records from WaterNSW and flows reported in the Water Technology (2011) report. As discussed in Section 8.4.1, the Water Technology (2011) report presents two datasets, SR&WSC-A and SR&WSC-B. Water Technology compared annual peak flows for significant flood events from 1905 to 1979, with the results summarised in Table 49.

Table 49: Yarrawonga Comparison of Annual Peak Flows (ML/d) for Significant Events (19051979)

Note: Flows rounded to the nearest 100 ML/d

1. As presented in Table 5-3 of the Water Technology (2011) report.

2. Flows from Gutteridge Haskins Davey (1986) have been added by WMAwater for comparison.

3. This year was not included in Table 5-3 of the Water Technology (2011) report (not regarded as a ‘significant’ event), however has been included since data for this event was contained in Gutteridge Haskins Davey (1986).

Water Technology (2011) analysed the two available historic datasets (SR&WSC-A and SR&WSC-B) and noted that significant differences occur in the peak flow estimates from the two datasets and the exact derivation of the flows was unclear. Water Technology (2011) observed that the SR&WSC-A revealed several inconsistencies with the 1974 and 1975 flood peaks being much larger than the estimates from the SR&WSC-B dataset, which aligned more closely with the gauge data. As such, Water Technology (2011) adopted the SR&WSC-B dataset. This is also the dataset that appears to have been adopted by Gutteridge Haskins Davey (1986) for peak flows prior to 1960.

For the current study, the SR&WSC-B dataset was adopted for the period from 1905-1937, with gauge data (derived from this study, rather than that presented in Water Technology (2011)) adopted for the period from 1938-2022. The 1917 event peak flow estimate (390,000 ML/d) is the largest within the gauge record, being substantially higher than the next highest event (1906 with an estimate of 264,000 ML/d). This event has a significant influence on the upper end of the FFA curve, particularly when considering the 1867 and 1870 historic events being larger than this. A discussion of this sensitivity is contained in Section 8.4.6 As such, a modelling exercise was undertaken to provide an estimate of the 1917 event, with details provided in Section 8.4.4. The modelling indicated a flow estimate of 340,000 ML/d is reasonable and this sits between the

Corowa, Howlong and Mulwala Flood Study

estimates in the SR&WSC-A and SR&WSC-B datasets. A flow of 340,000 ML/d was adopted for the 1917 event. The adopted AMS is contained in Table E2 of Appendix E

8.4.4.1917 Event Modelling

The estimation of the 1917 flow at Yarrawonga is extremely important for the upper end of the FFA estimates. In the SR&WSC-A and SR&WSC-B datasets, the flow estimates for the 1917 event are 306,000 ML/d and 390,000 ML/d, respectively. Water Technology (2011) obtained a flood hydrograph for the 1917 event that suggested a peak flow of 306,000 ML/d, aligning with the SR&WSC-A dataset. The flow estimate provided by Gutteridge Haskins and Davey (1986) indicated a peak flow of 390,000 ML/d, aligning with the SR&WSC-B dataset. The SR&WSC-B was adopted by Water Technology (2011), however, the reliability of this dataset for estimation of the 1917 event is unknown.

A modelling exercise was undertaken to estimate the 1917 event flow. From the Victoria Flood Database, there were two flood level points available near the confluence of the Murray River and Ovens River and three flood level points available downstream of Yarrawonga Weir (shown in Figure D26). The data indicated that these points were derived from “reasonable” or “detailed” contour and flood information with a “high” reliability The calibrated TUFLOW model for the 1974 – 1996 historic conditions was used to simulate the 1917 flow. Although it is difficult to determine what the floodplain conditions may have been at the time of the 1917 flood, the following modifications were made to the TUFLOW model:

• Removal of all bridge structures crossing the Murray River, except for the John Foord bridge at Corowa (constructed in 1892).

• Removal of the railway embankment across Lake Mulwala.

• Removal of Yarrawonga Weir, including the embankment and gate structure.

There is uncertainty about the floodplain conditions for the area now covered by Lake Mulwala. It is assumed that this area, prior to the construction of Yarrawonga Weir, would have looked similar to the Murray River floodplain upstream of the confluence with the Ovens River. This primarily consists of floodplain vegetation with some areas of floodplain grass. Two scenarios were run –one considering full floodplain vegetation (Mannings ‘n’ of 0.075 for historic conditions) and one considering full floodplain grass (Mannings ‘n’ of 0.045 for historic conditions). A rough estimate of the main river channel was also included, with a Mannings ‘n’ of 0.038, as per the historic conditions. These values represent an increase in roughness compared to the post-Yarrawonga Weir conditions, where the Mannings ‘n’ is considered to be 0.036 across Lake Mulwala.

The 2016 inflow hydrograph (for both the Murray River and Ovens River) was scaled until a satisfactory match was achieved to the 1917 flood level points. The results for the grassed floodplain conditions can be seen in Figure D26. Tabulated results are provided in Table 51.

Table 50: 1917 Event modelling Results

Corowa, Howlong and Mulwala Flood Study

1. Floodplain conditions for the Lake Mulwala area

The results indicate that the 1917 flow at the location of the Yarrawonga gauge was approximately 340,000 ML/d. There was a slight variation in peak flows (within 4,000 ML/d) depending on the adopted floodplain conditions for Lake Mulwala, however, the correct result would most likely be in between these two estimates, since floodplain conditions would have likely been a mix of floodplain vegetation and grass. The peak flood levels at the downstream points (C, D and E) were matched for both Lake Mulwala floodplain conditions tested, to within ±0.1 m. The variation in water level between the two floodplain conditions simulated was minimal (0.02 m) for these downstream points. At the upstream location, the floodplain condition has a significant influence on peak flood levels. The grassed conditions match the peak levels better, being approximately 0.4 m and 0.2 m higher. The vegetated conditions result in peak flood levels being overestimated by up to 1 m. There is, however, also uncertainty about the floodplain topography in 1917, the relative contribution of the Ovens River and how local hydraulics of this flow entering may have influenced flood levels at the recorded flood level locations. Based on the modelling undertaken, a peak flow estimate of 340,000 ML/d at Yarrawonga is considered reasonable.

8.4.5.

Low Flow Censoring

The AMS from 1905 to 2022 was adopted to undertake the FFA at Yarrawonga. In line with the findings from the Doctors Point FFA, low flows were also censored at Yarrawonga (see Section 8.1.4). The same threshold of 30,000 ML/d was adopted and found to be appropriate A multiple Grubbs-Beck test was undertaken, which indicated a threshold of 28,500 ML/d for filtering of PILFs. The adopted threshold of 30,000 ML/d results in one additional year being filtered, with a total of 33 years censored

8.4.6.Historic Event (High Flow) Censoring

The AMS from 1905 to 2022 was adopted to undertake the FFA at Yarrawonga. The three largest events on record are the 1867, 1870 and 1917 events (see Section 2.3). Through a comparison of peak flood heights, Water Technology (2011) assessed that the 1870 flood was clearly the largest flood on record, with the flood peaks of 1867 and 1917 being more difficult to distinguish. The 1867 flood, however, is generally accepted as the larger of the two, although flood heights were estimated to be the same (Water Technology 2011), as shown in Table 51. It was concluded that flow estimates of the 1867 and 1870 floods could not be derived due to a lack of rating curve

Corowa, Howlong and Mulwala Flood Study

data (Water Technology 2011). Although flow estimates cannot be assigned, the 1867 and 1870 floods can be included as censored events, being larger than the 1917 event.

Table 51: Yarrawonga Historic Event Gauge Heights (m)

While there is considerable discrepancy in the levels of the historic events, the 1867 and 1870 events are considered to as large or larger than the 1917 event. As such, the 1867 and 1870 events were included in the FFA as censored events larger than the 1917 event (340,000 ML/d).

Testing of the 1917 event flow, and the corresponding high flow threshold indicated that it has a substantial impact on the upper end of the curve, as shown in Table 52. Flows from 306,000 ML/d (the SR&WSC-A flow estimate) to 390,000 ML/d (the SR&WSC-B flow estimate) were tested. The resulting 1% AEP flow estimated range from 357,000 ML/d to 437,000 ML/d.

Table 52: Yarrawonga 1% AEP Flow Estimate Sensitivity to 1917 Flow and Corresponding High Flow Censoring Threshold (ML/d)

1917 Event1 1% AEP Flow Estimate2

1. The assumed 1917 flow and censoring threshold for the historic events of 1867 and 1870.

2. Adopting Log Pearson Type III distribution including censoring of low flows < 30,000 ML/d

It is also noted that there are several large events in the dataset prior to construction of the weir, which would also influence the upper end of the FFA fit. These events appear to be larger than the combined flows estimated in the Murray River and Ovens River, as demonstrated in Table 53 Only the AMS for Yarrawonga was provided in the Water Technology (2011) report for floods prior to 1938. The month of the flood was taken to be when the Murray River peaked, and the corresponding peak monthly flow for the Ovens River was obtained. It is noted that two early events do not have a month assigned, and as such it is assumed that coincidence of flood events occurs in the corresponding AMS for each gauge – a conservative assumption given the discussions in Section 9.1

Corowa, Howlong and Mulwala Flood Study

Table 53: Large flood events at Yarrawonga (pre-weir) and corresponding flows for the Murray River and Ovens River

1. Assumed month based on Muray River peak flows, where available

2. Based on peak flows at Yarrawonga gauge (derived from the AMS, month of peak unknown), except for the 1917 event, which is based on modelling as described in Section 8.4.4.

3. Based on peak flows at Corowa gauge where available, otherwise the Doctors Point/Union Bridge gauge (derived from the AMS, in italics).

4. Based on peak flows at Peechelba, correlated from Wangaratta (derived from the AMS where the month of the flood was unknown, otherwise based on monthly maximum flow).

This discussion is provided to demonstrate that there is some uncertainty in the flow estimates at Yarrawonga. Nonetheless, there is still a record of flows spanning over 100 years and the best available information was used to determine peak flow rates at Yarrawonga with a range of probabilities.

8.4.7.Probability Distribution

For this analysis a Bayesian maximum likelihood approach was adopted, allowing for the inclusion of censored events described above. The TUFLOW FLIKE software (version 5.0.251.0) developed by Kuczera (1999) was utilised in this study, as recommended in ARR 2019 (Ball et al 2019). Two probability distributions were tested, LPIII and GEV, as recommended in ARR 2019 (Ball et al 2019). The two distributions produced similar results up to the 2% AEP event, as shown in Table 54. Above the 2% AEP event, the GEV distribution produces a higher flow estimate.

Table 54: Yarrawonga Design Peak Flows (ML/d) with GEV and LPIII Distributions

Note: Flows rounded to the nearest 100 ML/d

8.4.8.Results

The LPIII distribution was adopted for this study, as it appeared to produce a slightly better fit upon visual inspection. The final FFA results are shown in Figure 39, and adopted flows are summarised in Table 55 below.

Table 55: Adopted Yarrawonga Design Peak Flows (ML/d)

Based on the FFA, the top 5 events, plus the calibration events, were assigned an annual exceedance probability (based on interpolation of the probability distribution). The results are shown in Table 56

Table 56: Yarrawonga Annual Exceedance Probability of Historic and Calibration Events Event (Rank)

8.4.9.

Discussion of Results

As discussed in Section 8.4.6, the 1917 estimate and censor threshold for the high flow events has a marked impact on the FFA results. The inclusion of the 1867 and 1870 events using the 1917 event asthe censor threshold is considered a reasonable approach (if the 1917 flow estimate is accurate). This results in a 1% AEP peak flow of approximately 390,000 ML/d. A comparison of this study’s flows with previous estimates is provided in Table 57.

Table 57: Yarrawonga Comparison of FFA Flows (ML/d)

Corowa, Howlong and Mulwala Flood Study

Note: Flows rounded to the nearest 100 ML/d

1. Based on an FFA undertaken for the Yarrawonga gauge with 1867 and 1870 events censored (above the 1917 event). A Generalised Pareto (GP) distribution was adopted. This is provided in Table 5-4 of the report.

2. Based on an FFA undertaken for the Yarrawonga gauge with only the 1870 event censored (above the 1917 event). A Generalised Pareto (GP) distribution was adopted. This is provided in Table 56 of the report. This was the adopted FFA in the study.

3. Based on an FFA undertaken on annual peak flows up to 1975.

4. The 1917 flow estimate was adopted as the 1% AEP flow rate, although the FFA undertaken resulted in a 1% AEP flow of 410,000 ML/d The flow rates here were also adopted by the NSW SES (2015).

The results are relatively consistent with previous estimates, in particular the adopted 1% AEP flow rate of 390,000 ML/d, which is similar to all previous studies. The flow rates across the range of probabilities displays consistency with the latest FFA estimates from Water Technology (2011), to within 5,000 ML/d. Water Technology (2011) states that the “study steering committee conducted lengthy discussion and debate regarding the appropriate peak flow estimates to be adopted. A consensus position was reached to adopt the peak flow estimates from the GP distribution with the inclusion of the 1870 event” (p. 19). The adoption of this method includes the 1870 event but neglects the 1867 event, which the report states to be of a similar magnitude to the 1917 event. If the 1867 event is included, the 1% AEP flow estimate increases to 445,000 ML/d, as shown in Table 57. This is much higher than the current estimate due to the assumption that the 1917 flow was 390,000 ML/d. The current study adopts a 1917 flow rate of 340,000 ML/d (based on modelling presented in Section 8.4.4) such that both of the significant historic events can be included (1867 and 1870) whilst arriving at the same FFA results. This is considered a more robust approach rather than simply neglecting historic events that are known to have occurred.

The Gutteridge Haskins Davey (1986) report FFA estimated a 1% AEP flow of 410,000 ML/d, however, a flow rate equivalent to the 1917 event of 390,000 ML/d was adopted. This flow aligns with that adopted by Water Technology (2011) and the current study. These flows are the same as those provided by the NSW SES (2015). The flows produced by an FFA using annual peak flows to 1975 were also similar (Australian Dams Alliance 1999).

As gauge heights were previously discussed (see Section 8.4.6), design levels have also been provided in Table 58. The Yarrawonga gauge is located downstream of Yarrawonga Weir (gauge zero at 115.036 mAHD), and hence levels are not representative of Lake Mulwala levels. While

Corowa, Howlong and Mulwala Flood Study

the Gutteridge Haskins Davey (1986) and NSW SES (2015) levels are the same, the URS (2009) 1% AEP level is slightly higher. The URS (2009) study does not provide any flow rates for comparison.

Table 58: Yarrawonga Gauge Design Flood Levels (m)

1. Derived from the corresponding levels for the FFA flows

2. Stated to be based on Gutteridge Haskins Davey (1986) and the 0.5% and 0.2% AEP events were extrapolated from the more frequent event levels.

3. It is unclear where these levels have come from, but they are thought to be from Gutteridge Haskins Davey (1986), given the consistency with the reported levels.

9. DESIGN FLOOD MODELLING

9.1. Murray River Design Flood Modelling

The Murray River design flood events were simulated by developing a design flood hydrograph for each event. The flood hydrograph consists of two main components – the peak flow and the hydrograph shape. Flood hydrographs were input into the TUFLOW model for the Murray River and Ovens River. The 20%, 10%, 5%, 2%, 1%, 0.5%, 0.2% AEP and PMF events were simulated. The development of the design flood hydrographs is detailed in the following sections.

9.1.1.Design Peak Flows

Design peak flows were derived from the FFA for the 20% AEP to 0.2% AEP events. For the Murray River, the FFA results at Doctors Point were adopted (Section 8.1.7). These are considered to be representative of flows at the upstream end of the Murray River TUFLOW model. The FFA at Corowa also provided similar results to the Doctors Point FFA, although it is recognised the Corowa FFA produced a slightly higher flow for the 0.5% and 0.2% AEP events. As such, a scaling factor was applied to the Doctors Point peak flows to account for this.

For the Ovens River, coincident design peak flows were selected based on the FFA results, however, these were scaled in order to match the FFA at Yarrawonga. Flooding at Yarrawonga can be attributed to the Murray River, the Ovens River, or a combination of both. An analysis of recent flood events producing flows above 100,000 ML/d at Yarrawonga and the contribution of both the Murray and Ovens River was undertaken, with a summary presented in Table 59

1. Based on peak flows at Corowa gauge where available, otherwise the Doctors Point/Union Bridge gauge

2. Based on peak flows at Peechelba – either gauge or correlated from Wangaratta (in italics)

3. Based on peak flows at Yarrawonga gauge

While a Murray River flood is typically also evident in the Ovens River, it does not necessarily

Table 59: Coincidence of Murray River and Ovens River Floods

Corowa, Howlong and Mulwala Flood Study

produce a flood peak commensurate with the Murray River peak (Murray dominated events). Conversely, an Ovens River event can occur without a flood occurring in the Murray River (Ovens dominated events). Even in the same year, the derived AMS for the Murray River and Ovens River may not be from the same event (for example 1974 and 2022, where the annual maximums for each river are from different flood peaks). It is only the 2016 event and potentially the 1996 event where coincidence occurs in both the timing and relative magnitude of the flood peaks.

The focus of this study is on the Murray River and hence the appropriate Murray River event is simulated. This is combined with the same event occurring simultaneously in the Ovens River, however, the peak flow was scaled in order to match the FFA flows at Yarrawonga. An Ovens River dominated event has not been simulated.

A summary of the adopted peak design flows for the Murray River is provided in Table 60

Table 60: Adopted Peak Design Inflows for the Murray River TUFLOW Model

1. Design flows based on the FFA at Doctors Point.

2. Scaling factor to match FFA at Corowa

3. Adopted design peak inflow

4. Design flows based on the FFA at Peechelba.

5. Scaling factor to match FFA at Yarrawonga (combined with Murray River flows)

6. Summation of the Murray River and Ovens River peak inflows (assumes no attenuation and complete coincidence of peaks)

7. Design flows based on the FFA at Yarrawonga.

8. Discussed in Section 9.1.3

9.1.2.Design Hydrograph Shape

The design flood hydrograph shape was adopted from the 2016 flood event for the 20% to 0.2% AEP events. In selecting a historic hydrograph for design flood event modelling, consideration should be given to its representation of typical flood events for that system, and the flood volume. The 2016 flood event was considered to be a reasonable representation of flood events on the Murray River. By comparison, the 1992 event had a very sharp flood peak while the 1996 event had a sustained peak (driven by consistent outflows from Hume Dam) – both of which are not considered to be representative. The 2016 flood had a duration of approximately 2 weeks (above 40,000 ML/d for the main flood peak) and a peak flow rate of 100,000 ML/d.

