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Table 1: Guidelines and reference documents
2.3 Policies, Legislation and Guidance 2.3.1 Implemented Guidelines and References
Table 1 presents the guidelines, manuals and technical reference documents used for this study. These documents detail best practice in regard to management of flood risk. They cover both best practice regarding the technical assessment of flood behaviour and flood risk, and, more generally, who has responsibility for managing flood risk and how this management is best achieved.
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Table 1: Guidelines and reference documents
Reference
Australian Emergency Management (AEM) Handbook Series, Managing the floodplain: A guide to best practice in flood risk management in Australia – AEM Handbook 7 AEM Handbook 7, Technical flood risk management guideline –Flood Hazard
Topic
Best practice
Flood hazard
AEM Handbook 7, Technical flood risk management guideline –Flood Emergency Response Classification AEM Handbook 7, Technical flood risk management guideline –Flood risk information to support land-use planning AEM Handbook 7, Technical flood risk management guideline –Assessing options and service levels for treating existing risk AEM Handbook 6, National Strategy for Disaster Resilience –community engagement framework Australian Rainfall & Runoff 2016 Emergency response
Land use
Mitigation options and service levels
Community engagement
Best practice
Section 733 of the Local Government Act, 1993
Flood prone land policy NSW Government’s Floodplain Development Manual (2005) Flood prone land policy and industry practice SES requirements from floodplain risk management process SES requirements Practical consideration of climate change Climate change Coincidence of Coastal Inundation and Catchment Flooding Coincidence
2.3.2 Review of Council Planning Policy
Local Environmental Plan Review
A Local Environmental Plan (LEP) is a statutory document developed to guide planning decisions for local government areas. LEP’s are primarily used as planning tool to aid the future of communities and to direct development in the study area.
The City of Canada Bay LEP was adopted in 2013. Section 6.8 - Flood Planning, addresses development on properties within the Flood Planning Area (FPA). The current study can be used in the development of a FPA for Exile Bay which will aid the application of these controls.
Development Control Plan Review
A Development Control Plan (DCP) is a non-statutory document which supports the planning controls in the LEP by providing detailed planning and design guidelines.
The City of Canada Bay DCP was adopted by Council in February 2017. Section C7 – Flooding Control uses a Flood Planning Matrix to outline the relevant Planning and Development Controls within the study area. This approach uses the land use and the level of flood risk at the site to determine the applicable Flood Planning Controls.
The outcomes of the current study can be used to determine the level of flood risk for properties within the Exile Bay catchment.
3. AVAILABLE DATA
3.1 Overview
A thorough review of available data has been undertaken for the Exile Bay catchment for the following key reasons:
• To obtain an understanding of how flooding has historically occurred in the study area; and • To gather as much information as possible to aid the development of modelling tools.
This process has involved examination of Council records, obtaining data to aid the hydrologic and hydraulic model development, reviewing previous flood related studies, procurement of historic rainfall records and engaging with the local community.
The following sections will provide detail on this process.
3.2 Previous Studies
There have been a range of relevant studies as listed and summarised below. These studies will aid the current flood study. The following sections summarises the related studies and the data used for the current study.
3.2.1 Exile Bay, St Lukes and William Street Flood Study (WMAwater, 2017) (Reference 4)
The Exile Bay, St Lukes and William Street Flood Study was prepared by WMAwater in 2017 for Burwood Council. This study defined the existing flood behaviour for the Burwood LGA catchments which flow into the City of Canada Bay LGA (with Parramatta Road as the boundary between Burwood and Canada Bay LGA’s). This study used a DRAINS/TUFLOW hydrologic and hydraulic modelling system to define flood affection for a full range of flood event probabilities in the Exile Bay catchment upstream of Parramatta Road.
The pit and pipe delineation presented in this study within the Burwood LGA has been used for the current study. Furthermore, the results from this study will be used in the current study for validation of the flood model.
3.2.2 WestConnex Technical Report Flood Mitigation Strategy (AECOM
Hyder Joint Venture, 2016) (Reference 20)
A flood investigation was undertaken as a part of the WestConnex M4 East Project to determine the flood impacts associated with the project for the 1% AEP and PMF events.
The WestConnex project is located at the upstream boundary of the current study area (at the intersection of Parramatta Road and Concord Road). As such, the current study has not incorporated the minor drainage features and on-site detention which were built for the WestConnex project as they will not affect the property flood affectation downstream of the project. Major trunk drainage elements through the WestConnex project have been included in the current study.
