4 minute read
Photo 2: Flood Photographs provided by Canada Bay Council
32-36 Burlington Road, Homebush Lower Parramatta River Flood Study
Powells Creek at Underwood Street Site Flood Study
Advertisement
B Lysenko Strathfield Ck February 1994 Determined design flood levels. 9
Willing & Partners Powells Creek to approximately Pomeroy Street February 1986 Determined design flood levels. 10
Tierney & Partners Powells Creek at Pomeroy Street November 1993 Determined design flood levels. 11
The references listed in Table 1 are of little value in the current study as they provide little historical data, and the results cannot be easily compared. The 2016 Powells Creek Revised Flood Study (Reference 2), however, is a comparable study to the current one and extensive use has been made of the data and results which were originally contained in the prior 1998 Powells Creek Flood Study (Reference 1).
The City of Canada Bay commissioned Jacobs to undertake the 2015 Concord West Precinct Master Plan Flood Study (Reference 3), however this only covered the City of Canada Bay LGA.
The 1998 Powells Creek Flood Study was undertaken under the NSW Government's Floodplain Management Program and used best practice techniques available at the time. A field survey was undertaken to provide approximately 100 cross sections of the creek channel as well as to collect historical flood height data. Some of the cross-section data have been used in the current study and the historical flood height data is provided in Section 2.10.
A comprehensive data search was undertaken including: • a review of previous studies. • interviews with residents. • discussions with Council Officers. • contact with SWC, the then Roads & Traffic Authority, the then State Rail Authority, the then Department of Land & Water Conservation and the UNSW. • review of aerial photographs. • provision of a questionnaire and review of all previous questionnaires. • obtaining height and rainfall data from the stream and rainfall gauges operated by the UNSW and SWC.
An ILSAX hydrologic model of the entire Powells and Saleyards Creeks catchment was constructed using ILSAX files from some of the studies listed in Table 1. Inflows from ILSAX were then input into the 1D HEC-RAS hydraulic model which determined flood levels and velocities. Flood extents were not defined; however, this was subsequently undertaken using the peak levels and ALS for the Strathfield LGA.
The ILSAX model was calibrated to the events of 3rd February, 7th February, 10th February, 17th February and 18th March 1990 using rainfall from two pluviometers at St Sabina College and at the Elva Street gauge. Calibration to the Elva Street gauge for the January 1996 event could not be undertaken as the gauge malfunctioned. The results are summarised in the 2016 Powells
Creek Revised Flood Study (Reference 2).
The study concluded that accuracy of the design flood data depended upon several factors including. • quality of the survey data. • downstream boundary conditions. • accuracy of design rainfall data. • ability of the models to accurately represent the channel hydraulics. • quantity and quality of available historical data.
The main factors affecting the accuracy of the design data were the ability of the models to simulate the channel hydraulics and the quantity and quality of the historical data. Based upon the above considerations the accuracy of the design flood levels was ±0.4 m. This could be improved if further calibration of the models to future flood events was undertaken.
This study provided a significant upgrade to the prior 1998 Powells Creek Flood Study (Reference 1). Its purpose was to define mainstream and overland (where there is no defined channel) flood behaviour under historical and existing floodplain conditions in the study area while addressing possible future variation in flood behaviour due to climate change and provide information for its management. The main features of this study compared to the prior 1998 study were: • The same historical rainfall and flood data was relied upon as there had been no floods of significance since 1998. • The modelling approach was similar, adopting flood frequency analysis of the historical flood record at the Elva Street gauge and incorporating a runoff routing approach to define flood levels, extents, and velocities across the entire Powells Creek catchment. • The flood frequency analysis (based on the same flood record) was re-done using updated approaches which analysed several different distribution procedures. • The 1998 study relied upon cross section data obtained from field survey to define the topography with the 2016 study relying upon Airborne Laser Scanning (ALS). ALS only became available since approximately the year 2000 and provides ground levels at approximately 1m spacing. It therefore provides a much more detailed and accurate definition of the topography, though cross section data was still used for definition of the lined channels. • In the 1998 study a HEC-RAS 1 dimensional (1D) computer model based on cross section data was adopted as the hydraulic model to determine design flood levels. In the 2016 study the 2D TUFLOW hydraulic model was adopted which relied upon defining the topography using a 2m-by-2m grid based on the ALS data. This change represents a significant upgrade to the modelling approach as it ensures accurate consideration of both the temporary floodplain storage and conveyance characteristics of the catchment. It also ensures more accurate definition of flow paths, velocities, flood depths and flood extents across the entire floodplain, rather than just at cross sections as in the 1998 study. • The ILSAX hydrologic model was adopted the 1998 study, and this was converted to a DRAINS hydrologic model for the 2016 study. However, as DRAINS uses the same basic hydrologic approach as ILSAX this change did not result in a significant change to the