Corowa, Howlong and Mulwala Flood Study

To ensure that the flood hydrograph volume is reasonable, a sensitivity analysis was undertaken to the hydrograph shape. Each of the calibration event hydrographs were tested with the 1% AEP design peak flow in the Murray River. Across the range of events, the peak flood level was within 0.02 m at Howlong and 0.06 m at Corowa. The range of peak flows at Corowa were all within 2%. These very different hydrograph shapes result in similar peak flood levels and flows at Howlong and Corowa. The 2016 and 2022 hydrographs were also tested with the adopted 1% AEP design peak flows for both Murray River and Ovens River. For these hydrographs there is consideration of hydrograph shape, but also the timing of peak flows between the two rivers. The resulting flood levels in Lake Mulwala were within 0.04 m and within 0.02 m downstream at the Yarrawonga gauge. Peak flows at Yarrawonga were within 1% of each other. It was concluded that the hydrograph shape does not significantly affect peak flood levels, and hence the Murray River floodplain in this reach is not sensitive to volume assumptions.

Adoption of the 2016 flood event also enables a real coincident event to be simulated for the Ovens River, since the Peechelba gauge was only operational for the 2016 and 2022 calibration events The 2016 event also produced a reasonable hydrograph shape for the Ovens River, with a peak flow of 90,000 ML/d which coincided with the Murray River peak. The timing of the Ovens River hydrograph compared to the Murray River hydrograph was adopted from the 2016 event calibration (see Section 7.8 for details), which accounts for travel time of the flood from the gauges (Doctors Point and Peechelba) to the TUFLOW model boundary. The 2016 hydrograph was trimmed to only represent the main flood peak. The first flood peak seen in Diagram 10 was removed from the design flood hydrograph. Sensitivity testing of the inclusion of this portion of the hydrograph indicated negligible change to peak flood levels at Corowa (within 0.01 m). The 2016 design hydrograph where Doctors Point flow rates were above 40,000 ML/d was adopted, with a total simulation time of 336 hours (14 days, plus 2 days ‘warm up’, as per the calibration events). The corresponding Ovens River hydrograph for that time period was also adopted.

The 2016 flood hydrographs for both the Murray River and Ovens River were scaled to match the peak flows discussed in Section 9.1.1, for the 20% AEP to 0.2% AEP events. The resulting hydrographs can be seen in Diagram 21 and Diagram 22 for the Murray River and Ovens River, respectively. These hydrographs are as applied at the TUFLOW model boundaries (accounting for travel time to the model boundary, as per the 2016 calibration event). The hydrographs do not include the 2 day warm-up period.

Corowa, Howlong and Mulwala Flood Study

Diagram 21: Murray River Design Flood Hydrographs for the 20% AEP to 0.2% AEP Events

Diagram 22: Ovens River Design Flood Hydrographs for the 20% AEP to 0.2% AEP Events

9.1.3.PMF Hydrograph

For the PMF, the peak flow estimate was derived from a paper presented at the 2011 Australian National Committee on Large Dams (ANCOLD) conference titled “Assessment of Hydrologic Risk

Corowa, Howlong and Mulwala Flood Study

for Hume Dam” (Nandakumar et al, 2011). The peak flow estimate for ‘Option D’ was adopted, being the deterministic PMF estimate with PMP rainfall, pre-burst and maximum temporal pattern, 0 mm and 1 mm/h initial and continuing losses, and reservoirs initially at FSL. This peak flow rate was reported to be 23,600 m3/s (approx. 2,040,000 ML/d) inflow into the dam and 14,900 m3/s (approx. 1,287,000 ML/d) outflow from the dam. It is understood that MDBA recently updated the PMF estimate for the Hume Dam (2022), however, these were not able to be obtained and the work presented by Nandakumar et al (2011) was adopted in lieu of this.

The reported inflow into the dam was used to derive an estimate of catchment inflows downstream of the dam, using the non-linear relationship presented in below:

Where: Q = peak flow (ML/d) A = catchment area (km2) a, b subscripts for upstream and downstream catchments

This relationship was found to be representative of the 1% AEP peak flow rates obtained via FFA for Doctors Point and Yarrawonga, as demonstrated below. The equation-derived flow of 370,000 ML/d is 5% lower than the FFA-derived flow of 390,000 ML/d.

A summary of the downstream catchments and peak flow rates derived from the equation above is presented in Table 61.

Table 61: Summary of downstream catchment areas and contributing peak PMF flows rates

1. Based on catchment areas to the relevant gauge, as estimated by WaterNSW using gauge site summary details

2. Based on Nandakumar et al (2011) for peak inflows into the dam

3. Used as the location to derive flows as this is downstream of the Doctors Point gauge and closer to the study area

For PMF inflows at the upstream boundary of the TUFLOW model, the estimated Hume Dam outflow of 1,287,000 ML/d was adopted from Nandakumar et al (2011) plus the additional inflow downstream of the dam to Albury (accounting primarily for the Kiewa River) of 200,000 ML/d (as per Table 61). This gives a total inflow into the TUFLOW model of approximately 1,487,000 ML/d. The adopted Ovens River inflow into the TUFLOW model was approximately 1,000,000 ML/d.

Corowa, Howlong and Mulwala Flood Study

This is the additional flow required to meet the estimated PMF flow at Yarrawonga of 2,487,000 ML/d (Hume Dam outflow plus 1,200,000 ML/d as per Table 61, of which 200,000 ML/d is already input at the upstream boundary to account for the Kiewa River). This is shown visually in Diagram 23.

Hume Dam Inflow

2,487,000ML/d

TUFLOW Model

1,487,000ML/d

1,000,000ML/d

Diagram 23: Schematic of PMF Inflows for the Murray River

2,040,000ML/d

1,287,000ML/d

Kiewa

200,000ML/d

The design flood hydrograph shape for the PMF event was adopted from the underlying research of Nandakumar et al (2011), representing the Hume Dam outflow. The Ovens River inflow hydrograph was also derived from the Hume Dam outflow hydrograph from Nandakumar et al (2011), in the absence of any other information. The hydrographs were scaled to match the peak flows presented in Section 9.1.1. The Ovens River hydrograph was shifted by 24 hours to allow for the travel time from the TUFLOW model Murray River boundary to the confluence with the Ovens River, such that the Murray River and Ovens River peaks would coincide. A total simulation time of 200 hours (approximately 8 days, plus 2 days ‘warm up’, as per the other design events) was adopted. The adopted hydrographs for the PMF event are shown in Diagram 24.

24: Design Flood Hydrographs for the PMF Event

9.2. Local Overland Design Flood Modelling

ARR 2019 guidelines (Ball et al 2019) for design flood modelling were adopted for this study. The new guidelines were first published in 2016, finalised in 2019 and present a significant update on the previous version published in 1987 (Pilgrim 1987). Since 1987, there have been numerous advances in the understanding of rainfall-runoff processes, technological advances and a larger set of recorded rainfall data available. This additional 30 years of data (from approximately 1985 to 2015), particularly for continuously recorded rainfall (pluviometers), allows for Australia-specific techniques and regionalised information to be used across the country. Specifically related to design flood modelling there is updated IFD information, design temporal patterns, areal reduction factors and rainfall losses to consider.

ARR 2019 guidelines were used to estimate the 20%, 10%, 5%, 2%, 1%, 0.5%, 0.2% AEP events. The PMF flows were derived using the Bureau of Meteorology’s Generalised Short Duration Method (GSDM, Bureau of Meteorology 2003) and Generalised Southeast Australia Method (GSAM, Bureau of Meteorology 2006) to estimate the probable maximum precipitation (PMP) over the local catchments. The design flood inputs and parameters that were used and the critical pattern duration selection method for the local overland flood models are outlined in the following sections.

9.2.1.Design Rainfall Depths

Intensity Frequency Duration (IFD) information was obtained from the BoM using the 2016 design rainfall data. A single IFD point at the catchment centroid of each town was adopted. The IFD

Diagram

Corowa, Howlong and Mulwala Flood Study

grids indicated that the changes in IFD across the catchment were typically within 5% and did not warrant spatially varying the IFDs. Even for the largest catchment, Majors Creek, the change in IFD was still typically within 5% throughout the entire Majors Creek catchment. The adopted design rainfall depths are shown in Table 62, Table 63 and Table 64 for the Howlong, Corowa and Mulwala catchments, respectively. Even between the study areas, the rainfall depths do not vary by more than approximately 5%.

Table 62: Design rainfall depths (mm) at the centroid of the Howlong catchment (including Majors Creek)

Note: Taken at centroid of the catchment, referencing IFD grid cell at 35.9875°S, 146.6375°E

Corowa, Howlong and Mulwala Flood Study

Table 63: Design rainfall depths (mm) at the centroid of the Corowa catchment

Note: Taken at centroid of the catchment, referencing IFD grid cell at 35.9875°S,

Table 64: Design rainfall depths (mm) at the centroid of the Mulwala catchment

Note: Taken at centroid of the catchment, referencing IFD grid cell at 35.9875°S, 146.0125°E

Design rainfalls for the PMP were derived using the BoM’s GSDM (Bureau of Meteorology 2003) and GSAM (Bureau of Meteorology 2006) The GSDM is valid for durations up to 3 hours in this

Corowa, Howlong and Mulwala Flood Study

part of Australia and durations from 24 to 96 hours are estimated from the GSAM. Interpolation is used for durations between 3 and 12 hours. GSDM parameters for each of the towns are shown in Table 65

Table 65: GSDM parameters

Parameter

Spatial Distribution

Ellipse ‘A’ using an area of 2 km2

Spatially distributed using Ellipses A - F Ellipse ‘A’ using an area of 2 km2

Ellipse ‘A’ using an area of 2 km2

In general, a ‘smooth’ terrain was adopted, with the exception of Majors Creek which was assigned 20% roughness due to the elevation changes associated with the nearby hills The GSDM requires rainfall to be distributed spatially using ellipses. Ellipse ‘A’, at the centre, has an area of 2.6 km2 and represents the region of highest rainfall. Given the nature of the towns and the focus on overland flow paths through urban areas, it was assumed that all the flow paths of interest would have an upstream catchment area less than this, and as such the ellipse ‘A’ rainfall was applied to all sub-catchments, adopting an area of 2 km2 to derive rainfall depths. For Majors Creek, the ellipses were used to spatially distribute the rainfall.

For the GSAM, the catchments fall within the ‘inland’ zone and a typical 2 km2 catchment (representing overland flow paths in the towns) was used to derive initial PMP rainfall depths for durations from 24 to 96 hours For Howlong, the entire Majors Creek catchment area was used to estimate rainfall depths. Topographical adjustment factors were applied (ranging from 1.08 – 1.13 across the catchments) and moisture adjustment factors (ranging from 0.73 – 0.8 across the catchments) using the relevant GSAM grids. Rainfall was distributed spatially as per the GSAM method.

9.2.2.Temporal Patterns

Temporal patterns are a hydrologic tool that describe how rain falls over time and are used in hydrograph estimation. Previously, with ARR 1987 guidelines (Pilgrim 1987), a single temporal pattern was adopted for each rainfall event duration. However, ARR 2019 (Ball et al 2019) discusses the potential inaccuracies with adopting a single temporal pattern and recommends an approach where an ensemble of different temporal patterns is investigated.

Temporal patterns for this study were obtained from the ARR 2019 data hub (Babister et al 2016, http://data.arr-software.org/). A summary of the data hub information at the catchment centroid for each of the towns is presented in Attachment A. The revised ARR 2019 temporal patterns attempt to address the key concerns practitioners found with the ARR 1987 temporal pattern approach. It

Corowa, Howlong and Mulwala Flood Study

is widely accepted that there are a large variety of temporal patterns possible for rainfall events of similar magnitude. This variation in temporal pattern can result in significant effects on the estimated peak flow. As such, the revised temporal patterns have adopted an ensemble of ten different temporal patterns for a particular design rainfall event and duration. Given the rainfallrunoff response can be quite catchment specific, using an ensemble of temporal patterns attempts to produce the median catchment response.

As hydrologic modelling has advanced, it is becoming increasingly important to use realistic temporal patterns. The ARR 1987 temporal patterns only provided a pattern of the most intense burst within a storm, whereas the ARR 2019 temporal patterns look at the entirety of the storm including pre-burst rainfall, the burst and post-burst rainfall. There can be significant variability in the burst loading distribution (i.e. depending on where 50% of the burst rainfall occurs an event can be defined as front, middle or back loaded). The ARR 2019 method divides Australia into 12 temporal pattern regions, with Federation Council falling within the Murray Basin region.

ARR 2019 provides 30 temporal patterns for each duration which are sub-divided into three temporal pattern bins based on the frequency of the events. Diagram 25 shows the three categories of bins (frequent, intermediate and rare) and corresponding AEP groups. The “very rare” bin is in the experimental stage and was not used in this flood study. There are ten temporal patterns for each AEP/duration in ARR 2019 that have been utilised in this study for the 20% AEP to 0.2% AEP events.

For the PMP events, the relevant temporal pattern was applied as per the GSDM or GSAM method.

9.2.3.Rainfall Losses

The initial losses adopted for the calibration events was 30 mm for the January 2022 event in Corowa and 10 mm for the March 2012 event in Howlong and Mulwala. It is only the January 2022 event that provided sufficient data for calibration, and as discussed in Section 7.14, the event showed little sensitivity to the adopted initial loss and this was not considered to be a ‘calibrated’ parameter. As such, for design flood modelling, the probability neutral burst initial losses from the ARR datahub (Babister et al 2016, http://data.arr-software.org/) were adopted, in line with recent advice from the NSW Government (NSW Office of Environment and Heritage 2019). These initial losses were sourced from the ARR datahub at the centroid of the catchment The losses vary with storm duration and AEP, however are generally in the range of 5 mm to 25 mm across the full range of AEPs and durations. The probability neutral burst initial losses at the centroid for each town catchment can be found in Attachment A

Diagram 25: Temporal Pattern Bins

Howlong and Mulwala Flood Study

For design continuing losses, the towns are covered by separate ARR data hub loss grid cells, with values ranging from 2.4 mm/h for Mulwala to 4.4 mm/h and 4.5 mm/h for Corowa and Howlong, respectively. Recent advice provided by the NSW Government (NSW Office of Environment and Heritage 2019) indicates that these losses should be factored by 0.4 for NSW catchments. This results in continuing losses of in the range of 0.96 mm/h to 1.8 mm/h across the towns These continuing losses were adopted for the calibration events also, as discussed in Section 7.14

The PMP event adopted an initial loss of 1 mm and continuing loss of 0 mm/h.

9.2.4.Areal Reduction Factors

The design rainfall estimates are based on point rainfalls and in reality, the catchment-average rainfall depth will be less. Areal reduction factors (ARFs) allow for the fact that larger catchments are less likely than smaller catchments to experience high intensity storms simultaneously over the whole catchment area. Given the nature of the towns and the focus on overland flow paths through urban areas, ARFs were not applied in the WBNM model. In accordance with ARR 2019 (Ball et al 2019), catchments with an area up to 1 km2 should not apply ARFs, and there is limited research on the applicability of ARFs to catchments that are less than 10 km2. The catchments for individual flow paths within the towns are not large enough to warrant the application of ARFs. Estimates of the ARFs for the largest of the catchments draining to individual flow paths within the towns was approximately 0.99, and hence it was considered reasonable to not apply ARFs.

The Majors Creek catchment, however, is large enough that ARFs should be considered. The ARFs were calculated in accordance with ARR 2019 (Ball et al 2019) and were found to be approximately 0.94-0.95 for the durations of interest. For the adopted storm event (see Section 9.2.5.4), the application of ARFs resulted in a negligible change in the 1% AEP peak flood levels through the majority of the township (within ±0.01 m), with a maximum reduction in peak flood level of 0.02 m. This change in peak flood level is not considered to be significant. The application of ARFs is complicated by the fact that Howlong can be affected by local catchments (for which ARFs should not apply) and Majors Creek (which ARFs can be applied), and different storm events and durations can affect which mechanism dominates in different parts of the town. For simplicity, no ARFs were adopted for Majors Creek

9.2.5.Critical Duration Assessment

9.2.5.1.

Approach

ARR 2019 (Ball et al 2019) requires an ensemble of temporal patterns to be run for each AEP and duration combination, and the ‘occasional’ approach was adopted for this study, as shown in Diagram 26

Diagram 26: Design modelling techniques for an ensemble of temporal patterns (Ball et al 2019)

This approach requires the ensemble of temporal patterns to be run in both the hydrologic and hydraulic models. This approach was adopted due to the complex nature of the shallow overland flow paths through the towns which means that flows at any point in the hydrologic model may not represent the actual flows arriving at that point due to hydraulic controls and cross-catchment flows. The towns also exhibit large areas of flood storage, which are driven by volume rather than flow.

9.2.5.2.

Critical Duration

The critical duration is the storm duration that best represents the flood behaviour (e.g. flow or level) for a specific design magnitude at a particular location. It is generally related to the catchment size, as flow takes longer to concentrate at the outlet from a larger catchment, as well as other considerations such as land use, shape, stream characteristics, etc.

With ARR 2019 methodology, the mean flow (or level) is computed from the ensemble of temporal patterns for each duration. The critical storm duration for a location of interest is then the design storm duration that produces the highest mean flow (or level). Where there are multiple locations of interest with different contributing catchment sizes, there can be multiple critical durations that need to be considered.

9.2.5.3.

Representative Storm Burst

Once the critical duration is established, it is usually desirable to select a representative design storm temporal pattern that reproduces this behaviour at all points of interest. This representative storm can then be used for determining design flood behaviour and for future modelling to inform floodplain management decisions. This is typically the storm that produces the next highest flow (or level) above the average (from the ensemble of temporal patterns) for the critical duration. In most cases, however, a representative storm does not necessarily need to be ofthe same duration as the critical duration, and there may be a number of storms that can represent the critical duration behaviour, potentially at multiple locations and even where the critical duration varies.

Adopting a range of critical durations across a catchment can complicate future analysis and the use of modelling tools if multiple storms need to be simulated to obtain the design flood behaviour for a particular event. Thus, it is preferable to adopt a single representative storm (or as few as

required) that is similar to the critical duration behaviour across the entire catchment for each event where possible.

9.2.5.4. Representative Storm Selection

To select the representative storm for each AEP for each of the towns, the WBNM hydrologic models were run for durations from 2 hours to 48 hours, with the ensemble of temporal patterns for the 20% AEP, 5% AEP and 1% AEP events (representative of each temporal pattern bin). Each of these storms was then simulated in the TUFLOW model. For each duration, a grid of the mean peak level at each grid cell was calculated for the 10 temporal patterns. A maximum envelope grid was then calculated taking the highest mean peak level for each grid cell for all durations. This shows the critical duration mean peak level at all flooded cells across the study area. The source of the peak mean level for each grid cell was mapped to show the variation in critical duration across the catchment.