3.2.3 Parramatta River Estuary Foreshore Management (Royal Haskoning
DHV, 2013) (Reference 8)
The Paramatta River Estuary Foreshore Management study was undertaken by Royal Haskoning DHV in 2013 and investigated actions for the repair and restoration of deteriorated and failed seawalls of the City of Canada Bay.
3.2.4 Beaconsfield Depot – Flooding Issues on Existing Properties (J.
Wyndam Prince, 2012) (Reference 5)
The flood study undertaken in (Reference 9) highlighted to Council that there were several floodaffected properties adjacent to the Beaconsfield Depot. Based on this analysis, a secondary study was commissioned to investigate measures to relieve the flood affectation at these properties. This study updated the hydrologic/hydraulic modelling system developed in Reference 9 using additional survey information. The study recommended an improved downstream piped system, the construction of an open concrete channel between Rothwell Park and Jessie Stewart Reserve the construction of a small retaining wall to minimise over floor flooding at five residential dwellings.
3.2.5 Proposed Re-Development of Beaconsfield Depot, Concord (Stage 2 – Flood Study Report) (J. Wyndam Prince, 2011) (Reference 9)
A flood study was commissioned by Council in 2011 to assess the optimum development yield for the Council owned Beaconsfield Depot located on Beaconsfield Lane, Concord. This site is located adjacent to the Main South Drain and, as such, analysis of flooding is an important part of determining the development potential. This study used a hydrologic (XP-RAFTS) and hydraulic (HEC-RAS) modelling system to assess several development options to maximise the development footprint and minimise potential adverse impacts on neighbouring properties. This study recommended two filling options for the depot site.
3.2.6 Intersection of Brewer Street and Majors Bay Road – Overland Flood
An overland flow investigation was undertaken by Taylor Thomson Whitting in 2010 at the intersection of Majors Bay Road and Brewer Street. This study used a hydrologic (DRAINS) and hydraulic (HEC-RAS) modelling system to assess flood mitigation solutions at the intersection. Recommendations from the study included: moving the entrance location of a commercial property that is subject to frequent inundation; Council purchasing the flood affected property, and finally installing floodway signage to increase community awareness.
This study provides peak flow rates and flood levels for the 100 year ARI event. As with the other studies, metrics (if available and relevant), will be used to create context for revised results.
3.2.7 Hydrological and Hydraulic Services for Proposed Development of
Concord Council’s Former Depot in Beaconsfield Avenue, Concord (Bankstown Civic Services Group, 2000) (Reference 10)
Bankstown Civic Services Group was engaged by Concord Council in 2000 to undertake an investigation into the flood levels and flood hazard for the 100 year ARI event at the Council Depot (Beaconsfield Avenue, Concord) for existing and proposed development conditions.
This study determined that the flood level and hazard impacts were beyond the desirable range for the proposed conditions however generally it was an improvement compared to the existing case.
The current study verified trunk structure sizes identified in this drainage investigation against the data provided by Council.
3.2.8 Overland Flow Investigation – Brewer Street/Majors Bay Road
An overland flow investigation was undertaken by Gardiner Willis & Associates in 1998 at the intersection of Majors Bay Road and Brewer Street. This study aimed to alleviate flood inundation at this intersection and flood affection for neighbouring commercial properties. This study used a hydrologic (XP-RAFTS) and hydraulic (HEC2 RAS) modelling system to assess flood affection at this location. The investigation recommended the construction of bunding, increase in drainage capacity and the construction of hydraulically efficient pits outside the Concord Centre.
This study provides peak flow rates and flood levels at various locations. As with the other studies, metrics (if available and relevant), will be used to create context for revised results.
3.2.9 Report on the Massey Park Gross Pollutant Trap (UTS Sydney, 1997) (Reference 12)
An investigation was undertaken into the construction of a Gross Pollutant Trap (GPT) along the Exile Bay Stormwater Channel by UTS Sydney 1997. This study reviewed all previous flood related studies and undertook community consultation regarding the installation of the GPT as there were concerns that it would exacerbate flooding. The investigation found using hydraulic (HEC-RAS) modelling that no additional flooding would occur if the GPT is installed.
This study used channel cross sections to undertake 1D hydraulic modelling in HEC-RAS. Council engaged a surveyor to undertake cross section survey of this channel in 2016 (see Section 3.3.2) and as such, the recent cross section survey will be used in preference to the cross sections from this study.
