3 Previous studies

3.1 Bulahdelah Flood Appraisal, NSW Public Works Department, 1991

This report was prepared for Great Lakes Shire Council, by the NSW Public Works Department (PWD) in October 1991 to assess flood behaviour for the Myall River floodplain in the vicinity of Bulahdelah (Public Works, 1991). The study area extended along the Myall River from 0.43 km downstream of the Pacific Highway bridge to about 2 km upstream of the bridge at Lee Street.

The study utilised the WBNM model with eight sub-catchments to produce the discharge hydrographs for the October 1985 and November 1987 flood events as well as 1%, 2% and 5% AEP events. WBNM hydrologic model was set using the lag parameter of C = 1.29 A onedimensional hydraulic model of MIKE 11 was established to study the flood behaviour across the study area. The Bulahdelah MIKE 11 model consisted of seven surveyed cross-sections (surveyed by the PWD in 1987 and 1988) and the Pacific Highwaybridge profile. The upstream and downstream boundaries were defined as discharge hydrographs at the confluence of the Crawford River and a stage hydrograph, respectively.

The study utilised daily rainfall and pluviography data to describe the areal and temporal distribution of rainfall No pluviograph station was located within the Myall River catchment therefore, the closest ones acquired for the study including Upper Johnson Creek, Nabiac, Maryville (Newcastle) Williamtown and Taree. Long term daily rainfall data were obtained from BoM station located at Bulahdelah Post Office and several stations located around the boundary of the catchment Further daily rainfall records were used to undertake the Flood Study from unofficial gauges located at Upper Myall (Tank Creek), Markwell (Deep Creek) and Rosenthal (Gloucester Road) No adequate details of the unofficial rainfall gauges are available. ARR 1987 methodology was used to determine the critical duration of design storm events, resulting in the critical duration of 36 hours for the 1% AEP event.

The study documented that the most information on flooding was available for 1985 and 1987 events (for further information on historical flood events see Section 2.2). Therefore, the hydraulic model was calibrated and verified using the October 1985 and November 1987 floods, respectively.

For the 1985 flood, an initial loss of 0 mm and a continuing loss of 2.5 mm/hr were adopted due to considerable rainfall prior to the event. An initial rainfall loss of 21 mm and continuing losses of 2.5 mm/hr were adopted for the 1987 flood and the design floods.

Sensitivity analysis was conducted to investigate the impacts of the tailwater conditions at the Muirs Creek confluence on the hydraulic model outcomes. The 1% AEP flood event hydrograph at the Highway Bridge was insensitive to the range of tailwater conditions. The 5% AEP flood event hydrograph at the Highway Bridge was relatively insensitive to the range of tailwater conditions, with 200 mm variation within the range of tested tailwater conditions. A single tailwater curve with a peak level of 3 m AHD was adopted for modelling the design events.

The key finds from the report are summarised below:

• A constant Manning’s roughness coefficient of 0.037 for the main channel and 0.05 for the floodplain resulted in the best outcomes.

• In 1% AEP event, a critical storm duration of 36 hours was found to produce a peak discharge of 2100, 750 and 1400 m3/s at the Bulahdelah bridge, Crawford River and upstream of Crawford River, respectively.

• It was documented that the 1897 flood event is the largest flood on record with a flood level of RL 5.5 m reported in the vicinity of the Highway Bridge.

• The simulations of 1% AEP event resulted in a flood level of approximately 50 mm higher than the 1897 event in the vicinity of the highway Bridge.

• Also, it was reported that the 1927 flood event is the second largest flood on record with a flood level of RL 5.0 m.

• The calculated flood level of a 2% AEP event resulted in a flood level of approximately 100 mm higher than the 1927 record.