Howlong

The critical duration maps for Howlong are shown in Figure 40, Figure 42 and Figure 44 for the 20% AEP, 5% AEP and 1% AEP events, respectively. The critical duration for Howlong is dominated by the 180 minute, 720 minute and 1440 minute storms in the town There are significant storage dominated areas throughout the town as well as the wider Majors Creek floodplain to the east of Kywong-Howlong Road. These areas, being driven primarily by runoff volume, require a long storm duration to fill them. Through a comparison of the peak flood level grid for each storm with the critical duration mean peak level across the entire study area, a representative storm was selected for each AEP event simulated. This was a temporal pattern from the 1440 minute (24 hour) storm duration and was able to represent the critical duration reasonably well across the entire town. The selection process focussed on accurately representing the critical duration behaviour in the town itself, while still being representative of the adjacent Majors Creek floodplain as well. The selected storms result in minimal variation in peak water level from the critical duration mean peak level. The selected storm typically results in peak flood levels being within ±0.03 m. This difference is shown in Figure 41, Figure 43 and Figure 45 for the 20% AEP, 5% AEP and 1% AEP events, respectively.

A similar, but simplified approach was undertaken for the Howlong PMF event, whereby a single storm was run for durations from 15 minutes to 72 hours. The results indicated that the 180 minute (3 hour) storm was critical across the majority of the town, with the south western portion having a shorter critical duration, as shown in Figure 46. For the purpose of this study, the 45 minute and 180 minute (3 hour) storms were selected as being representative of flooding across the study area. The maximum envelope of these two durations was taken to produce the PMF results across the study area.

The selected storms were considered representative for all design events within that temporal pattern bin (Diagram 25). The selected storms were adopted for modelling of the design flood events and processing of flood results (as described in Section 10). The adopted representative design storms for Howlong are summarised in Table 66.

Corowa, Howlong and Mulwala Flood Study

Table 66: Adopted Representative Design Storms for Howlong

Corowa

The critical duration maps for Corowa are shown in Figure 47, Figure 49 and Figure 51 for the 20% AEP, 5% AEP and 1% AEP events, respectively. The critical duration for Corowa is dominated by the 120 minute storm across the areas affected by shallow overland flows, while areas of ponding have critical durations around 540 minutes and 720 minutes. Through a comparison of the peak flood level grid for each storm with the critical duration mean peak level across the entire study area, a representative storm was selected for each AEP event simulated. This was a temporal pattern from the 720 minute (12 hour) storm duration for events up to the 5% AEP event and 1080 minute (18 hour) storm duration for events larger than this. These storms were able to represent the critical duration reasonably well across the entire town. The selected storms result in minimal variation in peak water level from the critical duration mean peak level. The selected storm typically results in peak flood levels being within ±0.03 m. This difference is shown in Figure 48, Figure 50 and Figure 52 for the 20% AEP, 5% AEP and 1% AEP events, respectively.

A similar, but simplified approach was undertaken for the Corowa PMF event, whereby a single storm was run for durations from 15 minutes to 72 hours. The results indicated that the 180 minute (3 hour) storm was critical across the majority of the town, with the shallow overland flow areas having a shorter critical duration, as shown in Figure 53. There was one flood storage area in the vicinity of Spring Drive that had a long critical duration (2160 minutes) For the purpose of this study, the 60 minute, 180 minute (3 hour) and 2160 minute (36 hour) storms were selected as being representative of flooding across the study area. The maximum envelope of these two durations was taken to produce the PMF results across the study area.

The selected storms were considered representative for all design events within that temporal pattern bin (Diagram 25). The selected storms were adopted for modelling of the design flood events and processing of flood results (as described in Section 10). The adopted representative design storms for Corowa are summarised in Table 67.

Corowa, Howlong and Mulwala Flood Study

Table 67: Adopted Representative Design Storms for Corowa

Mulwala

The critical duration maps for Mulwala are shown in Figure 54, Figure 56 and Figure 58 for the 20% AEP, 5% AEP and 1% AEP events, respectively. The critical duration for Mulwala varies between 180 minutes and 1440 minutes, depending on the AEP. Through a comparison of the peak flood level grid for each storm with the critical duration mean peak level across the entire study area, a representative storm was selected for each AEP event simulated. This was a temporal pattern from the 1440 minute (24 hour) storm duration for events up to the 5% AEP event and two durations for events larger than this – the 360 minute (6 hour) and 2880 minute (48 hour) storm durations For the larger events, two durations were needed to adequately represent flood behaviour across Mulwala. This is due to the large amounts of flood storage through the town requiring a long duration storm, but also areas of conveyance – particularly the channels downstream of Bayly Street. The maximum envelope of these two durations was taken to produce the results for rare events across the study area. It is noted that the long duration storm (2880 minutes) is at the upper end of the durations simulated. Running longer durations for the full range of temporal patterns was not considered viable due to the run times. However, the WBNM model was run for durations up to 7200 minutes (120 hours) and the results were analysed. Both peak flows and total volumes were assessed for key areas to determine a critical duration (as would be done with the ‘most common’ approach in Diagram 26). It was determined that the 2880 minute storm was critical for the majority of locations, but a 4320 minute (72 hour) storm was critical for some locations. A representative storm was selected for this duration and run in the TUFLOW model. The results indicated lower flood levels, or flood levels very similar to the adopted 2880 minute storm. As such, the adopted 2880 minute storm was considered appropriate.

These storms were able to represent the critical duration reasonably well across the entire town. The selected storms result in minimal variation in peak water level from the critical duration mean peak level. The selected storm typically results in peak flood levels being within ±0.03 m. This difference is shown in Figure 55, Figure 57 and Figure 59 for the 20% AEP, 5% AEP and 1% AEP events, respectively.

A similar, but simplified approach was undertaken for the Mulwala PMF event, whereby a single storm was run for durations from 15 minutes to 72 hours. The results indicated that the majority

Corowa, Howlong and Mulwala Flood Study

of the town had a critical duration in the range of 30 minutes to 180 minutes, with two flood storage areas having a longer critical duration (720 minutes and 2160 minutes), as shown in Figure 60 For the purpose of this study, the 60 minute, 180 minute (3 hour) and 2160 minute (36 hour) storms were selected as being representative of flooding across the study area. The maximum envelope of these two durations was taken to produce the PMF results across the study area.

The selected storms were considered representative for all design events within that temporal pattern bin (Diagram 25). The selected storms were adopted for modelling of the design flood events and processing of flood results (as described in Section 10). The adopted representative design storms for Corowa are summarised in Table 68.

Table 68: Adopted Representative Design Storms for Mulwala

9.3. Downstream Boundary and Initial Conditions

As outlined in Section 6.9.2, stage-discharge boundaries were applied for the downstream boundaries of each of the TUFLOW models. The exception to this was the local overland model for Mulwala, where a constant water level of 124.75 mAHD was adopted for all design events, representing an average lake level.

Each of the local overland TUFLOW models was assumed to be dry at the start of the storm. For the Murray River model, Lake Mulwala was assumed to be full to a level of 124.5 mAHD. Although this is slightly lower than the average lake level, this is the level at which the modelled gate operations allow for a stable water level at the start of the event where gates can remain closed. The initial warm-up period commences filling of Lake Mulwala and the gates begin their operation when the water level reaches 124.7 mAHD.

9.4. Blockage

ARR 2019 (Ball et al 2019) recommends applying blockage to hydraulic structures, and outlines a methodology to determine inlet blockage factors by considering debris availability, debris mobility, debris transportability and waterway opening of the structure. This assessment was undertaken considering the typical culvert structures found in the study area. These structures fell into two categories, for which AEP dependent blockages were estimated in accordance with ARR

Corowa, Howlong and Mulwala Flood Study

2019 procedures. The blockage factors from this analysis can be seen in Table 69

Table 69: Blockage assessment results of key hydraulic structures

1 Generally smaller structures with a diameter or width less than 1.2 m (the assumed L10), with typically a medium 1% AEP debris potential as they are located in the urban areas of the towns.

2 Generally larger structures with a diameter or width greater than or equal to 1.2 m (the assumed L10), with typically a high 1% AEP debris potential as they are located on major flow paths or receive flows from rural areas.

A single blockage factor was considered appropriate across the range of design flood events, with the adopted blockage factors outlined in Table 70. Blockage of large bridge structures crossing the Murray River had a nominal blockage factor of 5% applied. Pit inlets for the stormwater network were assumed to be blocked 50%.

Table 70: Adopted Blockage Factors

10. DESIGN FLOOD RESULTS

10.1.

Introduction

The 20%, 10%, 5%, 2%, 1%, 0.5% and 0.2% AEP and PMF events were simulated using the adopted Murray River and Ovens River hydrographs for the Murray River model. For the local overland flood behaviour, the adopted representative storms were run in the WBNM hydrologic model and the resulting flows were input into the TUFLOW hydraulic model to simulate flood behaviour for each of the towns. The results for the design flood events are presented in the following appendices:

Appendix F: Murray River

Appendix G: Howlong

Appendix H: Corowa

Appendix I: Mulwala

In each appendix, the following maps are provided:

• Peak flood depths and levels in Figure 1 to Figure 8;

• Peak flood velocities in Figure 9 to Figure 16;

• Hydraulic hazard in Figure 17 to Figure 19;

• Hydraulic categories in Figure 20 to Figure 22.

These results are available in electronic GIS and tabular format. The digital data should be used in preference to the figures in this report as they provide more detail. The figures are intended to provide an overview of the results and should not be relied upon for detailed information at individual properties. Property-level affectation should be confirmed by comparing the estimated design flood level(s) for the property with detailed ground survey undertaken by a registered surveyor.

Additional results are presented in the following tables and graphs:

• Peak flood levels, depths and flows at road crossings and key locations in Table J1, Table J2 and Table J3, respectively, of Appendix J;

• Peak water level profile for the Murray River in Figure K1 of Appendix K; and

• Stage hydrographs at road crossings in Figure K2 to Figure K44 of Appendix K

A discussion of these results is provided in the following sections.

10.2. Summary of Results

Murray River

The flood behaviour for the Murray River can be seen in the peak flood depth / level maps (Figure F1 to Figure F8) and peak velocity maps (Figure F9 to Figure F16). A graph of the peak water level profile along the Murray River is provided in Figure K1

At Howlong, the Murray River is largely contained within the lower floodplain, with the town being elevated above this. Backwater from the river extends up the creek bounded by Lowe Street and Hume Street, however, it does not cross Lowe Street in events up to and including the 0.2% AEP. Floodwater from the Murray River does not affect the Howlong township up to and including the 0.2% AEP event. In the PMF event, the Murray River extends across the south eastern portion of the Howlong township, with inundation roughly bounded by Sturt Street to the west, Hawkins Street to the north and Majors Creek to the east. Depths across the affected areas are typically 0.5 m to 1 m, with deeper areas up to 5 m along the creek line.

At Corowa, breakout flows from the Murray River affect the lower terrace area in events as frequent as the 20% AEP, with a flow path forming along the lower areas drained by box culverts under Bridge Road. The south eastern corner of River Road is also inundated in south Corowa. In the 10% AEP event a large portion of the lower terrace is inundated, with depths up to 2 m. Bridge Road is also overtopped at the northern end near Bangerang Park. In the 1% AEP event, the entire lower terrace is inundated by 1.5 m to 4 m. The majority of the developed areas on the upper terraces, however, are not affected. Flow only just begins to cross over the low point of Federation Avenue in the 0.2% AEP event, although most development still remains flood-free. In the PMF event, a significant portion of the town is affected. Flows break out of the Murray floodplain north of Federation Way and continue north (away from Corowa) and also to the south west, through the Whitehead Street Wetlands and affecting the Redlands Road industrial area and Corowa Airport. Only a very small amount of flow returns to the Murray River primarily via channels. Within the town, lower areas are inundated including Sanger Street up to River Street, Parliament Street up to Gray Street, the low lying area bounded by Gallipoli Street and Hermitage Street and River Street in south Corowa as well as areas south of Hay Street.

At Mulwala, floodwaters are contained within Lake Mulwala, being controlled by Yarrawonga Weir for frequent events. In the 2% AEP event, water begins to spill into the town at North Street and Erne Street, although the affectation is minimal. In the 1% AEP event, however, a large portion of the town is inundated with depths reaching 0.5 m to 1 m. This affects new development north of the Mulwala Canal as well as the low-lying area south of the Mulwala Canal. The majority of the town is inundated in the 0.5% AEP event and in the PMF event the flood depth through the town is approximately 3 m. Breakout flows typically continue to the north or are conveyed via the Bayly Street channels back to the Murray River.

Howlong Local Overland

At Howlong in the local overland events (including Majors Creek flooding), low lying portions of the town are inundated as well as a large area of the Majors Creek floodplain. In the 20% AEP event Majors Creek flows south, crossing Howlong-Burrumbuttock Road and also west, crossing Kywong-Howlong Road. These flows do not impact the main township. Within the town, water begins to pond along the remnant creek line through the north of the town (between Pell Street and Jude Street) and in a low lying area within and to the west of the Howlong Recreation Reserve. The local flow path through the southern portion of the town (adjacent to, and south of Victoria Street) begins to overtop roads and there are other low lying areas on the western side of the town where water also ponds in the 20% AEP event. Larger events are modelled to have a greater impact on the town, as ponding becomes deeper and more extensive through the town. In the 2%

Corowa, Howlong and Mulwala Flood Study

AEP event, water from Majors Creek begins to affect the town. In the 1% AEP event flood depths can typically reach up to 0.5 m at properties within the low-lying areas. In the PMF event, a significant portion of the town is inundated, the majority of the town is inundated with depths reaching over 1 m in some locations.

Corowa Local Overland

At Corowa in the local overland events, flooding is a mix of shallow overland flows and ponding at low points. In the 20% AEP event, ponding occurs at the Whitehead Street Wetlands, on land surrounding the Corowa Airport, north of Nixon Street (near Cemetery Road), within Sophia Close and Pinot Crescent, some areas within south Corowa (south of Spring Drive) and on the lower terraces of the Murray River floodplain. Larger events are modelled to have a greater impact on the town, as ponding becomes deeper and more extensive through the town. In particular the Redlands Road industrial area, Header World Avenue, the area around Sophia Close and Martin Street are impacted. In the PMF event there is extensive inundation of land to the north and west of the town, with significant ponding at Header World Avenue, Honour Avenue at the Whitehead Street Wetlands, the Southern Cross Care Karinya facility and Roden Street. The Sophia Close low point inundation extends north to Gallipoli Street, west to Sauvignon Drive and the intersection of Nixon Street and Railway Street, and spills across Federation Avenue, impacting properties downstream.

Mulwala Local Overland

At Mulwala in the local overland events, flooding is primarily driven by the low relief of the town such that ponding of water occurs in frequent events. In the 20% AEP event, ponding of water begins to affect new development off Tocumwal Road, an area around Payne Street and Hicks Street, an area around Manners Street and Nyncoola Circuit. In the western portion of the town there are isolated areas of ponding within low points in undulating terrain. Larger events are modelled to have a greater impact on the town, as ponding becomes deeper and more extensive through the town. In the 1% AEP event, flood depths are approximately 0.3 m to 0.5 m deep at those areas noted above. In the PMF event, the majority of the town is affected, with flood depths reaching 1 m deep, although typically 0.5 m deep.

10.3. Gauge Results

A comparison between the FFA results (see Section 8) and the design flood modelling results at the gauges was undertaken to verify the adopted design flows (see Section 9.1). A comparison is provided for the Corowa gauge and Yarrawonga gauge in Table 71

Table 71: Design Flow Comparison at Gauges

Corowa, Howlong and Mulwala Flood Study

The results indicate that the modelled flows closely match the FFA derived flows (typically within 3%). The 20% AEP modelled flow at Corowa is slightly higher (by 6%) than the FFA due to the adopted Doctors Point FFA flow being higher than at Corowa and the fact that the flows were not scaled down. Flows from Doctors Point were only scaled up for rarer events to enable a better match to FFA flows at Corowa. The modelled flows at Yarrawonga begin to deviate from the FFA flows at the 1% AEP event. This is due to flows breaking out through Mulwala. For example, in the 1% AEP event, approximately 10,500 ML/d peak flow is modelled to break out through the town, accounting for the 9,300 ML deficit at the Yarrawonga gauge, located downstream of Yarrawonga Weir. The difference between the modelled and FFA flows at the Yarrawonga gauge is greater for larger events, as more flow breaks out through Mulwala. This is discussed further in Section 10.4

Design flood levels at the gauges within the study area are provided in Table 72 It is recognised that design flood levels are typically higher than that previously adopted upstream of Yarrawonga Weir, and lower downstream of the weir. At Howlong, the previous 1% AEP design flood level was 7.14 m (1975 flood, as reported in URS 2009 and NSW SES 2015) and the current 1% AEP design flood level is 7.57 m. At Corowa, previous design flood levels (see Table 45) for the 1% AEP event were 8.825 m (Gutteridge Haskins Davey 1986), 9.31 m (URS 2009) and 8.8 m (NSW SES 2015), whereas this study has estimated a level of 9.41 m. At Yarrawonga (downstream of the weir), previous design flood levels (see Table 58) for the 1% AEP event were 9.76 m (Gutteridge Haskins Davey 1986), 9.83 m (URS 2009) and 9.76 m (NSW SES 2015), whereas this study has estimated a level of 9.29 m.

Table 72: Design Flood Levels at Gauges

Corowa, Howlong and Mulwala Flood Study

The flood classifications for the Corowa and Yarrawonga gauges and the approximate AEP of the event are summarised in Table 73

Table 73: Flood Classifications at Gauges

10.4. Yarrawonga Weir

The performance of Yarrawonga Weir was assessed in the model to ensure correct representation and operation of the gates for the design flood events. The gates are implemented as an operational structure (details in Section 6.8) and their operation was validated for the 2016 and 2022 flood events (see Sections 7.8 and 7.9, respectively). For the design flood events, since Mulwala is modelled to be impacted by the 1% AEP Murray River event, the performance of the weir was checked. In the 1% AEP event, the gates are modelled to be fully open fairly early in the simulation. The opening of the gates (elevation of the bottom of the gate) is shown in Diagram 27, together with the flow hydrograph and water levels immediately upstream and downstream of the weir. The gates progressively open as the flood level in the lake rises. The gates fully open (bottom elevation of 127 mAHD) at approximately 95 hours in the simulation, when the flow through the gates is approximately 285,000 ML/d and the water level in the lake is approximately 125.1 mAHD. The gates remain fully open until the water level drops below 124.5 mAHD. This confirms that when the peak of the flood occurs, the gates are fully open and do not operationally restrict flows through the weir.

