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Floodplain Risk Management Study

Sans Souci Drainage Catchments Floodplain Risk Management Study

This report covers the Floodplain Risk Management Study for the Sans Souci Drainage Catchments No. 1, 2 and 3 in the City of Rockdale. It forms the second stage of a Floodplain Risk Management Plan (FRMP) for the three catchments. The objectives of this study include:

  • to reduce the flood hazard and risk to people and property in the existing community in a cost effective and sustainable way and to ensure future development is controlled in a manner consistent with the flood hazard and risk
  • to reduce private and public losses due to flooding.
  • to protect and where possible enhance the river and floodplain environment.
  • to be consistent with the objectives of relevant State policies.
  • to ensure that the floodplain management plan can be fully integrated with Council’s existing corporate, business and strategic plans, meets Council’s obligations under the Local Government Act, 1993, and has the support of the local community.
  • to ensure that actions arising out of the management plan are sustainable in social, environmental, ecological and economic terms.
  • ensure that the floodplain management plan can be fully integrated with the local emergency management plan and other relevant catchment management plans.
  • program for implementation, taking account of the priorities, staging, funding, responsibilities, constraints and monitoring.

The Floodplain Risk Management Study was prepared in accordance with principles set out in the NSW Government's “Floodplain Management Manual (January 2001)”.

Piped Drainage and Overland Flow paths

A DRAINS urban drainage model was set up on computer to model the existing piped stormwater drainage system in each catchment. DRAINS is a development of an earlier urban drainage model known as ILSAX. The model was set up with drainage system attribute information supplied by Council and catchment parameters deemed appropriate for the catchments based on previous studies in the area.

Council provided a database of past flooding reports. These data mainly cover residents' reports of flooding of habitable areas, non-habitable building areas, or yards. Further information was obtained in the second Questionnaire (see Appendix B).

Within the limits of the supplied information, the DRAINS model was adjusted to model the flood storage and flood depths at each of the trapped low points where flood storage is known to occur. The results of the preliminary model were checked against the historic storm flooding records.

Catchment Hydrology

The MIKE-11 model was used for hydraulic study of the main drains. This model is considered to be more suitable than DRAINS for modelling open channels, particularly where they are subject to tailwater effects, flat grades and large overbank storages. However the transfer of flow data from DRAINS to MIKE-11 was a difficult and time-consuming exercise.

Main Drainage Channels

The MIKE-11 model was used for hydraulic study of the main drains and flow paths, as described in Appendix D. MIKE-11 is an unsteady-state quasi one-dimensional model which is widely used for flood studies. This model was selected in order to accurately model the effects of floodplain storage and high ocean tailwater levels.

Model runs were initially undertaken to calibrate the model for the 1975 flood. The calibration process is described in Appendix D. The calibrated model was then re-run for the 1% AEP and PMF events with existing catchment conditions. The XP-RAFTS and MIKE-11 models were also used to calculate flood profiles for other floods with 50%, 20%, 10%, 5% and 2% AEP for flood damage analysis. The resulting flood profiles on the main channels are shown in Figure 8.
Flood profiles were not prepared for the small bayside catchments as they are dominated by ocean flooding.

A summary map of existing conditions was prepared indicating 1% AEP flood levels and estimated flood extent based on available survey data. The map is shown in Figure 9. Digital data as well as a larger scale set of A1 size maps has been supplied separately to Council.

Additional Information

Field Value
Title Sans Souci Drainage Catchments Floodplain Risk Management Study
Publication Date 1 February 2005
Themes Land and Resource Management
Spatial Extent
Map data © OpenStreetMap contributors
Council/LGA Bayside Council
Author/ Prepared by Cardno Willing (NSW) Pty Ltd
Publish date 5 July 2017
Update date 26 July 2017
River Basin 213 - Sydney Coast and Georges River
Place Name Sans Souci
Approval State Approved
Submitted for approval 5 July 2017
Submitted by John Silk
Approved 26 July 2017
Approved by plattns
Data Comment

Report only; no other data. Legacy data; may have been superseded.

Identifier efd76c21-0cec-4f67-8c55-e6ebbfb36ecc