The review of the Bulahdelah Flood Appraisal revealed that the WBNM and MIKE11 models utilised in the study are very limited in terms of the number of sub-catchments to represent the study area and cross-sections to represent the 1D model. Therefore, there is a need for establishing hydrologic and hydraulic models with adequate details to appropriately represent the study area

3.2 Frys Creek Flood Study, NSW Public Works Department, 1994

This report was prepared for the Great Lakes Shire Council, by the NSW Public Works Department in July 1994 to assess flood behaviour in the lower reaches of the Frys Creek (Public Works, 1994)

The study adopted and adapted the hydrological WBNM model and the one-dimensional hydraulic model of MIKE 11 developed for the completion of Bulahdelah Flood Appraisal (1991). To meet the objectives of the study, the MIKE 11 model was extended to include Frys Creek. Figure 3 1 showed the model schematization including the branches, weirs and culverts. The hydraulic model was calibrated using the recorded flood level in November 1987 event near Lee Street, Frys Creek Bridge and Markwell Road Bridge The simulated peak water level, discharge and velocity results for the 1% and 5% AEP events are tabulated in Table 3.1 to Table 3.3.

3.3 Myall River Floodplain Risk Management Study for Bulahdelah, Department of Land and Water Conservation, 2002

The Floodplain Risk Management Study Report was completed for the Great Lakes Shire Council, by the Department of Land and Water Conservation in August 2002 to assess the potential floodplain risk management measures to alleviate the flooding issues caused by mainstream flooding at Bulahdelah (Department of Land and Water Conservation, 2002) The study area covers the floodplains of the Myall River and its main tributaries including Myall River from the Markwell Road Bridge to downstream of the Pacific highway Bridge, Frys Creek from upstream of the Markwell Road Bridge to the Myall River and Crawford River from the confluence with Wild Cattle Creek to the Myall River. The study only investigated the mainstream flooding due to the Myall River and its main tributaries.

The key finds from the report are summarised below:

• The major flood events can have a significant impact on Bulahdelah including significant property damages and danger to personal safety

• A range of flood levels was observed from 5.6 m AHD to 6.1 m AHD from upstream of the Pacific Highway to Lee Street at Bulahdelah. The peak average mainstream flow velocity varied from 3.1 m/s in the vicinity of Lee Street to 1.3 m/s upstream of the confluence of the Crawford River. Peak average overbank velocities on the eastern side of the Myall River is ranging from 0.4 to 0.8 m/s. Flood levels for the 1% and 2% AEP events were similar to the 1897 and 1927 flood levels in this vicinity

• In an extreme event, the flood level is generally in the order of 1.8 m higher than the 1% AEP event.

• In the 1% AEP event, around 45 residential and 7 non-residential properties are subject to inundation above the floor level resulting in the potential flood damage of $1,700,000.

• In an extreme event, the number of residential properties subject to inundation above the floor level is likely to double.

• The main channels of Myall River, Crawford River and Frys Creek are categorised as floodway. The remaining floodplain area at Bulahdelah was categorised as flood storage and some areas as flood fringe.

• This study recommended a voluntary house raising and voluntary purchase scheme, updating the Council’s 1985 Flood Management Policy and associated development controls, installation of a flood warning system with associated emergency management planning, community education and awareness.

The study tabulated the simulated peak water level results for the 5%, 2%, 1% AEP and extreme events reported in Table 3.4 Also, peak flow for the Myall River and tributaries for the 5%, 2%, 1% AEP were presented in Table 3.5

3.4 Myall River Floodplain Risk Management Plan for Bulahdelah, Department of Infrastructure, Planning and Natural Resources, 2003

This report was prepared for the Great Lakes Shire Council, by the Department of Infrastructure, Planning and Natural Resources in August 2003 to assess the potential floodplain risk management measures to alleviate the flooding issues at Bulahdelah (Department of Infrastructure, Planning and Natural Resources, 2003). The flood management measures recommended in this plan tabulated in Table 3.6

Table 3.6 Proposed flood management measures.

1 (Undertaken immediately)

2a (Undertaken as soon as subsided funds are available)

2b

2c

Updating the development controls and S149 certificates

Flood response plan

Flood warning system

Flood education and awareness

2d Ongoing data collection

3 (Undertaken every 5 years) Review of the Floodplain Management Plan (2003).