Corowa, Howlong and Mulwala Flood Study

Diagram 27: Yarrawonga Weir Operation in the 1% AEP event

It can also be seen in Diagram 27 that the downstream water level is approximately 1.3 m below the upstream water level. This aligns with design assumptions of 1.15 m head loss and observations from the 1975 event (estimated 1.42 m head loss). The combined discharge through the gates is approximately 380,000 ML/d at a headwater level of approximately 126.15 mAHD. This aligns with the water level vs discharge curve provided in the Australian Dams Alliance (1999) report and discussed further below.

The flow through the gates was checked against the stage-discharge curve estimated by Australian Dams Alliance (1999). The comparison is shown in Diagram 28. The stage-discharge curve from TUFLOW was derived when from when the gates are fully open (i.e. not operationally restricting flows and free flow is possible) from the 1% AEP and PMF events. There is good agreement for elevations from 124 mAHD to 125 mAHD. Above this, for flows in excess of 300,000 ML/d, there is a slight deviation in the curves, with the TUFLOW model estimating a slightly higher discharge for the same level. The difference in discharge is no greater than 4% at any point on the curve and shows consistency with the theoretical stage-discharge curve.

Corowa, Howlong and Mulwala Flood Study

The Australian Dams Alliance (1999) report states a number of levels and flows for the weir, as shown in Table 74 The report states that the weir was designed considering the 1917 event, the maximum recorded flood at the time of the weir’s original design with a flow of 207,800 ML/d (not 340,000 ML/d assumed in this study’s FFA). The source of this flow estimate is not provided, although the report does state that this figure was questioned by the Senior Engineer for Storages. The report states that the flow was factored by 1.5 to derive a design discharge of 306,000 ML/d. The gates were then designed accordingly, with an actual discharge calculated to be 314,700 ML/d. A review of the design discharge for the structure in November 1975 then revised this estimate to be 329,000 ML/d. A revised discharge curve was then prepared for the Australian Dams Alliance (1999) study, which indicated a design flood discharge of approximately 343,000 ML/d. With a 1% flow of 390,000 ML/d approaching the weir, it is expected that the weir would not be able to freely passthis flow with outflanking ofthe weir (including through theMulwala township) occurring

Diagram 28: Yarrawonga Weir Stage-Discharge Curve Comparison

Corowa, Howlong and Mulwala Flood Study

Table 74: Yarrawonga Weir Design Levels and Discharges (free flow conditions)

Estimated performance of the weir during the October 1975 flood.

124.9

127.5

127.5 490,200

Full Supply Level. Estimated to be a 1 in 30 AEP flow.

Design Flood Level (i.e. the weir was designed assuming an afflux of 1.07 m resulting from a head of 125.85 mAHD). Estimated to be approximately a 1 in 60 or 1 in 70 AEP event. Above this level, the report states that Yarrawonga will be inundated as flows outflank the weir. It does not mention Mulwala.

Bottom of gates. Estimated to be approximately 1 in 180 or 1 in 200 AEP. This assumes the weir is not outflanked by flood flow. It is conceivable that flow could outflank the weir in the order of 50,000 ML/d to 500,000 ML/d. This means that flow could potentially be in the range of 445,500 to 945,000 ML/d before flow reaches the bottom of the gates.

Weir crest, assuming weir is not outflanked by flood flow. This induces orifice flow conditions. It is expected that the weir would only be overtopped by flows in excess of 1,000,000 ML/d (assuming approximately 500,000 ML/d through the gates and 500,000 ML/d across the floodplain.

Weir crest, assuming weir is not outflanked by flood flow. This induces orifice flow conditions. It is expected that the weir would only be overtopped by flows in excess of 1,000,000 ML/d (assuming approximately 500,000 ML/d through the gates and 500,000 ML/d across the floodplain.

1. Immediately upstream of the gates (away from drawdown influence).

Source: Australian Dams Alliance 1999

The gate performance in the model aligns with the information provided in the Yarrawonga Weir Review of Flood Security (Australian Dams Alliance 1999). Design peak water levels at various points within Lake Mulwala are provided in Table 75 For frequent events up to and including the 10% AEP event, the water level remains at FSL (124.9 mAHD) with a constant water level from the open lake to the weir. In the 5% AEP event, a slight gradient in the water level occurs, with the open lake level being approximately 0.2 m higher than water levels closer to the weir. As water flows from the open lake down to the weir, the water level decreases, indicating that the natural topographic constriction of the Lake Mulwala outlet, in conjunction with Yarrawonga Weir cause these higher water levels in the lake. In particular, water levels rise high enough in the 2% AEP event such that floodwaters begin to impact the Mulwala township (approximately 126.0 mAHD) The 2% AEP flow rate of approximately 330,000 ML/d is in the range of estimates of the design discharge that Yarrawonga Weir was designed for. The afflux caused by the constriction (both natural and man-made) at the weir increases with the size of the event. With the PMF event, the modelling estimates a water level difference of 1 m between Mulwala and the weir. A sensitivity indicating the influence of these structures within this portion of the lake is provided in Section 11.8.

Table 75: Design Flood Levels (mAHD) in Lake Mulwala

Corowa, Howlong and Mulwala Flood Study

1. Located downstream of the Murray River and Ovens River confluence.

The current 1% AEP adopted flood level for Lake Mulwala is 125.86 mAHD (URS 2009). This appears to be from the Gutteridge Haskins Davey (1986) report, although the derivation of this level is not clear in the report. It may be that this was estimated to be the theoretical water level upstream of the weir for the 1% AEP design flow of 390,000 ML/d at the time of the study. The stage-discharge curve provided by the Australian Dams Alliance (1999) indicates a flow of approximately 350,000 ML/d would achieve a water level close to 125.86 mAHD upstream of the weir. Using the Australian Dams Alliance (1999) stage-discharge curve for the weir, a flow of approximately 390,000 ML/d (adopted 1% AEP flow) would produce a water level immediately upstream of approximately 126.3 mAHD. The flood model indicates a slightly lower level of 126.1 mAHD (due to the difference in rating curves discussed above). In either case, the existing 1% AEP level is likely too low for this flow rate. In addition to this, the 2D TUFLOW hydraulic model indicates that water levels in the lake may be approximately 0.6 m higher than immediately upstream of the weir (as described above). The 1% AEP Murray River flood level estimated in this study (approximately 126.74 mAHD) is almost 0.9 m higher than the previous 1% AEP flood level estimate (125.86 mAHD). A summary of previous design flood levels for Mulwala is provided in Table 76

Table 76: Previous Lake Mulwala Design Flood Levels (m)

Yarrawonga Weir

Event

Design Flood Level (Australian Dams Alliance 1999)

Gutteridge Haskins Davey1 1986

2 2009

1. This may be from estimates of the headwater level of the weir passing 390,000 ML/d.

2. Stated to be based on Gutteridge Haskins Davey (1986), with the 0.5% and 0.2% AEP events extrapolated from the more frequent event flows (and presumably converted to levels) based upon flood frequency curves in a 2008 draft of the Water Technology (2011) report.

A summary of breakout flows through Mulwala is contained in Table 77. In the PMF event, there are substantial breakout flows to the north of Lake Mulwala (east of Mulwala) and also south through Yarrawonga. These breakout flows are not captured in Table 77

Table 77: Breakout flows through Lake Mulwala Event

1. Located downstream of the Murray River and Ovens River confluence

2. Additional breakouts occur from Lake Mulwala in the PMF event not captured here. Only 680,000 ML/d makes it downstream of the weir to the Yarrawonga gauge.

10.5. Hydraulic Hazard Categorisation

Hydraulic hazard is a measure of the potential risk to life and property damage from flooding Hydraulic hazard is typically determined by considering the depth and velocity of floodwaters. In recent years, there have been several developments in the classification of flood hazard. Research has been undertaken to assess the hazard to people, vehicles and buildings based on flood depth, velocity and velocity depth product. ARR 2019 (Ball et al 2019) contains updated recommendations regarding the categorisation of flood hazard. A summary of this categorisation is provided in Diagram 29. This categorisation is based on an extensive literature review and laboratory testing. It considers hazard to people, vehicles and buildings to develop 6 categories of flood hazard based on flood depth, velocity and depth-velocity product.

Corowa, Howlong and Mulwala Flood Study

Diagram 29: General flood hazard vulnerability curves (Source: Ball et al 2019)

The following 6 classes of hazard are defined:

• H1 – Generally safe for vehicles, people and buildings;

• H2 – Unsafe for small vehicles;

• H3 – Unsafe for vehicles, children and the elderly;

• H4 – Unsafe for vehicles and people;

• H5 – Unsafe for vehicles and people. All building types vulnerable to structural damage. Some less robust building types vulnerable to failure; and

• H6 – Unsafe for vehicles and people. All building types considered vulnerable to failure.

Murray River

The hydraulic hazard categorisations using the ARR 2019 classification for the Murray River are mapped in Figure F17, Figure F18 and Figure F19 in Appendix F, for the 5% AEP, 1% AEP and PMF events, respectively. The majority of the Murray River floodplain is H5 and H6 in the 5% and 1% AEP events. In the 1% AEP event, the breakout through Mulwala is up to H3 hazard. In the PMF event, the floodplain is primarily H6, with breakouts through Corowa being H3 and H4, and breakouts through Mulwala being H5.

Howlong Local Overland

Corowa, Howlong and Mulwala Flood Study

The hydraulic hazard categorisations using the ARR 2019 classification for Howlong, including Majors Creek, are mapped in Figure G17, Figure G18 and Figure G19 in Appendix G, for the 5% AEP, 1% AEP and PMF events, respectively. The hazard remains H1 for much of the shallow overland flows surrounding Howlong in the 5% AEP and 1% AEP events. On the Majors Creek floodplain the hazard reaches H3 in the flood storage areas and H5 and H6 in conveyance areas. Through the town, the hazard is typically H3 on the Victoria Street flow path and the Jude Street remnant channel. In the PMF event a large portion of the Majors Creek floodplain is H5 hazard and flood affected areas of the Howlong town are typically H3 hazard with H4 and H5 on the Victoria Street watercourse and Jude Street remnant channel.

Corowa Local Overland

The hydraulic hazard categorisations using the ARR 2019 classification for Corowa are mapped in Figure H17, Figure H18 and Figure H19 in Appendix H, for the 5% AEP, 1% AEP and PMF events, respectively. The hazard remains H1 for much of the shallow overland flows through Corowa in the 5% AEP and 1% AEP events. The hazard is H2 and H3 in key areas of flooding such as Header World Avenue, Sophia Close and in two areas in south Corowa. In the PMF event the hazard is typically H3 in flood affected areas of the town and up to H4 hazard in those key areas where deeper ponding is modelled.

Mulwala Local Overland

The hydraulic hazard categorisations using the ARR 2019 classification for Mulwala are mapped in Figure I17, Figure I18 and Figure I19 in Appendix I, for the 5% AEP, 1% AEP and PMF events, respectively. The hazard remains H1 for much of the town inundated by shallow depths in the 5% AEP and 1% AEP events. The hazard is H2 and H3 along roads in Mulwala where deeper flooding is modelled, typically with H2 on the surrounding lots. In the PMF event, a large portion of the town is exposed to H3 hazard, with H4 and H5 hazard reached in some locations where deeper ponding is modelled.

10.6. Hydraulic Categorisation

Hydraulic categorisation involves mapping the floodplain to indicate which areas are most important for the conveyance of floodwaters and the temporary storage of floodwaters. This can help in planning decisions about which parts of the floodplain are suitable for development, and which areas need to be left as-is to ensure that flooding impacts are not worsened compared to existing conditions.

The Flood Risk Management Manual (NSW Department of Planning and Environment 2023) defines three hydraulic categories which can be applied to different areas of the floodplain depending on the flood function:

• Floodways;

• Flood Storage; and

• Flood Fringe

Howlong and Mulwala Flood Study

Floodways are generally areas which convey a significant portion of water during floods and are particularly sensitive to changes that impact flow conveyance. They often align with naturally defined channels. Flood storage areas are located outside of floodways and generally store a significant proportion of the volume of water. Flood behaviour in these areas is sensitive to changes that impact on the storage of water during a flood. Flood fringe areas are within the extent of flooding for a particular event but are outside floodway and flood storage areas. The flood fringe is not sensitive to changes in either flow conveyance or storage.

There is no quantitative definition of these three categories or accepted approach to differentiate between the various classifications. The delineation of these areas is somewhat subjective based on knowledge of an area and flood behaviour, hydraulic modelling and previous experience in categorising flood function. A number of approaches, such as that of Howells et al (2003), rely on combinations of velocity and depth criteria to define the floodway.

For this study, hydraulic categories were defined by the following criteria and is considered to be a reasonable representation of the flood function of this catchment:

• Floodway is defined as areas where:

o the peak value of velocity multiplied by depth (V x D) > 0.2 m2/s, AND peak velocity > 0.2 m/s, OR

o peak velocity > 1.0 m/s AND peak depth > 0 m;

The remainder of the floodplain is either Flood Storage or Flood Fringe;

• Flood Storage comprises areas outside the floodway where peak depth > 0.5 m; and

• Flood Fringe comprises areas outside the Floodway where peak depth < 0.5 m.

The adopted parameters are consistent with those derived for Boree Creek, another town with the Federation Council LGA (WMAwater 2022). In the local overland models for Howlong, Corowa, and Mulwala, the above criteria did not produce a continuous floodway in some defined channels and creeks. The flood function based on the indicator method above were used to inform where defined significant flow paths were located. These flow paths were digitised and categorised as floodways, such as the Jude Street channel in Howlong, the Airport channels in Corowa and the Bayly Street channels in Mulwala.

In the riverine model for the Murray River, the floodway from the indicator method above was generalised and tested in the hydraulic model with an encroachment analysis. The encroachment analysis showed that the parameters used in the indicator method were appropriate, however minor adjustments to the floodway was made near Mulwala in order to achieve a maximum increase in peak flood level of approximately 0.1 m This encroachment analysis was undertaken for the 1% AEP event.

The hydraulic categories defined in this study are considered to be ‘preliminary’ and subject to review and refinement in a subsequent Floodplain Risk Management Study.

Murray River

Figure F20 presents the preliminary hydraulic categorisation of the Murray River in the 1% AEP

Corowa, Howlong and Mulwala Flood Study

riverine event. In the 1% AEP event, the Murray River inundates the entire lower floodplain and most of this area was categorised as a floodway. The majority of areas in Howlong and Corowa are beyond the 1% AEP riverine flood extent and only some properties located at lower elevations near the river are considered flood storage or floodway. In Corowa, there are some caravan parks, which are within the floodway on the lower terrace. In the 1% AEP riverine event, Mulwala Channel is categorised as a floodway with the rest of the town being either flood fringe or flood storage. Areas north of Tocumwal Road, along North Street, Barooga Road, Erne Street and Payne St are categorised as flood storage. Other areas are categorised as flood fringe.

Howlong

Figure G20, Figure G21 and Figure G22 presents the preliminary hydraulic categorisation in Howlong for the 5% AEP, 1% AEP and PMF local catchment rainfall events, respectively. In the 5% AEP and 1% AEP event, most areas in the Howlong are considered flood fringe. The Victoria Street watercourse flows west through properties along Clarke Street is a categorised as a floodway. The remnant channel which meanders through properties along Pell Street, Kennedy Street and Jude Street is mostly categorised as flood storage in the 1% AEP and floodway in the PMF event. Along this flow path, a channel which flows from Jude Street to Kennedy Street is categorised as a floodway. Outside of the main township, the main channels of Majors Creek south of Howlong-Burrumbuttock Road are mostly categorised as floodway in the 5% AEP and 1% AEP event until a flood storage area is reached at Jude Road (east of Kywong-Howlong Road). In the PMF event, most watercourses and roads are categorised as floodway, as well as a significant portion of the southeastern portion of the town.

Corowa

Figure H20, Figure H21, and Figure H22 present the preliminary hydraulic categorisation in Corowa in the 5% AEP, 1% AEP and PMF local catchment rainfall events, respectively. In the 1% AEP event, most areas are considered as flood fringe, with some flood storage in Pinot Crescent, Sophia Close and Hermitage Drive. The channels which flow from Corowa Airport to the Murray River are categorised as floodway. In the PMF event, most properties remain as flood fringe and a number of roads become floodways. The area categorised as flood storage near Pinot Crescent and Sophia Close expands substantially.

Mulwala

Figure I20, Figure I21 and Figure I22 present the preliminary hydraulic categorisation in Mulwala in the 5% AEP, 1% AEP and PMF local catchment rainfall events, respectively. In the 5% AEP and 1% AEP events, most areas are categorised as flood fringe, with some flood storage in along Savernake Road. A channel flowing towards the basin north of Acacia Drive as well as two channels from Bayly Street are categorised as floodways. In the PMF event, flood storage areas cover a larger area, including developed areas of the town and a number of roads become floodways. In particular, Lucan Street, Murray Street, Erne Street south of the Mulwala Channel; and Acacia Drive, Kingfisher Drive and Grevillea Terrace north of the Mulwala Channel.

10.7. Flood Emergency Response Planning

10.7.1. Murray River

10.7.1.1.

Property Inundation

At Howlong, properties are generally flood-free up to and including the 0.2% AEP event from Murray River inundation. In the PMF event, however, the southeastern portion of the town is inundated, affecting a substantial number of properties with depths up to 1 m.

At Corowa, the lower terrace area is affected by Murray River events as frequent as the 20% AEP. Properties at risk on the lower terrace include the Ball Park Caravan Park, Corowa Caravan Park, Corowa and Wahgunyah Rowing Clubs, Corowa Rutherglan Football Club, Corowa Aquatic Centre, Corowa Lawn Tennis Club, Corowa Civic Bowls Club and the Corowa Shire Civic Building, in addition to other open air sporting and playground facilities. The only other area affected by events up to the 0.2% AEP are buildings located at a low elevation to the east of River Street in south Corowa. In the PMF event, a significant number of properties are affected throughout Corowa and south Corowa.

At Mulwala, in the 2% AEP Murray River event, only half a dozen properties are affected. In the 1% AEP event, a significant portion of the town is inundated with depths in the range of 0.5 m to 1 m. This worsens in the 0.5% and 0.2% AEP events. In the PMF event, most properties in Mulwala would be inundated, with the exception of some properties located at higher elevations to the south of Bayly Street.

Further information regarding property inundation is provided in Section 12 where the flood damages assessment is discussed.

