The Cooks River and Coxs Creek Flood Study was initiated as a result of flooding of roads and residential areas, most recently in August 1986, February 1990 and March 1991. This report has been prepared by WMAwater for Strathfield Municipal Council and the Department of Environment, Climate Change and Water (DECCW) under the guidance of Council's floodplain management committee. A Draft of this Flood Study was placed on public exhibition in June 2010 but no significant feedback was obtained from the public.
The specific aims of the Cooks River and Coxs Creek Flood Study are to: define flood behaviour in terms of flood levels, depths, velocities, flows and extents within the Cooks River and Coxs Creek catchment study area, prepare flood hazard and flood extent mapping, prepare suitable models of the catchment and floodplain for use in a subsequent Floodplain Risk Management Study. to assess the adequacy and capacity of Council’s existing pipe network and quantify overland flows, to consider the potential effects of a climate change induced increase in design rainfall intensities.
Description of Creek System (Section 2 of report): The study area lies on both sides of the Cooks River and Coxs Creek within the Strathfield Municipal Council Local Government Area (LGA). Runoff from the Bankstown and Canterbury LGA enters the study area to the west of Roberts Road and south of Juno Parade. Both the Cooks River and the Coxs Creek channels are concrete lined except where the Cooks River enters Strathfield Golf Course. Upstream of the golf course the river reduces to become a small semi natural channel. Upstream of the Enfield Marshalling Yards Coxs Creek continues as an open concrete lined channel for a few kilometres. The majority of the study area is residential developments comprising detached homes which are drained by Council's stormwater drainage system.
The land usage within the study area comprises of a mix of residential, industrial and commercial developments together with significant amounts of open space (parks, Strathfield Golf Course, Freshwater Park) and the Enfield Marshalling Yards at Strathfield South/Greenacre.
Past Flooding Problems (Sections 2 and 3 of report): A number of past reports have been prepared for Council to identify, quantify and manage existing flood problem areas (High Street, Augusta Street and others). As a result Council has undertaken works to reduce the problem. Council has also introduced development controls to ensure that adequate studies are undertaken to ensure new development are constructed in a manner compatible with the flood risk and will not exacerbate an existing flood problem.
Available Data (Section 3 of report): The Cooks River Flood Study was completed in February 2009 (Report prepared for Sydney Water) and provides design flood levels along the Cooks River to Centenary Drive and Coxs Creek to downstream of the Enfield Marshalling Yards.
Airborne Laser Scanning (ALS) survey (provides a very accurate and detailed definition of the ground surface) was available for the entire study area and was used to determine catchment areas as well as to define the topography for the hydraulic models. Council provided details on the pit and pipe network within Strathfield and the adjoining Bankstown Council parts of the study area.
Previous reports were available to describe historical flooding in the catchment and further data was obtained as part of the present study through distribution of a Newsletter and Questionnaire. Historical rainfall was also collected.
Approach (Section 4 of report) : A DRAINS hydrologic model (converts rainfall to runoff) was established for the study area to provide inflow hydrographs to the TUFLOW 2D hydraulic model which was used to determine design flood levels, depths, velocities and extents.
Calibration to Historical Flood Levels (Section 6.4 of report): Due to the lack of available data a rigorous calibration (matching of actual flood height data to that produced from the models and so verifying their accuracy) of the TUFLOW model could not be undertaken. This situation is typical of all urban catchments where there are limited flood records available (no instruments measuring water level and as the flood happens very quickly residents may not actually see the floodwaters – thus reliance has to be made on debris marks or such. Questionnaires were sent out as part of this study to allow residents to advise of past flood events and data). However a limited calibration was undertaken based on recorded flood levels. This generally indicates that the results from TUFLOW are similar to historical data. However immediately following the next major flood both rainfall and flood level data should be collected and used to further verify the results.
Determination of Design Flood Flows and Levels (Section 7 of report): Design rainfall data from Strathfield Council and design rainfall patterns from Australian Rainfall and Runoff were obtained and input to the modelling procedure to obtain the design flood data. Flooding is a combination of runoff from the study area catchment as well as from high levels in the Cooks River and Coxs Creek. Detailed mapping was undertaken for the full range of design events (2, 5, 10, 20, 100 and 200 year ARI design storms and the Probable Maximum Flood) with the results provided as maps showing: Cooks River and Coxs Creek flooding taken from the Sydney Water Cooks River Flood
Study of 2009 (Figures 11a to d), Flood contours, extents and depths from the study area (Figures 13a to g), Flood profiles (Figures 14a to 14d), Hazard and hydraulic classification (Figures 15a to 15g).
Accuracy of Design Flood Levels and Extents (Sections 7.3 and 7.4): Sensitivity analyses (to assess the effects of changing various model parameters) were undertaken of both the DRAINS and TUFLOW model results. Part of this analysis was to assess the effects of possible increases in design rainfall (10%, 20% and 30%) due to climate change. The results indicate that the average increase (based on a comparison of the peak level at the inlet pits) in the 100 year ARI event is:
- low level rainfall increase of 10% = +0.03m,
- medium level rainfall increase of 20% = +0.05m,
- high level rainfall increase of 30% = +0.06m.
However the results do show a significant variation between locations.
Due to the limited quantity and quality of the calibration data available and in view of the sensitivity analyses, it is estimated that the order of accuracy of the design flood levels is up to ±0.5m, however in many places the order of accuracy will be ±0.3m. The accuracy of the flood extent largely depends on the slope of the land and may vary from of the order of 1m to 10m. These orders of accuracy are typical of such studies and can only be improved upon with additional observed flood data to refine the model calibration and more detailed and accurate definition of the terrain.