3.2.10 Main South Drain – Investigation of Stormwater System Parramatta Road to John Street (Ledingham Hensby Oxley & Partners, 1992) (Reference 13)
An investigation into the drainage system, known as the Main South Drain (shown in Figure 1), between John Street and Paramatta Road was undertaken by Leginham Hensby Oxley & Partners in 1992. This study reviewed the capacity of the existing drainage system and developed strategies to improve the drainage system. The investigation recommended implementing floor level and
development controls for the flood affected properties, enhancing the 20 year ARI drainage capacity and facilitating an overland flow path for 100 year ARI overland flows.
The current study verified trunk structure sizes identified in this drainage investigation against the data provided by Council.
3.2.11 Sydney Storms November 1984 (Public Works, 1985) (Reference 17)
Between the 5th and 12th of November 1984, the Sydney metropolitan area was affected by a series of severe storms which caused widespread flooding and damage to public and private property. As such, Public Works undertook an analysis of these storms in 1985.
The report provides an excellent overview of the November 1984 event. Analysis of historical rainfall records (see Section 3.4.2) found that the nearby pluviometer gauges (Greenlees Park Bowling Club and Homebush SP0041) were not operational at the time. Due to the significant spatial variation of rainfall (determined in Section 3.4.2), use of pluviometer data from distant gauges would not be an accurate representation of rainfall in the study area.
3.2.12 Stormwater Drainage capacity assessment within the Municipality of Concord (E. S. Rowe & Ennis, 1973) (Reference 16)
A stormwater drainage capacity assessment was undertaken by E. S. Rowe & Ennis for the Department of Public works in 1973. This assessment reviewed the drainage system in the Exile Bay and Majors Bay catchments and recommended improvements where necessary. The report recommends that Council lower the level of the weir in the Exile Bay stormwater channel to the High Water Spring Tide level and install flood gates with the aim of ultimately removing the weir over ten years.
Since the time of this report, there have been significant changes in the catchment and also in standard hydrologic practices. As such, the study is of little utility.
3.2.13 History of Canals and Bridges (Various Councils and Public Works)
The history of Canals and Bridges included various documents pertaining to the study area. Some key elements include the approval for the construction of the Massey Park stormwater channel in March 1947 and the subsequent transfer of ownership from the Department of Public Works NSW to the Concord Municipal Council in 1947. These documents also included anecdotal evidence of flash flooding occurring in the study area in November 1972 with an approximate magnitude of 50 year ARI. It was noted that the weir within the stormwater channel at Massey Park may have exacerbated the flooding in this event.
3.3 Hydraulic Model Data
3.3.1 LiDAR Data
LiDAR data is used to define the topography of the study areas in the hydrologic and hydraulic models. In the hydrologic model, LiDAR data is used to delineate sub-catchments. In the hydraulic models, ground points in the LiDAR set are sampled to form an elevation grid. The grid is one of the key model inputs and is used to simulate the flow of runoff from one grid cell to the next in the floodplain.
LiDAR data was obtained from the NSW Government Spatial Services and used in the current study. Metadata indicates the LiDAR survey was undertaken in 2013. The data is best practice with an accuracy of ± 0.15 m (1st confidence interval) in the vertical direction and ± 0.4 m (1st confidence interval) in the horizontal direction. The 1 m LiDAR raster product is produced using a Triangular Irregular Network (TIN) method of averaging ground heights to formulate a surface which can then be sampled to generate a regular grid.
LiDAR data used in the current study is shown in Figure 2.
3.3.2 Ground Level and Bathymetric Survey
Ground level survey was undertaken at a number of locations throughout the catchment including the intersection of Majors Bay Road and Davidson Avenue in 2008 and Brewer Street and Spring Street in 2018. Historically, these locations have been subject to frequent inundation from overland flows. This survey was used to incorporate key hydraulic features such as the kerb, gutter median strip and roundabout. This survey is incorporated into the current study using breaklines which denote hydraulic controls.
The intersection of Favelle Street and Davidson Avenue underwent some resurfacing works in 2015 and as such, this data has been incorporated into the hydraulic model for the current study.
In 2016, Council commissioned detailed ground level survey of Massey Park Golf Course, Edwards Park, Greenlees Park and Jessie Stewart Reserve. Council have identified topographical changes in this area subsequent to the LiDAR capture in 2013 and as such, this data has been incorporated into the hydraulic model as a ground level surface.
As a part of the 2016 survey, the Saltwater Creek channel bathymetry was also surveyed at regular intervals (approximately 8 m) from Ian Parade to the creek outlet into Exile Bay. Since LiDAR data cannot penetrate water, this survey information is key to representing the Saltwater Creek channel shape. These cross sections have been incorporated into the hydraulic model (see Section 7.5).
The ground level survey data in the current study is shown in Figure 3.