10.7.1.2. Road Inundation

Road inundation was assessed for major crossings of the Murray River. These crossings are shown in the design flood level profile plot in Figure K1. The majority of bridges have flood immunity up to and including the 0.2% AEP, and are only overtopped in the PMF event. This includes River Road, Federation Way, the John Foord Bridge and the railway crossing of Lake Mulwala. In the PMF event, however, the depth of inundation exceeds 1 m at these crossings.

The road with the lowest level of flood immunity is Bridge Road, the approach road to the John Foord Bridge at Corowa. This low-level road is inundated in the 10% AEP event (Corowa gauge level of approximately 8.0 m), cutting off access across the Murray River even though the bridge remains flood-free. The low-level crossing of Melbourne Street over Lake Mulwala also has a lower level of immunity, although is only just estimated to be inundated in the 1% AEP event, although likely still trafficable. Access would be cut off, however, in the 0.5% AEP event and greater. A summary of peak flood depths at these road crossings is provided in Table 75

Table 78: Design Flood Depths for Murray River Crossings

Event River Road

Note: NF = Not Flooded

10.7.1.3.

Corowa, Howlong and Mulwala Flood Study

Federation Way John Foord Bridge Bridge Road Melbourne Street Railway

Flood Emergency Response Classification

The Flood Risk Management Manual (NSW Department of Planning and Environment 2023) requires flood studies to address the management of continuing flood risk to both existing and future development areas. As continuing flood risk varies across the floodplain, so does the type and scale of the emergency response problem and therefore the information necessary for effective Emergency Response Planning (ERP). Classification provides an indication of the vulnerability of the community in flood emergency response and identifies the type and scale of information needed by the NSW SES to assist in ERP.

The Flood Emergency Response Classification (FERC) for the study area was undertaken in accordance with the Australian Disaster Resilience Handbook 7 Managing the Floodplain: A guide to best practice flood risk management in Australia (Commonwealth of Australia 2017). FERC classifications consider flood affected communities as those in which the normal functioning of services is altered, either directly or indirectly, and results in the need for external assistance. This impact relates directly to the operational issues of evacuation, resupply and rescue, which is coordinated by the SES.

The ERP classification for the three towns were defined using the Murray River PMF flood event and can be seen in Figure F21. The classification has been undertaken on a precinct basis rather than lot-by-lot and is targeted at highlighting those areas which may require evacuation or assistance during a flood event. However, these classifications may vary depending on local flood characteristics and resultant flood behaviour. These categories are described in Diagram 30 below.

Diagram 30: Flow Chart for Determining Flood Emergency Response Classifications (Commonwealth of Australia 2017)

At Howlong, the southeastern portion of the town is classified as flood affected, isolated and submerged (FIS) due to inundation from the Murray River. There is an area of land south of the Riverina Highway, between Ashford Road and Majors Creek that is classified as flood affected, isolated and elevated (FIE). The western and northern portion of the town (approximately west of Sturt Street and north of Hawkins Street) remains flood free, with no classification although these areas are likely to experience indirect consequences (IC) due to service disruptions. Most notably, the Riverina Highway will be cut off to the east.

At Corowa, the town is largely surrounded by floodwaters in the PMF event. The area affected is classified as flood affected, isolated and submerged (FIS). The main town areas that are elevated are classified as flood affected with an exit route via roads (FER).This route consists of Cemetery Road, Nixon Street, Enfield Street and Spring Drive. It is noted that floodwaters are modelled to just overtop Spring Drive in the PMF due to Murray breakout flows returning to the river, however, the Murray River model does not have fine scale drainage features such as the airport channels and culverts under Spring Drive, such that it is likely that this road would still be trafficable. Access to Fergusons Road, Stuckeys Road and Freyers Road would be maintained via Spring Drive, although the extent of the Murray breakout in this rural area to the west of Corowa (and hence trafficability of these roads) was not assessed.

At Mulwala, the majority of the town is inundated in the Murray River PMF event, and as such is classified as flood affected, isolated and submerged (FIS). There are some elevated areas of Mulwala located to the southwest of the main town that are classified as flood affected, isolated and elevated (FIE), including Club Mulwala and The Yarrawonga Mulwala Golf Club Resort.

Mulwala

10.7.1.4. Gauges

Corowa, Howlong and Mulwala Flood Study

The water level gauges located at each of the towns provides real time data on the water levels in the Murray River. However, these levels should be correlated with inundation of certain roads, facilities or areas to assist with understanding the impact of Murray River inundation, the need for evacuation and planning of evacuation routes. The following describes key levels at the gauges for each town.

There is also substantial warning time available due to the travel time of the flood from Albury to the towns. Relevant timing was provided in Table 2 and further discussion was provided in Section 7.13. A summary of travel times based on actual flood events is provided in Table 79. It was discussed that the travel time has increased in recent years.

Table 79: Flood Peak Travel Times from Hume Dam

1. Based on the 1974, 1975 and 1992 flood events

2. Based on the 2016 and 2022 flood events

3. Based on the Yarrawonga gauge. Travel time is harder to distinguish due to gate operations.

Howlong

At Howlong, the water level gauge is located on the western side of the town. The lowest point is where the local creek discharges into the Murray River near Lowe Street. Lowe Street begins to be affected by backwater when the gauge level reaches approximately 8.9 m. Properties in the vicinity of Lowe Street begin to be affected above floor at a gauge level of approximately 10 m.

Corowa

At Corowa, the water level gauge is located just upstream of the John Foord bridge. This provides a good indication of water levels that could potentially inundate the lower terrace of Corowa. When the gauge level reaches approximately 6.4 m, the lower terrace begins to be affected, with water flowing through the area by the time the gauge reaches approximately 7.0 m. Water begins to overtop Bridge Road when the gauge reaches approximately 8.0 m. The upper terrace only begins to be affected once the gauge level reaches approximately 11.0 m.

Mulwala

At Mulwala, the water level gauge for Lake Mulwala is located near the Yarrawonga Main Channel. This gauge provides an accurate level for the lake in frequent flood events, however, in large flood events, there can be a gradient between the open lake level and the gauge level. The lowest point of Mulwala, at Erne Street, begins to be affected when the lake level reaches approximately 125.3 mAHD, in which case the gauge may read approximately 125.05 mAHD. Properties may begin to be affected above floor level when the lake level reaches approximately 126.0 mAHD,

Corowa, Howlong and Mulwala Flood Study

and the gauge may read approximately 125.8 mAHD. A large portion of the town may be inundated by the time the lake level reaches 126.4 mAHD, or a gauge level of approximately 126.1 mAHD. The Melbourne Street Bridge is estimated to be overtopped when the gauge reaches approximately 126.4 mAHD.

10.7.2. Local Overland Flows

10.7.2.1.

Property Inundation

Howlong

At Howlong, local overland flooding is modelled to affect properties in the vicinity of Hume Street (near the Howlong Recreation Reserve) and Victoria Street, as well as some properties along the Victoria Street flow path through the south of the town in events as frequent as the 20% AEP. In the 1% AEP event, a large number of properties are affected by flooding in these areas, with shallow overland flows covering a substantial area. The Jude Street remnant creek through the north of the town also affects a number of properties. In the PMF event, it is only the highest points of the town that remain flood free, including Sarah Circuit, an area around Lowe Street, Hoddle Street and Larmer Street, and the southwestern corner of the town.

Corowa

At Corowa, local overland flooding is modelled to affect properties between Wanstead Street and Gallipoli Street, and in the vicinity of the Gallipoli Street and Federation Avenue intersection (although very shallow) in events as frequent as the 20% AEP. Properties around Martin Street also have shallow inundation in the 20% AEP event. Deeper flooding in a low point crossing Adams Street, Church Street and Hume Street also affects properties. In the 1% AEP event, flooding in these areas affects a larger number of properties. Flood affectation is most notable around Header World Avenue, Sophia Close. In the PMF event, the Sophia Close and Pinot Crescent low point expands to affect a significant number of properties and a number of areas through south Corowa are also affected.

Mulwala

At Mulwala, local overland flooding is modelled to affect properties in events as frequent as the 20% AEP, including properties around Damian Crescent, Hicks Street, Payne Street, Lucan Street and Manners Street, although the inundation above ground is typically shallow. In the 1% AEP event, flooding at these low points becomes more extensive and a number of properties are affected. In the PMF event, it is primarily only properties that front Lake Mulwala and within the large-lot residential area to the southwest of the town that are not affected.

Further information regarding property inundation is provided in Section 12 where the flood damages assessment is discussed.

10.7.2.2.

Road Inundation

Corowa, Howlong and Mulwala Flood Study

There are numerous local roads throughout the towns that are subject to inundation in a local overland flood event. The inundation is typically shallow as overland flows are conveyed along road corridors or where water ponds at low points. Peak flood levels, depths and flows at key locations are provided in Table J1, Table J2 and Table J3 in Appendix J, respectively. Levels, depths and flows were provided where appropriate for the location. For example, flows are not reported at trapped low points and flood depths are not provided where shallow flows cross a road at a number of locations. Water level time series are provided at some of these key locations, which indicate the flood immunity of the road, the rate of rise and duration of inundation. Due to the flood storage nature of many areas of the towns and the storm durations simulated, a full time series (for the derivation of duration of inundation) was not possible. The locations of these road crossings where results are provided are shown in Figure 61, Figure 62 and Figure 63 for Howlong, Corowa and Mulwala, respectively.

10.7.2.3. Flood Emergency Response Classification

Howlong

The FERC (see Section 10.7.1.3 for description) for Howlong is shown in Figure G23 and was undertaken with consideration of the PMF event. A large portion of the town is classified as flood affected, isolated and submerged (FIS). There are two areas that remain flood affected and isolated, but elevated (FIE). These areas are around Sarah Circuit and Gilson Place, and the around Lowe Street. The southwest portion of the town is classified as rising road access, since River Road is accessible via Clarke Street West, which provides access over the Murray River. While this road is likely to be accessible in a local overland PMF event, it is unknown the extent of inundation of roads on the southern side (Victoria) that would occur and whether these roads would be trafficable.

Corowa

The FERC (see Section 10.7.1.3 for description) for Corowa is shown in Figure H23 and was undertaken with consideration of the PMF event. The town is surrounded by floodwater, with these areas classified as flood affected, isolated and submerged (FIS). This includes areas around Header World Avenue, The Whitehead Street Wetlands, Sophia Close, Edward Street, the Corowa Airport and parts of south Corowa. In between these areas are flood affected areas that are isolated and elevated (FIE). This is primarily due to the inundation of major roads into and out of the town, including Honour Avenue and Spring Drive. There is one area at Federation Way that is classified as flood affected with an exit route via roads (FER). This is because access to Federation Way is possible such that Wahgunyah may be accessible, although the extent of inundation south of the Murray River in a local PMF event (and hence trafficability) is unknown.

Mulwala

The FERC (see Section 10.7.1.3 for description) for Mulwala is shown in Figure I23 and was undertaken with consideration of the PMF event. A large portion of the residential lots within

Mulwala are flood affected, isolated and submerged (FIS). There are some elevated areas, primarily along the Lake Mulwala waterfront that are flood affected, isolated and elevated (FIE). Adi Avenue and some of the residential properties located to the southwest of the main town are also isolated and elevated (FIE). A portion of land, including Club Mulwala and the Yarrawonga Mulwala Golf Club Resort are flood affected with an exit route via roads (FER), as Weir Road and Melbourne Street are accessible, providing a route to Yarrawonga, although the extent of inundation in Yarrawonga in a local PMF event (and hence trafficability) was not modelled in this study. There are also two areas classified as flood affected with an exit route via an overland path (FEO) that would be accessible on foot, if required. This includes one area to the south of Mulwala that covers the golf course, where traversing the golf course on foot to the FER area is possible, and one area covering parts of the Department of Defence land that is included in the flood model.

10.8. Flood Planning Area

10.8.1. Background

Land use planning is an effective means of minimising flood risk and damages from flooding. Land use planning for flooding can be achieved through the use of:

• A Flood Planning Area (FPA), which identifies land that is subject to flood related development controls; and

• A Flood Planning Level (FPL), which identifies the minimum floor level applied to development proposals within the FPA.

Defining FPAs and FPLs in urban areas can be complicated by the variability of flow conditions between mainstream and local overland flow. Traditional approaches developed for riverine or “mainstream” flow areas often cannot be applied in steeper urban overland flow areas. Additionally, defining the area of flood affectation due to overland flow (which by its nature includes shallow flow) involves determining at which point flow is significant enough to be classified as “flooding” rather than just a drainage or local runoff issue. In some areas of overland flow, the difference in peak flood level between events of varying magnitude can be so minor that applying the typical freeboard can result in an FPL greater than the PMF level.

The FPA should include properties where development would result in impacts on flood behaviour in the surrounding area and in areas of high hazard where there is a risk to safety or life. The FPL is determined in addition to this with the purpose of decreasing the likelihood of damage such as over-floor flooding of buildings.

The Flood Risk Management Manual (NSW Department of Planning and Environment 2023) identifies that the FPL is generally be based on the 1% AEP event plus an appropriate freeboard (typically 0.5 m). However, it also recognises that different freeboards may be deemed appropriate due to local conditions provided adequate justification is provided.

Further consideration of flood planning areas and levels is typically undertaken as part of the Floodplain Risk Management Study to determine what should be included in the Floodplain Risk Management Plan and as such, the FPA derived as part of this study is considered to be preliminary.

10.8.2.

Methodology

The methodology used for defining the flood planning divides the flood area between “mainstream” and “overland” flooding areas as follows:

• Mainstream flooding: For the purpose of this study, it is assumed that mainstream flooding is due to the Murray River only. While there are a number of flow paths and creeks within the towns that drain reasonably large catchments (and may be traditionally considered “mainstream” flooding) these are not included for the mainstream FPA. Further discussion is provided below. For the Murray River, the topography of the floodplain allows for the traditional approach of adding freeboard to the flood surface and the extent then “stretched” to intersect with the land. The FPA for the Murray River was defined as the 1% AEP peak flood level plus 0.5 m freeboard, with the level extended perpendicular to the flow direction.

• Overland flooding: Overland flooding for the purpose of the FPA is defined as those areas modelled in the local overland model. It is noted that these areas include some significant creeks (such as Majors Creek), however, these areas were not treated the same way as the “mainstream” FPA described above. The addition of freeboard and stretching in these areas generally produces an over-estimate of the land subject to flood risk. This is because the stretching extends across land in a way that would not actually occur even with significant additional flow from a much larger storm. It may even extend beyond the modelled PMF extent. It is therefore appropriate to not apply freeboard for the purpose of defining the FPA for overland flooding. As an example, some key areas of ponding within the towns and the associated flood depths are shown in Table 80. This table indicates that the flood risk does not scale with increasing rarity of the event. If 0.5 m freeboard were added to these areas (or even 0.3 m freeboard in some cases), the resulting flood level would be greater than the PMF level. This is also exacerbated by the relatively flat nature of the towns. For example, much of the Mulwala township is within 126 mAHD and 126.5 mAHD. The addition of freeboard on any flood surface encompasses a significant portion of the town As such, the 0.2% AEP event was adopted as a proxy for the 1% AEP event with a ‘freeboard’ added to account for uncertainties. Utilising an actual event results in a more reasonable flood extent than ‘stretching’ a hypothetical flood surface. The following filtering was applied to remove shallow overland flows not associated with a continuous flow path:

o Depth Filter – Exclude results below 150 mm depth; and

o Small Pond Filter – Remove isolated ‘puddles’ or ‘orphans’ smaller than 100 m2 .

Table 80: Approximate flood depths at key areas of ponding

The resultant extent of the preliminary mainstream Murray River FPA can be seen in Figure F22

Corowa, Howlong and Mulwala Flood Study

and the preliminary overland FPA for Howlong, Corowa and Mulwala is provided in Figure G24, Figure H24 and Figure I24, respectively.

10.8.3. PMF Event

In July 2021, the NSW Government implemented updates to the Flood Prone Land Package. The purpose of the package is to increase flood resilience in New South Wales, reduce loss of life and property damage. The package provides Councils additional land use planning tools to manage flood risk beyond the 1% AEP flood event and strengthen evacuation consideration in land use planning. The updates included amendments to Schedule 4 of Environmental Planning and Assessment Regulation including changes to Clause 7A(1), Clause 7A(2). These amendments (now contained in Clause 9 of Schedule 2) require Councils to note on Section 10.7 certificates if the land is within the FPA (Clause 9(1)) or between the FPA and PMF (Clause 9(2)) and subject to flood related development controls.

For the purpose of identification of those properties that are outside the FPA, but within the PMF and subject to flood related development controls (for development comprising hazardous materials / industry, sensitive, vulnerable or critical uses), the full Murray River PMF extent was used. For the local overland PMF, the extent was filtered in a similar manner to the derivation of the overland FPA. The following filtering was applied:

o Depth Filter – Exclude results below 150 mm depth; and

o Small Pond Filter – Remove isolated ‘puddles’ or ‘orphans’ smaller than 100 m2

The resultant preliminary extent of the Murray River PMF for flood planning controls can be seen in Figure F23, and the preliminary overland PMF extent for flood planning controls for Howlong, Corowa and Mulwala is provided in Figure G25, Figure H25 and Figure I25, respectively.

10.9. Advice on Land-Use Planning Considering Flooding

It is considered good practice (and a requirement of the Environmental Planning and Assessment Act 1979) to permit land use and development that is compatible with the nature of flooding in a particular area. For example, it is wise to limit use and development of land that is classified as floodway, since these are areas of conveyance and not only pose significant risks to humans, but any development in these areas can shift flood risks to other areas.

10.9.1. Existing Flood Planning Controls

Federation Council implements flood-related planning controls in the study area via the Corowa Local Environmental Plan (LEP) 2012 and the Corowa Shire Development Control Plan (DCP) 2013. The LEP specifies that land is subject to flood-related restrictions if it is within the flood planning area for any type of development (Clause 5.21). The LEP was prepared under the Standard Instrument LEP program and incorporates the revised flood clause introduced as part of the NSW Government’s Flood Prone Land Package that commences on 14 July 2021. The Flood Prone Land Package also included a second optional clause ‘Special Flood Consideration’ which provides Councils the mechanism to apply development controls to land outside the FPA but within the PMF. This clause is specific to land with a significant risk to life, sensitive, vulnerable

Corowa, Howlong and Mulwala Flood Study

or critical uses, or land with hazardous materials or industry. Federation Council has not adopted this clause.