4 Voluntary house raising of all below 2% AEP flood to 0.5 m above the peak flood level in 1% AEP event.

5 Voluntary purchase of all below 5% AEP flood level that can’t be raised

3.5 Bulahdelah Upgrading the Pacific Highway, Parsons Brinckerhoff, 2004

This report was prepared for the Road and Traffic Authority (RTA), by Parsons Brinckerhoff in November 2004 to assess the flood impact associated with the construction of the new bridge and embankments associated with the Bulahdelah Bypass due to the highway upgrade.

The study utilised the MIKE 11 model prepared for the Myall River Floodplain Risk Management Study for Bulahdelah by the department of Land and Water Conservation. The model was based on surveyed cross-sections and calibrated against 1987 flood event by adjusting the roughness coefficient. Roughness coefficient of 0.09 and 0.06 were applied in the overbanks and in the channel in the vicinity of the bends downstream of the proposed bridge. Moreover, a steady-state one-dimensional backwater analysis software HEC-RAS was used to assess the impact of the proposed structures crossing the Myall River and floodplain.

The

study investigated the impact of several bridge options on flood flows in the Myall River

The report documented the simulated water level for the adopted option including a 192 m bridge opening, approximately 204 m2 of culvert openings in the embankment of the proposed Upgrade and 23 m2 of culvert openings in the embankment of the existing highway

3.6 Lower Myall River and Myall Lakes Flood Study, BMT, 2015

This report was completed for the Great Lakes Council by BMT in June 2015 to define the existing flood behaviour in the lake and river and establish the basis for subsequent floodplain management measures (BMT, 2015).

The study documented that the lower Myall River catchment covers an area of approximately 900 km2 and is located north of Port Stephens in the Great Lakes Council Local Government Area (LGA). Myall River is the major contributor to the lake system which drains an area of approximately445 km2 and enters the lake system at the north-western side of the Broadwater. The Lake system is drained by the 28 km long Lower Myall River which exits the Broadwater at Tamboy and drains into Port Stephens at Corrie Island via Corrie Creek and Paddy Marrs Inlet.

The study utilised RAFTS-XP hydrologic model with 61 sub-catchments to produce the discharge hydrographs. A two-dimensional hydraulic model of TUFLOW was established to study the flood behaviour across the study area. The TUFLOW model was developed using a 20 m grid to represent the Lower Myall River channel and floodplain and a 50 m grid to represent the Lakes and Myall channel and floodplain. The upstream boundary was defined as flow versus time inputs at major sub-catchments inflow points and along the modelled watercourses. The downstream boundary was modelled corresponding to the tidal water level of Port Stephens.

Myall Lake system and Lower Myall River is flooded by a combination of fluvial and ocean storm events. The following historical floods and their associated peak water levels were identified by MHL (1993) and are based on PWD (1980):

• 1890s - 3.7 m AHD (Myall Lakes);

• 1927 - 2.7 to 3.2 m AHD (Myall Lakes);

• 1963 - 2.2 m AHD (Tamboy, Myall Lakes);

• March 1977 - 1.2 m AHD (Bombah Point, Myall Lakes);

• March 1978 - 1.31 m AHD (Bombah Point, Myall Lakes); and

• May 1978 - 1.3 m AHD (Bombah Point, Myall Lakes).

Inspection of the Bombah Point water level gauge from July 2001 to May 2012 indicates several other significant flood events as presented in Table 3 7

* This is an estimated value as discussed in Section 5.5.1.2 of Lower Myall River and Myall Lake Flood Study.

The model was calibrated against July 2011 flood event and validated against May 2003 and April 2008 flood events. During the Flood Study, a significant flood event (in early March 2013) occurred and was used as a validation event. In addition to the four flood events, a tidal calibration exercise was undertaken using data collected by MHL in September 2009. An initial rainfall loss of 15 mm and continuing losses of 0.5 mm/hr were adopted for May 2003, April 2008 and July 2011 floods and the design floods. For the 2013 flood, an initial loss of 0 mm and a continuing loss of 0.1 mm/hr were adopted due to considerable rainfall on the 23rd February 2013.

