East Hills Stormwater Catchment Flood Study

Study Area

The study area is located within the Sydney suburb of East Hills. The study area is a small
urban catchment centred on the East Hills Railway station, which drains through a
stormwater pipe network to the Georges River. Much of the study area is also affected by
flooding from the Georges River, where a series of blockwall “finger levees” have been
constructed to reduce flood velocities and the hazard experienced to the occupants of
riverfront dwellings. The catchment is wholly within the Bankstown local government area.

Report Structure

This report is divided into two parts. The first part provides background on the study and
further discussion of the available data, modelling approach and results from the flood model
that was established to analyse flooding within the catchment. The second part is a technical
report that provides additional detail concerning the flood model, model results and flood
mapping, which is included as an appendix.

Outcomes from the Study

Outcomes from this study include:
(i) a database of all drainage assets within the study area;
(ii) establishment of a computer model capable of assessing flood behaviour;
(iii) information on flood behaviour under existing catchment conditions; and
(iv) a model that can be used to assess flood mitigation options and future development
proposals.

Database of Drainage Assets

All data collected for the study has been included within a GIS database. This allows the
data to be spatially represented across the study area and allows for easy retrieval of the
data as required. Information in the database includes data for some 88 stormwater pits and
83 stormwater drainage pipelines.

Other catchment data, including aerial photography, property cadastre, building footprints
and the terrain surface (based on ALS survey) is also represented in the database.

Computer Modelling

A numerical computer model was developed for the catchment to simulate flood behaviour,
using the computer program known as TUFLOW. Surface flows are represented in the
model through a 2-dimensional grid covering the entire study area. The stormwater pipe
network is included as 1-dimensional elements within this grid.

Full details of the modelling approach, modelling parameters and other assumptions are
included in the Flood Model Report, which is included in Appendix A.

Existing Flood Behaviour

Design flood behaviour has been computed for a range of floods, ranging from relatively
frequent events to more extreme floods, under existing (2007) catchment conditions.
The model produces a grid of results over the study area providing data on flood levels,
flood depths and flood velocities. Flood level contours have also been prepared showing
contours of equal flood heights throughout the study area. This data is provided digitally and
can be overlaid on base mapping such as aerial photos and cadastral plans showing
property boundaries.

All flood model results have been provided to Council for incorporation into their GIS
computer system. Much of this information is also included as A4 sized plans included in
Appendix A.

It is intended to develop a database of properties that are at risk of being affected by
flooding as part of the floodplain management study, which is the next phase of the
investigations. This will define the problem areas within the catchment and allow an
assessment of potential flood mitigation options.

Tags: East Hills, hydraulic model, hydrologic model
Organization: Canterbury-Bankstown Council