The LEP outlines the overall objectives and nature of these restrictions and the DCP supports the implementation of the LEP objectives, providing specific guidance for design and assessment of proposed developments. Federation Council currently operates with the LEP and DCP from the former Corowa Shire Council and Urana Shire Council. The Corowa Shire DCP governs the study area of Howlong, Corowa, and Mulwala and has been adopted by Council since February 2013. Section 11 of the DCP specifies flood-related development controls that apply to land affected by flooding. The DCP controls and the associated flood planning area maps in the LEP that delineate land affected by flooding focus on mainstream Murray River flooding only.

It is understood that Council is currently preparing a comprehensive LEP and DCP for the Federation Council LGA. A high-level review of the existing Corowa Shire DCP has been undertaken as part of this study and the outcomes are as follows:

• The DCP adopts a matrix approach, with controls and performance criteria varying based on the type of development (such as residential, commercial and recreational uses) and the flood risk precinct (high, medium or low). This approach, which aligns controls with the level of hazard and inherent vulnerabilities of different development types, is considered consistent with current best practice.

• The current DCP does not describe a flood planning level or flood planning area, however planning controls are based on flood risk precincts within the PMF extent. The flood planning area is specified in the LEP.

• The FPA is typically the land at or below the flood planning level, where the flood planning level is typically the 1% AEP event plus a freeboard. The current DCP specifies that the freeboard is 500 mm. As each township is relatively flat, a 500 mm freeboard may not be appropriate and may result in levels above the PMF. The Federation Villages Floodplain Risk Management Study and Plan (WMAwater 2022) recommended a 300 mm freeboard instead of a 500 mm freeboard.

• DCP specifies controls based on a Dam Crest Flood (DCF) and riverine events (from previous modelling). DCP should be updated accordingly considering overland flow and the modelling and results produced as part of this study. The current DCP controls are not all readily applicable to overland flooding.

The flood-related development controls specified in the DCP for the Murray River cover minimum floor levels, building components (flood compatible materials), structural soundness (to ensure buildings can withstand flood forces), flood impacts (not making flooding worse for neighbouring properties) and consideration of safety for people, vehicles and the environment These controls are considered suitable for mainstream flooding, however, would require revision to be fully applicable to overland flow events. DCP updates considering overland flooding are outlined in Table 81 below, in addition to several other general items.

Table 81: Flood-related Development Control Considerations

Aspect/Control Contained in LEP/DCP

Terminology

Partial

Flood Planning Level No

Flood Planning Area Yes

Consideration of flood affectation and land use

Yes

Minimum Floor Level Partial

Flood Proofing Yes

Flood Impacts Partial

Evacuation Partial

Comment

Update reference to outdated documents, including the 2023 Floodplain Risk Management Manual and ARR 2019.

DCF acronym is used, but not explicitly defined. It also does not refer to a particular report/model/dam.

Update flood risk precinct definitions to match the ARR 2019 hazard categories.

Update definition for caravan parks according to Local Government (Manufactured Home Estates, Caravan Parks, Camping Grounds and Moveable Dwellings) Regulation 2021.

LEP has since been updated to exclude definition of the FPL. DCP should be updated to include definition of the FPL.

Ensure map is available on Council’s website if separate from the DCP, since changes to the NSW Government planning framework in relation to flooding has removed the FPA overlay from the LEP. Flood planning area to be updated based on results from this flood study.

A matrix approach is used considering the flood hazard (low to high) of the site and land use to apply flood-related development controls.

A matrix approach used to specify minimum floor levels per development type and flood risk precinct. The freeboard (where applicable), however, is set at 500 mm. A variable freeboard is recommended for overland flow flooding, and may depend on the flood risk precinct and development type. Sensitive uses in the low flood risk precinct utilise the DCF as the minimum floor level. This should be updated to the PMF for overland flow flooding.

Consideration of flood compatible building materials, electrical components, structural soundness and storage of hazardous materials are included in the DCP.

The DCP requires all flood affected developments to submit a flood impact statement which includes flood effects. DCP stipulates that the proposed development does not result in increased flooding elsewhere in the floodplain and outlines requirements of a flood impact assessment. Two assessment methods are provided. The cross-section analysis method (method 1) is not considered suitable for overland flow flooding and the development affected by overland flows should be assessed using the computer models developed as part of this study (method 2). The assessment of flood requirements, including consideration of a range of floods events, the change in flood storage, the change in flood conveyance, cumulative impacts and reporting requirements is considered appropriate.

A matrix approach used to specify evacuation requirements per development type and flood risk precinct.

Aspect/Control Contained in LEP/DCP

Fencing No

Carparking No

Special Flood Considerations No

Future Climate No

Comment

In high flood risk precincts, residential and commercial properties are required to have reliable pedestrian and vehicle access during a 1% AEP event flood and to a location above the DCF. For overland flow flooding, this should refer to the PMF event.

Evacuation requirements in the DCP should be updated to require consideration of overland flow flooding in which sufficient warning time may not be available, including where shelter in place may be appropriate.

Fencing and landform requirements are typically prescribed to prevent boundary and internal fences from obstructing natural path of overland flow. There are no prescriptive controls for fencing.

The DCP does not specify controls for carparking areas, including basement carparks.

The LEP currently does not include the Special Flood Considerations clause. Changes to the NSW Government planning framework in relation to flooding allows Council the opportunity to include a second clause within their LEPs which applies to land between the FPA and the PMF extent and considers sensitive and hazardous uses in addition to those uses which may have evacuation constraints. This inclusion empowers Council to apply controls that ensure the developers of such facilities appropriately consider and plan for the full range of flood risk at the site, so as to reduce potential property damages and minimise the risk to life in future flood events. There are controls in the DCP to this effect (for the low flood risk precinct), which should be applied by opting in for the Special Flood Considerations clause in the LEP. This would also require a map of the area to which this clause applies to be available in Council’s DCP.

The DCP does not consider climate change. The DCP should be updated to incorporate climate change in two ways. Firstly, the climate change should be considered as part of flood impact assessment, where climate change impacts should be modelled to manage risk of future climate change. Secondly, development controls should be integrated with consideration of climate change. It is recommended that Council includes climate change in flood-related development controls considering best available climate change data to combat future increased rainfall intensity.

The use of flood risk precincts is common across NSW. This approach to categorising the flooding provides some breakdown of flood risk and constraints, however, does not clearly link the precincts to all flood related development constraints described in the flood planning constraint category (FPCC) approach, which is outlined below. The flood risk precinct approach does not

Howlong and Mulwala Flood Study

explicitly consider some of the emergency planning constraints such as evacuation, flood warning, rate of rise and duration. The shorter response time of local overland flow events means that there may be little warning time and rapid rises in floodwaters (as experienced in the Corowa 2022 event). The duration of inundation may be short-lived, however, could also be prolonged in flood storage areas with little drainage capacity. While the flood risk precinct approach is considered appropriate, the FPCC approach provides further detail and considers a range of flood related development constraints.

10.9.2. Flood Planning Constraint Categories

Guideline 7-5 of the Australian Disaster Resilience Handbook Collection (Australian Institute of Disaster Resilience 2017) recommends using FPCCs to better inform land use planning activities. These categories condense the wealth of flood information produced in a flood study and classify the floodplain into areas with similar degrees of constraint. These FPCCs can be used in high level assessments of land use planning to inform and support decisions. For detailed land use planning activities, it is recommended that the flood behaviour across the range of flood events be considered, depending on the level of constraint.

The Australian Disaster Resilience Handbook Collection (Australian Institute of Disaster Resilience 2017) recommends the use of four constraint categories. It is recommended that isolation potential also be considered for the high constraint category. This could include areas classified as ‘isolated’ (see Sections 10.7.1.3 and 10.7.2.3 for details). Isolation has not been considered in the preliminary FPCCs defined for the study area. In land use planning for greenfield areas, it is assumed that any development would be accompanied by new roads and access routes which may change the isolation potential of the land. In areas that are already developed, the isolation potential has been defined using Flood Emergency Response Classifications (see Sections 10.7.1.3 and 10.7.2.3), and land use planning activities should consider these in addition to the preliminary FPCCs.

The constraints adopted are outlined in Table 82. The associated FPCC map for the Murray River is provided in Figure F24 and local overland flooding FPCC maps are provided in Figure G26, Figure H26 and Figure I26 for Howlong, Corowa and Mulwala, respectively.

Table 82: Flood Planning Constraint Categories

FPCC Constraints Implications

Floodway and flood storage areas in the 1% AEP event

FPCC 1

H6 hazard in the 1% AEP event

Floodway in the 0.2% AEP event

H5 flood hazard in the 1% AEP event

FPCC 2

H6 flood hazard in the 0.2% AEP event

FPCC 3 Within the FPA

Any development is likely to affect flood behaviour in the 1% AEP event and cause impacts elsewhere.

Hazardous conditions considered unsafe for vehicles and people, all types of buildings considered vulnerable to structural failure.

People and buildings in these areas may be affected by dangerous floodwaters in rarer events.

Hazardous conditions considered unsafe for vehicles and people, and all buildings vulnerable to structural damage.

Hazardous conditions develop in rare events which may have implications for the development and its occupants.

Considerations

Majority of developments and uses have adverse impacts on flood behaviour or are vulnerable. Consider limiting uses and developments to those that are compatible with flood function and hazard.

Many uses and developments will be vulnerable. Consider limiting new uses to those compatible with flood function and hazard (including rarer flood flows) or consider treatments to reduce the hazard (such as filling). Consider the need for additional development control conditions to reduce the effect of flooding on the development and its occupants.

FPCC 4

Within the PMF extent

Hazardous conditions may exist creating issues for vehicles and people. Structural damage to buildings is unlikely.

Standard land use and development controls aimed at reducing damage and the exposure of the development to flooding are likely to be suitable. Consider additional conditions for emergency response facilities, key community infrastructure and land uses with vulnerable users.

Emergency response may rely on key community facilities such as emergency hospitals, emergency management headquarters and evacuation centres operating during an event. Recovery may rely on key utility services being able to be readily re-established after an event.

Consider the need for conditions for emergency response facilities, key community infrastructure and land uses with vulnerable users.

11. SENSITIVITY ANALYSIS

11.1. Overview

Corowa, Howlong and Mulwala Flood Study

A number of sensitivity analyses were undertaken to establish the variation in design flood levels and flows that may occur if different parameter assumptions were made. These sensitivity scenarios are summarised in Table 83

Table 83: Overview of Sensitivity Analyses

Scenario

Catchment Lag (C) - 20% + 20%

Mannings ‘n’ roughness - 20% + 20%

Blockage No Blockage High Blockage

Downstream boundary condition (curve slope) x 0.5 x 2

Climate Change (for the 1% AEP event) 0.5% AEP 0.2% AEP

Murray River Historic Conditions Various

The change in flood level across the study area for each scenario compared to the adopted design 1% AEP flood events are provided in Appendix L.

11.2. Rainfall Losses

Rainfall losses were adopted from the ARR 2019 data hub (see Section 9.2.3). Initial losses were taken from the data hub’s probability neutral burst initial losses, which vary based on the AEP and duration of the storm. The continuing loss adopted was 0.96 mm/h to 1.8 mm/h across the towns, based on the factored data hub loss values. A sensitivity analysis was undertaken for both initial loss and continuing loss. The initial loss was altered to simulate both no initial loss (representing a very wet catchment at the start of the storm) and double the initial loss (representing a very dry catchment at the start of the storm). The continuing loss was altered to simulate an increase and decrease in continuing loss by 50%.

The change in peak flood levels for the 1% AEP event with no initial losses simulated are presented in Figure L1 to Figure L3 for the overland flow event for Howlong, Corowa and Mulwala, respectively. The reduction in initial loss has minimal impact (<20 mm flood level difference) to most areas of Corowa and Mulwala. At Howlong, peak flood levels increase by approximately 100 mm along the watercourses along Kennedy Street and Victoria Street as well as through Majors Creek.

The change in peak flood levels for the 1% AEP event with doubled initial losses simulated are presented in Figure L4 to Figure L6 for the overland flow event for Howlong, Corowa and Mulwala, respectively. The increase in initial loss has minimal impact (<20 mm flood level difference) to most areas of Corowa and Mulwala, with only some flood levels decreasing by less than 100 mm in downstream areas. At Howlong, peak flood levels decrease by up to 100 mm in the watercourse

Corowa, Howlong and Mulwala Flood Study

along Kennedy Street. In the watercourse along Victoria Street, peak flood levels decrease by up to 300 mm. Majors Creek and its floodplain has peak flood levels typically decrease by up to 100 mm.

The change in peak flood levels for the 1% AEP event with continuing losses decreased by 50% are presented in Figure L7 to Figure L9 for the overland flow event for Howlong, Corowa and Mulwala, respectively. Reduction in continuing loss has minimal impact on most overland flow areas in the townships. Flood levels increase up to 100 mm in some isolated areas in the townships mainly in flood storage areas where continuing losses contribute to the total volume of runoff generated.

The change in peak flood levels for the 1% AEP event with continuing losses increased by 50% are presented in Figure L10 to Figure L12 for the overland flow event for Howlong, Corowa and Mulwala, respectively. Increase in continuing loss has minimal impact on most overland flow areas in the townships. Flood levels decrease up to 100 mm in some isolated areas in the townships mainly in flood storage areas where continuing losses contribute to the total volume of runoff generated.

11.3. Catchment Lag

Catchment lag factor (termed ‘C’ in the WBNM model) delays and attenuates runoff response to rainfall. The ‘C’ lag factor of 1.6 was adopted as it is the recommended default value for an ungauged catchment in NSW. The adopted lag factor was increased and decreased by approximately 20% (1.9 and 1.3, respectively) for this sensitivity analysis. The decrease in the lag factor generally slightly increases and speeds up the peak catchment flows, while an increase in the lag factor results in a slight decrease and delay in peak flows. This was undertaken for Corowa only, as the change to catchment flows was minimal and a similar result is expected for Howlong and Mulwala. The change in peak flood levels for the 1% AEP event for Corowa with the lag factor decreased and increased by 20% are presented in Figure L13 and Figure L14, respectively. Corowa is not sensitive to variation in the catchment lag factor with peak flood levels changing by less than 20 mm. There is some very minor flood extent increase as catchment lag is decreased. Conversely, there is some very minor flood extent reduction as catchment lag is increased.

11.4. Mannings ‘n’ Roughness

The adopted Mannings ‘n’ roughness coefficients for the design flood events are shown in Table 16. For sensitivity analyses, the Mannings ‘n’ roughness coefficient was increased and decreased by 20% for all land types across the study area. The changes in peak flood levels with decreasing and increasing the Mannings ‘n’ roughness values for the 1% AEP event are shown in Figure L15 to Figure L22

For the riverine event in the Murray River, the results indicate that decreasing the surface roughness results in lower peak flood levels across most of the river Flood levels decrease along the banks of the river at Howlong by 200 mm to 300 mm, at Corowa by 300 mm to 500 mm and at Mulwala by 100 mm to 200 mm. Overall, the three townships are largely insensitive to reductions in Mannings ‘n’ roughness in the overland flow events, with exception of some minor

flood level decreases (up to 100 mm) in Majors Creek.

Corowa, Howlong and Mulwala Flood Study

Conversely, in the riverine event, increasing the surface roughness results in higher peak flood levels across the Murray River floodplain. Flood levels increase along the banks of the river at Howlong by 200 mm to 300 mm, at Corowa by 300 mm to 500 mm, and at Mulwala by 100 to 200 mm. Overall, the three townships are largely insensitive to increases in Mannings ‘n’ roughness in the overland flow events, with exception of some minor flood level increases (up to 100 mm) in Majors Creek.

11.5. Structure Blockage

Sensitivity of the adopted blockage factors (see Table 70) of hydraulic structures for the design events were assessed via two scenarios for the 1% AEP event. The first assumed no blockage, whereas the second increased blockage. The blockages applied in design flood events and the sensitivity scenarios are outlined in Table 84

Table 84: Structure Blockage Sensitivity

The changes in peak flood level in the no blockage scenario are shown in Figure L23 to Figure L26. For the Murray River, it is only two areas that have changes greater than 20 mm – a backwater area in between Corowa and Mulwala, and in the vicinity of the Mulwala Canal. There are minor reductions in flood level with no blockage (< 50 mm) in these areas There are minimal impacts in most areas of the townships in the 1% AEP overland event. Flood levels reduce typically by up to 100 mm just upstream of key hydraulic structures and there are minimal flood level increases (<20 mm) on the downstream reaches.

The changes in peak flood level in the increased blockage scenario are shown in Figure L27 to Figure L30 For the Murray River, the two areas noted above have minor flood level increases (< 50 mm). In Howlong, flood levels increase by up to 100 mm in the flow path along Victoria Street (where the local watercourse through the town is) and in the Howlong Country Golf Club. In Corowa, most areas in the township have minimal impact (<20 mm), except for the residential area near Sophia Close and Martin Street (up to 70 mm) and the commercial area around Header World Avenue (up to 160 mm). In Mulwala, flooded areas west of the railway line have flood level increases of up to 30 mm while in some areas north of Savernake Road and south of Bayly Street have increases of up to 200 mm

While overall most of the study area has low sensitivity to the changes in structure blockage, there are still some locations which are more sensitive to blockage. These areas are:

• The flow path along Victoria Street in Howlong

• Residential area near Sophia Close and Martin Street in Corowa

• Commercial area at Header World Ave in Corowa

• Commercial and residential areas west of the railway line in Mulwala

11.6. Downstream Boundary Conditions

For the design flood events, a stage-discharge relationship was applied to the outlet of the overland flow that continues west in Howlong, to the north and west in Corowa, and to the north in Mulwala. Overland flow discharges to the Murray River are also simulated with stage-discharge relationships except for Lake Mulwala, where a static water level was applied. The Murray River downstream boundary is also applied as a stage-discharge relationship with an adopted slope of 1%. Other breakout flows that occur in the PMF also have stage-discharge relationships applied. The stage-discharge relationship is determined within the TUFLOW software based on the specified slope. To test the sensitivity of this stage-discharge relationship, the slope was halved increased doubled, and tested for both the Murray River and Corowa overland 1% AEP events. Corowa was adopted as a representative town with the impacts expected to be similar for Howlong and Mulwala.

The change in peak flood level due to decreasing the downstream boundary is shown in Figure L31 and Figure L32 for the Murray River and Corowa, respectively. Decreasing the slope results in less water being allowed to exit the model. The change in peak flood level for the Murray River is increased by up to 50 mm, however this is highly localised. The more sensitive area is the Mulwala Canal, where overtopping floodwater is allowed to enter the canal, and subsequently flow out of the model domain. Increased flood levels in the channel inundate adjacent areas. In the area where the breakout leaves the model overland, the change is typically less than 30 mm and does not affect flood levels in the town. In Corowa, the change in peak flood level is typically within 20 mm, with only a small area that drains north having levels increased by less than 30 mm.