Lake water level at the start of the July 2011 event is approximately 0.5 m AHD with a peak lake level of 1.27 m AHD occurring on the 25th July 2011. Lake water level at the start of the event on the 25th May is 0.78 m AHD with a peak lake level of 1.4 m AHD occurring on the 29th May. Lake level at the start of the event on the 18th April 2008 is 0.25 m AHD with a peak lake level of approximately 1.45 m AHD occurring on the 26th April 2008. Lake water level at the start of the event is approximately 0.8 m AHD with a peak lake level of 1.75 m AHD occurring on the 4th March 2013. The Bulahdelah water level gauge on the Myall River just downstream of the Pacific Highway Bridge is able to provide information on catchment response and at this location, the river water level is responsive to rainfall intensity, though is also affected by a tailwater from the Lakes.

Initial water levels in the Myall Lakes system for design flood events have been derived based on an analysis of water level statistics at Bombah Point between July 2001 and April 2012. An initial lake level of 0.5 m AHD was selected (for both catchment and ocean events) in consultation with OEH and GLC and matches the lake level exceeded only 10% of the time. A range of design flood conditions including the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% AEP events and an extreme event (similar to PMF) were modelled and the results of simulated peak flood levels at several locations were documented Table 3.8 and Table 3.9 presented the peak flood levels at Myall Lake and Bombah Broadwater for three scenarios including catchment derived design events, ocean derived design events and combined conditions

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The key finds from the report are summarised below:

• Results indicate that peak flood levels upstream of Monkey Jacket are attributed to catchment derived design flood events, while ocean derived flood events are more significant downstream of Monkey Jacket.

• Coincident ocean and catchment flood events cause a negligible increase in peak levels within the lake; however, depending on the timing of peak tides may increase peak flood levels below Kangaroo Island by up to 0.2 m.

• Impacts of climate change (including increases in rainfall intensity and sea-level rise) on flood risk along the Lower Myall were investigated. The outcomes revealed that an increase in mean sea level due to sea-level rise may pose a significant flood risk problem along the Tea Gardens peninsula, where ground elevations of only 1.5 m AHD are common. Low lying areas in Hawks Nest (with ground elevations below 2.0 m AHD) will also be affected by predicted SLR.

3.7 Flood Emergency Response Plan Bulahdelah Caravan Park, Cardno, 2017

This report was prepared by Cardno in April 2017 on behalf of Mr Gary Ekert and Mrs Kim Ekert of the proposed Bulahdelah Caravan Park This report wasprepared for the development of the Bulahdelah Caravan Park to ensure the flood risk for the visitors and staff is managed appropriately.

The caravan park is located in the middle of the township of Bulahdelah, 90 m upstream of the Old Pacific Highway Bridge. The Caravan Park is located on the banks of the Myall River, opposite the confluence with the Crawford River.

The study tabulated the flood levels in the vicinity of the Bulahdelah Caravan Park site based on Flood Appraisal (1991) and Myall River FRMS (2002) - see Table 3.10. Table 3.10 Flood levels in the vicinity of the Bulahdelah Caravan Park.

Hazard and hydraulic category maps revealed that most of the site lies within the high hazard area highlighting that occupation of the site poses a significant flood risk.

The study analysed the Bulahdelah Flood Appraisal outcomes on the 5% and 1% AEP water level time series at the Bulahdelah Bridge just downstream of the Caravan Park The analysis determined the average rate of 0.5 m/hr of water level rise for the 5% and 1% AEP design events Due to the insufficient data documented in the previous studies on extreme events, the study adopted an extreme rate of rising of 2 m/hr which is four times greater than the rate of rising of the 5% and 1% AEP design events

The study concluded that:

• There is sufficient time and a safe route to evacuate both the Stage 1 and 2 development of the Bulahdelah Caravan Park based on active monitoring of river water levels

• Raising of the proposed internal road connecting Stage 1 and 2 sites, to a finished level of 4.0 m AHD

• Adoption of the route east of the site through the main access onto Stroud Street, south to Bulahdelah Way and east to Meade Street was recommended.

• An evacuation procedure was recommended to be implemented based on a range of factors discussed in detail in the report.

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