The change in peak flood level due to increasing the downstream boundary slope is shown in Figure L33 and Figure L34 and for the Murray River and Corowa, respectively. Increasing the slope results in more water being allowed to exit the model. The change in peak flood level for the Murray River, however, is minor, with only localised impacts up to 30 mm at the Murray River downstream boundary. There are decreases at the boundary where the Lake Mulwala breakout is allowed to leave the model. Decreased flood levels in the channel result in less water overtopping and inundating land to the northwest of Mulwala. The decrease in flood level is up to 200 mm. In other areas where the breakout flow is allowed to exit the model overland the decrease is up to 30 mm. At Corowa, the change in peak water level is negligible, being less than 20 mm across the whole study area.

The sensitivity results indicate that the changes are typically minor and do not affect flood behaviour across the urbanised areas.

11.7. Climate Change

Climate change is expected to increase sea levels and rainfall intensities. It is typical practice in catchment flood studies under the NSW flood program to model scenarios incorporating the effects of these impacts from climate change to understand the potential future changes in flood behaviour.

Various projections of the likely increases to sea levels are available, however, receiving waters of the Murray River are not influenced by the ocean in the study area. Any increase in design flood rainfall intensities will increase the frequency, depth and extent of inundation across the catchment. The primary driver for this change is under a warmer climate, the atmosphere can hold more water, and hence more rainfall can occur in any given storm event. The design rainfall information currently provided by the BoM is based on historical climate data and does not currently include any allowance for likely increases to rainfall intensity in the future. ARR 2019 (Ball et al 2019) provides some guidance about consideration of the impacts of climate change on design rainfall intensities. It suggests assuming that rainfall intensities can be assumed to scale up by 5% per degree of average surface warming.

Projected increases to evaporation under a warmer climate are also an important consideration because increased evaporation would lead to generally drier catchment conditions, resulting in lower runoff from rainfall. Mean annual rainfall is projected to decrease, which will also result in generally dryer catchment conditions and potentially lower average dam storage levels. This may be a significant factor for the Murray River catchment.

The current NSW State Government’s advice recommends sensitivity analysis on flood modelling should be undertaken to develop an understanding of the effect of various levels of change in the hydrologic regime on the study area (NSW Department of Planning and Environment 2023). To understand potential changes to flood behaviour due to increased intensity of rainfall, the 0.5% AEP and 0.2% AEP events were compared with the 1% AEP event, as suggested in the NSW Flood Risk Management Manual (NSW Department of Planning and Environment 2023). These events provide an indication of how 1% AEP flood levels would change if the rainfall intensity increased to the point that it matches either the current 0.5% AEP (a 13-16% increase in intensity for the adopted critical durations for the towns) or 0.2% AEP (a 32-35% increase in intensity for the adopted critical durations for the towns). The change in peak flood levels, comparing the 0.5% AEP event and 0.2% AEP with the 1% AEP event can be seen in Figure L35 to Figure L42

In comparison to the 1% AEP event, the 0.5% AEP flood levels for the Murray River are higher by approximately 300 mm at Howlong and Mulwala, and 400 mm at Corowa. Across the Murray River floodplain, flood levels increase by up to 500 mm. In comparison, overland flow events have minor flood level increases (<100 mm) on main flow paths such as Majors Creek and in flood storage areas

Similarly, the 0.2% AEP flood levels for the Murray River are higher than the 1% AEP levels by more than 500 mm across the entire floodplain and up to 1.1 m. Flood levels increase by approximately 600 mm at Howlong, 900 mm at Corowa and 700 mm at Mulwala, with a larger breakout occurring through Mulwala. In overland flow events, flood levels increase typically up to 200 mm with areas that are newly flooded. Some flood storage areas to the west of the Mulwala township have increased flood levels by up to 350 mm.

These results indicate that the Murray River may be highly sensitive to climate change, although the long duration storms required to produce flooding in the Murray River catchment, in addition to the overall drier conditions may result in runoff increases that are not as high as those expected

for the local catchments. The overland flow flooding is less sensitive to increased rainfall intensity, although there are still substantial changes on major flow paths and in flood storage areas.

11.8. Historic Conditions

As development occurs over time, the Murray River floodplain has changed. Some of the key changes include structures that cross the waterway such as bridges and weirs, and changes to floodplain vegetation due to clearing, cropping, grazing and more recently re-vegetation. These changes to floodplain conditions cause impacts to flood behaviour and the effect of these are explored in this section.

11.8.1.

Historic Floodplain Roughness

As discussed in Section 7.13, there have been recent changes to the Murray River floodplain that have caused a shift in the rating curve at Corowa. These changes were assumed to be attributed to floodplain vegetation and as such, two models were produced – one that was calibrated to events from 1975 to 1996 (the ‘historic’ model) and one that was calibrated to 2016 and 2022 events (the ‘current conditions’ model). This sensitivity analysis simulated the 1% AEP Murray River event in the ‘historic’ model, rather than the ‘current conditions’ model. The Mannings ‘n’ value for floodplain grass was reduced from 0.05 to 0.045, floodplain vegetation reduced from 0.09 to 0.075 and the Murray River channel reduced from 0.048 to 0.038. The change in peak flood level is shown in Figure L43. The results indicate that peak flood levels reduce by approximately 100-150 mm at Howlong and approximately 300 mm at Corowa. There is negligible change in the downstream reach due to the presence of Lake Mulwala, which is insensitive to the Mannings ‘n’ roughness. Over the entire floodplain, the historic conditions result in 1% AEP flood levels being up to 350 mm lower than current conditions.

11.8.2.

Influence of Murray River Structures

There are several key bridge structures that cross the Murray River floodplain, including River Road at Howlong, Federation Way and Bridge Road (John Foord Bridge) at Corowa, and Melbourne Street and the railway line at Mulwala. A sensitivity analysis was conducted by removing these bridge structures (retaining any embankments). The change in the 1% AEP flood level can be seen in Figure L44. The results show that the reduction is up to 20 mm at River Road, approximately 30 mm at Federation Way, approximately 10 mm at John Foord Bridge and 40 mm in Lake Mulwala (Melbourne Street and the Railway). These are relatively small changes given the size of the Murray River floodplain.

An additional scenario was modelled whereby the embankments associated with linear infrastructure across the Murray floodplain were removed. This includes the removal of Federation Way embankment and the railway embankment. Note that the John Foord Bridge was retained for this run, and it is noted that it does not cause significant afflux (approximately 10 mm, as discussed above). The removal of the Federation Way embankment results in a decrease in upstream flood levels by approximately 200 mm, which diminishes to zero as you approach Howlong. There are only very localised changes to downstream flood levels such that there are no changesto flood levels onthe lowerterrace ofCorowa. The flood level in LakeMulwala remains

very similar to the run with no bridges (approximately 40 mm), indicating that the railway embankment is not a significant constraint to flow from the lake to the weir.

11.8.3. Removal of Yarrawonga Weir

Yarrawonga Weir is a significant barrier to flow and is designed to retain water within Lake Mulwala. The gates, however, allow water to be passed downstream and are operated such that they aim to pass any flood through the lake to the downstream areas. However, as discussed in Section 10.4, the gates were designed to pass a flow less than the current 1% AEP flow. As such, the gates cause some impediment to flow. In this scenario, Yarrawonga Weir was removed, in addition to the removal of the bridge structures and the railway embankment to enable a clear path through the natural constriction. The change in the 1% AEP flood level is shown in Figure L46. This indicates a reduction in peak flood levels of up to 1.6 m immediately upstream of the weir, with reductions of approximately 1.3 m in the open lake at Mulwala. Most notably, the breakout through Mulwala no longer occurs. The reduction in flood level extends upstream to the just above the confluence with the Ovens River. The peak flood level downstream of the weir increases by approximately 100 mm and persists to the model boundary.

The above scenario assumes current floodplain conditions – that is, the adopted roughness for Lake Mulwala (0.036) would be valid if the weir was removed. While this may be true immediately after the weir was removed, it would not be true ofhistoric conditions (pre-weir) or future conditions if the weir was removed. The floodplain vegetation would re-grow where the lake currently is. It is not known exactly what the floodplain looked like in the past, or what it would look like if the weir was removed and re-vegetation occurred and so two additional scenarios were run – one assuming the floodplain would be grassed (Mannings ‘n’ of 0.05) and one assuming the floodplain would be vegetated with trees and shrubs (Mannings ‘n’ of 0.09). In reality, the floodplain would be a mix of both of these, as it is for areas upstream of Lake Mulwala. As such, these two scenarios provide an upper and lower bound on a more realistic impact of the weir removal.

The 1% AEP peak flood level impact of the weir removal (and bridges and embankments) assuming the floodplain would be grassed is shown in Figure L47. With the floodplain grassed, the decrease in Lake Mulwala is approximately 1 m and no breakout through Mulwala occurs. The reduction immediately upstream of the weir remains at approximately 1.6 m. There are flood level increases of up to 200 mm in the vicinity of the confluence with the Ovens River as floodwaters flow through grassed areas rather than the existing lake bed conditions. Downstream of the weir the flood level increases are less than 100 mm.

The change in 1% AEP peak flood level assuming full vegetation is shown in Figure L48. This shows a very different impact, with the reduction in Lake Mulwala being approximately 300 mm and the breakout through Mulwala still occurring. The reduction immediately upstream of the weir remains at approximately 1.6 m. There are increases in flood levels upstream of Mulwala of up to 1.1 m that centre aroundthe confluence with theOvens River (approximatestart of Lake Mulwala). The increase in flood level downstream of the weir is less than 40 mm.

The results indicate that Yarrawonga Weir can have a significant influence on peak flood levels, however, if the weir was removed, the state of the floodplain has a marked impact on flood

Howlong and Mulwala Flood Study

behaviour. In grassed conditions, flood levels at Mulwala may decrease by up to 1 m, however, with fully vegetated conditions the flood level decrease may only be approximately 0.3 m, with the breakout through Mulwala still occurring and upstream water levels increasing significantly.

11.8.4. 1917 Floodplain Conditions

To simulate an actual historic condition (floodplain conditions estimated for 1917), this scenario the changes to floodplain roughness (Section 11.8.1) and removal of Yarrawonga Weir and Murray River structures (except for the John Foord Bridge, see Section 11.8.3). This model is the setup used to simulate the 1917 floodplain conditions (described in Section 8.4.6). As discussed in Section 11.8.3, the assumption about floodplain conditions without the weir in place is important. Without knowing what the floodplain looked like in 2017, two scenarios were modelled – one assuming it was grassed (historic Mannings ‘n’ of 0.045) and one assuming it was fully vegetated (historic Mannings ‘n’ of 0.09).

The change in 1% AEP peak flood level with 1917 conditions assuming Lake Mulwala was grassed is presented in Figure L49. Peak flood levels reduce at Howlong by up to 150 mm and at Corowa by up to 600 mm upstream of Federation Way and approximately 300 mm downstream. At the confluence with the Ovens River, peak flood levels increase by less than 50 mm. At Mulwala peak flood levels reduce by approximately 1.1 m and the breakout through Mulwala no longer occurs. Peak flood levels downstream increase by up to 100 mm.

The change in 1% AEP peak flood level with 1917 conditions assuming Lake Mulwala was fully vegetated is presented in Figure L50 At the confluence with the Ovens River, peak flood levels increase by up to 0.9 m. At Mulwala peak flood levels reduce by approximately 0.6 m and the breakout through Mulwala is significantly reduced, particularly on the southern side of the Mulwala Canal. Peak flood levels downstream increase by up to 60 mm.

12. ECONOMIC IMPACTS OF FLOODING

12.1. Background

The impact of flooding can be quantified through the calculation of flood damages. Flood damage calculations do not include all impacts associated with flooding. They do, however, provide a basis for assessing the economic loss of flooding and also provide a non-subjective means of assessing the merit of flood mitigation works such as detention basins, levees, drainage enhancement etc. The quantification of flood damages is an important part of the floodplain risk management process. By quantifying flood damage for a range of design events, appropriate costeffective management measures can be analysed in terms of their benefits (reduction in damages) versus the cost of implementation. The cost of damage and the degree of disruption to the community caused by flooding depends upon many factors including:

• The magnitude (depth, velocity and duration) of the flood,

• Land use and susceptibility to damages,

• Awareness of the community to flooding,

• Effective warning time,

• The availability of an evacuation plan or damage minimisation program,

• Physical factors such as failure of services (sewerage), flood borne debris, sedimentation, and

• The types of assets and infrastructure affected.

The estimation of flood damages tends to focus on the physical impact of damages on the human environment, but there is also a need to consider the ecological cost and benefits associated with flooding. Flood damages can be defined as being tangible or intangible. Tangible damages are those for which a monetary value can be easily assigned (for example damage to buildings, infrastructure, furnishings, goods or stock), while intangible damages are those to which a monetary value cannot easily be attributed (for example social costs such as increased levels of mental stress, loss of sentimental items, inconvenience to people, injury or loss of life). Types of flood damages are shown in Diagram 31

The assessment of flood damages not only quantifies potential costs due to flooding but also identifies when properties are likely to become flood affected by either flooding on the property or by over floor flooding.

The total likely damages in any given flood event are difficult to quantify precisely, given the variable nature of flooding and the property and content values of houses affected. Design flood damages are estimated to obtain an indication of the magnitude of the flood problem and compare the economic effectiveness of proposed mitigation options. Understanding the total damages prevented over the life of a mitigation option in relation to current damages, or to an alternative option, can assist in the decision-making process.

Diagram 31: Flood Damages Categories (including damage and losses

from permanent inundation)

FINANCIAL

Costs which can be expressed in dollars.

Damage caused by floodwaters coming into contact with items. This can be expressed as "Potential" (max. damage) and "Actual" (reduced damages due to moving items).

Contents of Buildings:

Clothes, Carpets, Furniture, Valuables, Fittings, Appliances

External Items: Vehicles, Laundries, Caravans, Sheds, Tools, Gardens, Fences

Physical Damage to Buildings: Gyprock, Cupboards, Scour of Footings, Houses becoming buoyant (floating off footings)

Clean Carpets, Walls, Clothes; Re-instate Furniture; Remove Mud and Debris

Loss of wages, Living costs (temporary accomodation and food), Time to repair/replace damaged items

SOCIAL

Costs which cannot be expressed in dollars, eg:

- stress, - loss of life, - serious injury, - depression, - inconvenience, - insecurity.

Costs associated with the flood event occurring, but not as readily quantifiable.

Not Applicable

Contents of Buildings:

Clothes, Carpets, Furniture, Valuables, Fittings, Appliances

External Items: Vehicles, Sheds (stables/barns), Machinery, Tools, Fences, Feed storage, Saddles, Crops &/or Stock, Irrigation Systems

Physical Damage to Structures: Damage to Homestead, Sheds, Access tracks, Protection levees

Clean Homestead and Out-buildings; Remove Debris; Dispose of affected crops &/or stock

Loss of Farm Production and Income, Re-instatement of Pastures, Supplementary feeding of stock (by hand or outside agistment), Stock movement/ transport, Living costs (temporary accomodation and food)

Sowing or harvesting of Crops, Sale of Stock (at depreciated value or dependent on market influences)

Contents of Buildings:

Products, Stock, Fittings, Tools, Machinery, Raw Materials

Physical Damage to Buildings External Items: Vehicles, Machinery, Display, Raw Materials/Stockpiles, Fences

Dispose of damaged products, stock, materials; Cleaning and Re-instatement

Loss of Productivity and Income, Bank Interest Charges

Loss of existing &/or Potential Trade

12.2. Approach

Estimation of flood damage has focussed on residential and community buildings in the study area using guidelines issued by the NSW Government (NSW Department of Planning and Environment 2023) and recognised damage assessment methodologies. The most common approach to present flood damage data is in the form of flood-damage curves for a range of property types, i.e. residential, commercial, public property, public utilities etc. These relate flood damage to depth of flooding above a threshold level (usually floor level). The estimation of damage is based upon a flood level relative to the floor level of a property. These damage curves are then factored 5.19% (according to the consumer price index) to adjust the damages from its initial estimates (in 2022) to current day dollars. Additionally, these damages are varied for different regions in the state. The study area is located within the Central Land Division and requires a regional cost adjustment factor of an additional 10%.

The assumed parameters and flood damage curve assumptions are outlined in the following sections.

12.2.1. Property Database

A property database was assembled using the available data, since it is not cost-effective to undertake detailed topographic survey of all or even a portion of flood prone properties across the study area. Floor levels of properties were estimated based on the following approach:

1. Determine properties affected by the 1% AEP flood extent (for both Murray River and local overland flooding) for inclusion in the property database and estimate the height of the floor level above the ground level for these properties by undertaking a ‘windscreen survey’, utilising Google Street View where available. This involved looking at features such as number of steps into the building, number of bricks to the floor level or other visible features which can be used to provide an estimate of the difference between the floor level and adjacent ground level. For properties where it was difficult to estimate the floor height above ground due to obstructions, the lower level of confidence in the estimate was noted in the database.

2. Based on the above analysis, an indicative average floor level height above adjacent ground levels was determined. It was found that the average height above ground was between 0.17 m and 0.21 m (across the towns), with the median height being 0.15 m for all the towns. A typical height above ground of 0.15 m was adopted for all towns.

3. Determine additional properties flood affected up to the PMF and add these to the property database.

4. Use GIS analysis to determine the ground level adjacent to each building within the property database using LiDAR data (2020 LiDAR at 1 m grid resolution)

5. Estimate the floor level using, in order of preference:

• Estimated floor level from ground level and task 1 (typically those within the 1% AEP extent where floor levels were visible from Google Street View).

• Estimated floor level from ground level and task 2 (typically those properties outside the 1% AEP extent but within the PMF extent).

The level of accuracy for the estimated floor heights is considered suitable for two reasons. Firstly,

Corowa, Howlong and Mulwala Flood Study

the estimation of property damage due to flooding is inherently difficult to estimate, given the large variation in building types, their contents, the duration of flooding and other factors, and so the accuracy of floor heights should be in line with the accuracy and applicability of the flood damage curves. Secondly, the economic damages assessment is only intended to be used as an estimate of the entire study area flood affectation and not on a per-property basis.

The property points can be seen in Figure 64, Figure 65 and Figure 66 for Howlong, Corowa and Mulwala, respectively. The total number of properties within the database is as follows:

• Howlong: 911

• Corowa: 1,651

• Mulwala: 1,378

12.2.2.

Residential Flood Damages

Tangible flood damages are comprised of two basic categories; direct and indirect damages (refer Diagram 31). Direct damages are caused by floodwaters wetting goods and possessions thereby damaging them and resulting in either costs to replace or repair or in a reduction to their value. Direct damages are further classified as either internal (damage to the contents of a building including carpets, furniture), structural (referring to the structural fabric of a building such as foundations, walls, floors, windows) or external (damage to all items outside the building such as cars, garages). Indirect damages are the additional financial losses caused by the flood for example the cost of temporary accommodation, loss of wages by employees etc.

Given the variability of flooding, property and content values, the total likely damages figure in any given flood event is useful to get a feel for the magnitude of the flood problem, however it is of little value for absolute economic evaluation. Flood damages estimates are also useful when studying the economic effectiveness of proposed mitigation options. Understanding the total damages prevented over the life of the option in relation to current damages, or to an alternative option, can assist in the decision-making process.

The standard way of expressing flood damages is in terms of average annual damages (AAD). AAD represents the equivalent average damages that would be experienced by the community on an annual basis, by taking into account the probability of a flood occurrence. This means the smaller floods, which occur more frequently, are given a greater weighting than the rare catastrophic floods.

In order to quantify the damages caused by inundation for existing development, the floor level database was used (see Section 12.2.1) in conjunction with modelled flood level information to calculate damages. The flood damages assessment was undertaken for existing development in accordance with current NSW Government guidelines (NSW Department of Planning and Environment 2023) The damages were calculated using a number of height-damage curves which relate the depth of water above the floor with tangible damages. Each component of tangible damages is allocated a maximum value and a maximum depth at which this value occurs. Any flood depths greater than this allocated value do not incur additional damages as it is assumed that, by this level, all potential damages have already occurred.

12.2.2.1.

Direct Internal Damages

Internal damages were assumed to follow the default damages of $550 per square metre (in 2022 dollars) adopted in the guideline (NSW Department of Planning and Environment 2023) for residential properties The actual damage to contents in an event can be reduced by actions taken during the warning time available in response to a flood threat. These actions may include raising goods and furniture, moving valuable items to the kitchen benchtop, onto tables, or up to the second storey, and taking some valuables as part of evacuation, if possible. The default value of 0.9 for the actual to potential damage ratio in the guideline (NSW Department of Planning and Environment 2023) was adopted for this study area.

12.2.2.2. Direct Structural Damages

Structural damages were assumed to follow the default damages relationships to the dwelling size and number of storeys adopted in the guideline (NSW Department of Planning and Environment 2023) Damage per m2 is assumed to be $2,280 for single storey houses and $2,620 for double storey houses and $2,730 for units and $2,620 for townhouses. As the dwelling size has not been obtained, all houses were assumed to have the default size of 220 m2 and units and townhouses were assumed to be 100 m2 and 160 m2, respectively. In floods larger than the 1% AEP event there is the possibility that some buildings may collapse or have to be demolished. The cost of these damages have not been included in the analysis.

12.2.2.3.

Direct External Damages

The default external damages of $17,000 (in 2022 dollars) in the guideline (NSW Department of Planning and Environment 2023) were adopted. This fixed external damage value was applied when the flood depth above ground level exceeded 300 mm or was above the habitable floor level.

12.2.2.4.

Indirect Damages

Indirect damages were assumed to follow the default damage relationship in the guideline (NSW Department of Planning and Environment 2023). That is, for residential clean-up costs of 4,500 (in 2022 dollars) and relocations costs of $441 per week (in 2022 dollars) will apply if over floor inundation exists. Non-residential indirect costs, which cover clean-up costs and loss of trading are 30% of the direct damages.

12.2.3. Non-residential Buildings

12.2.3.1.

Caravan Parks

The guideline (NSW Department of Planning and Environment 2023) does not provide further guidance on the estimation of flood damages to caravan parks. There are several caravan parks within the study area which are near major watercourses and are flood affected. To ensure damages to caravan parks were adequately considered, structural damages to caravan parks were estimated by adopting the residential structural damages curve of a small (22 m2) single storey house. Caravan parks were assumed to have no internal or intangible damages. External damages associated with caravan parks were assumed to follow the same relationship as

Corowa, Howlong and Mulwala Flood Study

residential external damages in Section 12.2.2.3 Caravan parks are seasonal in nature and it is difficult to estimate the potential number of sites that may be occupied. Although caravans (longterm and short-term) and/or cabins on site may be moved with adequate warning time, there are still costs associated with this when a flood occurs. The estimation of flood damages for caravan parks should be reviewed in the Floodplain Risk Management Study when assessing particular flood risk migration measures to ensure a reasonable representation of damage costs (and hence benefits for mitigation measures)

12.2.3.2. Commercial Properties and Public Buildings

Damage curves for commercial, industrial, and public buildings were adopted from the guideline (NSW Department of Planning and Environment 2023). Direct damages (accounting for structural and contents damage) to these buildings are based on the value classification of the building as well as the floor area.

Commercial and industrial buildings are classified as low to medium, medium/default, and medium to high. The low to medium damage curves are factored by 0.6 of the default and medium to high damage curves are factored by 1.5. Commercial and industrial buildings were used the medium/default damage curve as no further information on these buildings had been provided. As no information on floor area of each commercial and industrial building was provided, the default area of 418 m2 was adopted. Actual to potential damage ratio was assumed to be 0.9.

Public buildings were classified as low/default and medium to high categories. The low/default damage curves for public buildings were assumed to be 40% of the medium/default commercial damage curve, whereas medium to high public buildings damage curve were assumed to be the same as the medium/default commercial damage curve.

12.2.4. Intangible Damages

Intangible damages were assumed to follow the default damage relationship in the guideline (NSW Department of Planning and Environment 2023) These intangible damages cover social and wellbeing impacts of flooding to the community These intangible damages have been incorporated in this assessment and were found to contribute only a small portion of the total flood damages (<10%).

12.3. Estimated Flood Damages

An estimation of the number of properties impacted (flooding occurring on the property within 4 m of the dwelling/building or 2 model grid cells), number of properties with above floor flooding and total damage costs for each modelled flood event was undertaken for each of the model areas Properties estimated to be flooded above floor due to Murray River inundation for Howlong, Corowa and Mulwala can be seen in Figure 64, Figure 65 and Figure 66, respectively. Properties estimated to be flooded above floor due to local overland flooding for Howlong, Corowa and Mulwala can be seen in Figure 67, Figure 68 and Figure 69, respectively.

A typical measure used to estimate flood damages over a range of flood events is the Annual Average Damage (AAD). AAD represents the equivalent average damages that would be experienced by the community on an annual basis, by taking into account the probability of a flood occurrence over the long term. The AAD value is determined by multiplying the damages that can occur in a given flood by the probability of that flood actually occurring in a given year, and then summing across a rangeof floods. This method allows smaller floods, which occur more frequently to be given a greater weighting than the larger catastrophic floods that only occur rarely The AAD for the existing case thenprovides a benchmark by which to assess the merit of flood management options.

A summary of the flood damages is provided in Table 85 to Table 90. The damages associated with the Murray River event and overland flow event have been presented separately for each township. Residential damages and the total damages (which include residential, commercial and public buildings, along with infrastructure damages) are provided separately. The total number of properties affected is also presented in these tables. The number of lots affected indicates that the flood level was higher than the ground level near the building on the property and the number of lots affected above floor indicates that the flood level was higher than the floor level. ‘Lots’ includes individual sites located within caravan parks (for cabins and/or caravans). These were included in the assessment particularly for Corowa, where Murray River flooding on the lower terrace primarily affects three caravan parks. In recent events, it has been the caravan parks that have sustained the most damage and incurred costs associated with flood damage.

Howlong is largely unaffected by the Murray River until the PMF event. The AAD due to Murray River flooding at Howlong is small (approximately $133,000 or $300 per flood affected property in the PMF). Overland flow events in Howlong typically have a gradual increase in the number of properties affected with increasing flood magnitude. Most of the flood damage (43%) of the flood damage is caused by more frequent events. The AAD due to overland flow (including Majors Creek) at Howlong is much larger than the Murray River, being approximately $2.6M, or $3,300 per flood affected property in the PMF

Corowa is affected in Murray River events mainly due to the caravan parks located on the lower terrace of the floodplain. The number of properties affected increase from 249 to 315 from the 10% AEP to 0.2% AEP and is fairly consistent across the events. This indicates that there are few residential properties that become inundated up to the 0.2% AEP and most of the damages are incurred at the caravan parks. In the PMF event, the number of affected properties is substantially higher. The AAD due to Murray River flooding at Corowa is approximately $10.2M, or $8,800 per flood affected property (including caravan park sites) in the PMF event. Overland flow events in Corowa typically have a gradual increase in the number of properties affected with increasing flood magnitude. Most of the flood damage (53%) is caused by more frequent events. The AAD due to overland flow at Corowa is much less than Murray River, at approximately $1.8M, or $2,000 per flood affected property in the PMF

Mulwala is largely unaffected by frequent Murray River events, with no flood damages up to and including the 5% AEP event. In the 2% AEP event, properties begin to be affected The AAD due to Murray River flooding at Mulwala is approximately $3.3M, or $2,400 per flood affected property in the PMF, with most of the flood damage caused in the rare events, with the PMF contributing

Corowa, Howlong and Mulwala Flood Study

approximately 30%. Overland flow events in Mulwala typically have a gradual increase in the number of properties affected with increasing flood magnitude. Most of the flood damage of the flood damage is caused by more frequent events, with the 20% AEP event contributing 50% The AAD due to overland flow at Mulwala is approximately $2.2M, or $2,700 per flood affected property in the PMF.

Table 85: Summary of Estimated Murray River Flood Damages at Howlong

and Mulwala Flood Study

1. Includes individual cabin and caravan sites within caravan parks

Table 86: Summary of Estimated Overland Flow Flood Damages at Howlong

1. Includes individual

and

Table 87: Summary of Estimated Murray River Flood Damages at Corowa

1. Includes individual

Table 88: Summary of Estimated Overland Flow Flood Damages at Corowa

1. Includes

Table 89: Summary of Estimated Murray River Flood Damages at Mulwala

1. Includes individual cabin and caravan sites within

1. Includes

Corowa, Howlong and Mulwala Flood Study

The estimation of tangible flood damages is a high-level exercise, intended to capture the catchment-scale flood damages. It can provide a good indication of the average flood damage across a catchment. The accuracy of the results at individual properties can be affected by vagaries such as the variability in the flood level across the property, the location of the sampled flood level for the property, whether the floor level is consistent or varies through the building. This variability tends to average out across the catchment, particularly if many properties are considered.

13. REFERENCES

Corowa, Howlong and Mulwala Flood Study

ABS (Australian Bureau of Statistics) 2021, 2016 Census QuickStats, Online <https://www.abs.gov.au/websitedbs/D3310114.nsf/Home/2016%20QuickStats>, Accessed 12/01/22.

Alluvium 2019, Yarrawonga Framework Plan: Stormwater Drainage Strategy, prepared for Moira Shire Council, September 2019.

Australian Dams Alliance 1999, Yarrawonga Weir Review of Flood Security, Report No. DC99200, October 1999.

Australian Institute of Disaster Resilience 2017, Managing the Floodplain: A Guide to Best Practice Flood Risk Management in Australia, Commonwealth of Australia, 2017.

Australian Water Technologies 1999, Lake Mulwala Bathymetry Model, prepared for GoulburnMurray Water, July 1999.

Austroads 2018, Guide to Bridge Technology Part 8: Hydraulic Design of Waterway Structures, Second Edition.

Babister M & Barton C (Editors) 2012, Australian Rainfall and Runoff Revision Project 15: Two Dimensional Modelling in Urban and Rural Floodplains- Stage 1&2, Report P15/S1/009, Engineers Australia

Babister, M., Trim, A., Testoni, I. & Retallick, M 2016, The Australian Rainfall & Runoff Datahub, 37th Hydrology and Water Resources Symposium, Queenstown NZ.

Ball J, Babister M, Nathan R, Weeks W, Weinmann E, Retallick M, Testoni I, (Editors) 2019, Australian Rainfall and Runoff: A Guide to Flood Estimation, Commonwealth of Australia, Australia

BMT WBM 2018, TUFLOW Classic/HPC User Manual, Build 2018-03-AD.

Boyd, MJ, Rigby, EH, Van Drie, R & Schymitzek, I 2012, Watershed Bounded Network Model User Guide, June 2012.

Bureau of Meteorology 2003, The Estimation of Probable Maximum Precipitation in Australia: Generalised Short Duration Method, June 2003.

Bureau of Meteorology 2006, Guidebook to the Estimation Probable Maximum Precipitation: Generalised Southeast Australia Method, October 2006.

Commonwealth of Australia 2017, Australian Disaster Resilience Handbook 7 Managing the Floodplain: A Guide to Best Practice in Flood Risk Management in Australia, Australian Institute for Disaster Resilience.

Corowa, Howlong and Mulwala Flood Study

Corowa Free Press 2022a, Water waist-deep in Corowa street, new homes impacted (online), <https://www.bordermail.com.au/story/7601423/water-was-waist-deep-in-corowastreet-new-homes-impacted/>, published 31 January 2022, accessed 21/03/22.

Corowa Free Press 2022b, Border rocked by wild weather (online), <https://www.corowafreepress.com.au/news/border-rocked-by-wild-weather/>, published 1 February 2022, accessed 21/03/22.

Howells, L, McLuckie, D, Collings, G. and Lawson, N 2003, ‘Defining the Floodway – Can One Size Fit All?’, Floodplain Management Authorities of NSW 43rd Annual Conference, Forbes, February 2003.

GHD 2008, Report for Howlong Drainage Strategy: Drainage Report, prepared for Corowa Shire Council, February 2008.

GHD 2012, Albury City to Greater Hume Murray River Flood Study, prepared for Albury City Council and Greater Hume Shire Council, March 2012.

GHD 2019a, Stormwater Options Assessment – John Street, Corowa, prepared for Federation Council, Final Report, March 2019.

GHD 2019b, Mulwala Drainage Improvement Project: Stage 1 Scoping Study, prepared for Federation Council, Final Report, March 2019.

GHD 2019c, Mulwala Stormwater Stage 2 Options Study, prepared for Federation Council, Final Report, October 2019.

Goulburn-Murray Water 2021, Yarrawonga Weir, Online <https://www.g-mwater.com.au/waterresources/catchments/storages/murray/yarrawongaweir>, Accessed 23/12/21.

Gutteridge Haskins Davey 1985, Howlong Drainage Study, prepared for Hume Shire Council, April 1985.

Gutteridge Haskins Davey 1986, Murray River Flood Plan Management Study, Detailed Report and Flood Plan Atlas, prepared by Gutteridge Haskins and Davey Pty Ltd, Cameron McNamara Pty Ltd and Laurie Montgomerie and Pettit Pty Ltd for Rural Water Commission of Victoria and Water Resources Commission of New South Wales, December 1986.

Kuczera, G 1999, ‘Comprehensive at-site flood frequency analysis using Monte Carlo Bayesian inference’, Water Resources Research, vol 35, no. 5, pp. 1551-1558.

MDBA (Murray Darling Basin Authority), Snags and Resnagging, Fact Sheet (online), <https://www.mdba.gov.au/publications/brochures-factsheets/fact-sheet-snagsresnagging>, accessed 13/02/23.

Corowa, Howlong and Mulwala Flood Study

MDBA (Murray Darling Basin Authority), Resnagging: Risk Management Issues, Fact Sheet (online), <https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0008/634058/Lachlanresnagging-brochure.pdf>, accessed 13/02/23.

MDBA (Murray Darling Basin Authority) 2016, Yarrawonga Weir Flood Incident Management Plan, Document No. 3257637.

MDBA (Murray Darling Basin Authority) 2019, Hume to Yarrawonga Hydrodynamic Model, February 2019.

MDBA (Murray Darling Basin Authority) 2020, Weirs and Locks, Online <https://www.mdba.gov.au/water-management/infrastructure/weirs-locks>, Accessed 23/12/21

MDBA (Murray Darling Basin Authority) 2021a, Joint media release: Yarrawonga Weir crossing to close in October 2021, Online <https://www.mdba.gov.au/media/mr/joint-mediarelease-yarrawonga-weir-crossing-close-october-2021>, Accessed 12/01/22

MDBA (Murray Darling Basin Authority) 2021b, Managing floods at Hume Dam, Online <https://www.mdba.gov.au/water-management/infrastructure/hume-dam/managingfloods>, Accessed 12/01/22

Nanakumar, N, Green, J, Nathan, R, Sih, K & Wilson, R 2011, Assessment of Hydrologic Risk for Hume Dam, ANCOLD Conference 2011.

NSW Department of Planning and Environment 2023, Flood risk management measures: Flood risk management guideline MM01, June 2023.

NSW Government 2005, Floodplain Development Manual: The management of flood liable land, April 2005.

NSW Office of Environment and Heritage 2019, Review of ARR Design Inputs for NSW, Final Report, WMAwater, February 2019.

NSW SES (State Emergency Service) 2015, Corowa Shire Local Flood Plan, a sub-plan of the Corowa Shire Local Emergency Management Plan

Pilgrim DH (Editor in Chief) 1987, Australian Rainfall and Runoff – A Guide to Flood Estimation, Institution of Engineers, Australia.

River Murray Commission 1977, Parliamentary Paper No. 98/1977 Annual Report 1975-76, presented 2 June 1977.

Spiire 2018, South Corowa Stormwater Drainage Strategy, prepared for Federation Council, 13 December 2018.

Corowa, Howlong and Mulwala Flood Study

URS 2009, Corowa, Howlong & Mulwala Floodplain Risk Management Study and Plan, prepared for Corowa Shire Council, 23 March 2009.

Victoria State Government 2018, Pre 1900 Floods, Online <https://www.floodvictoria.vic.gov.au/learn-about-flooding/flood-history/pre-1900floods>, Accessed 12/01/22

Water Technology 2011, Murray River Regional Flood Study Dicks/Seppelts levees to downstream of the Ulupna Creek confluence, prepared for Goulburn Broken Catchment Management Authority, Berrigan Shire Council and Moira Shire Council, November 2011.

Water Technology 2017, Wangaratta Urban Waterways Flood Investigation, prepared for North East Catchment Management Authority, August 2017.

Water Technology 2024, South Albury Levee Upgrade – Murray River Flood Study, prepared for Albury City Council, June 2024.

WMAwater 2016, Albury Floodplain Risk Management Study and Plan, prepared for Albury City Council, March 2016.

WMAwater 2022, Federation Villages Floodplain Risk Management Study and Plan, prepared for Federation Council, March 2022.Figure F20