Punchbowl Catchment Flood Study

Study Area

The study area is located in south west Sydney and includes parts of the suburbs of Punchbowl,
Mount Lewis and Greenacre. It is an urban catchment that drains through a stormwater pipe network
and the Sydney Water open concrete channel drainage system in a north-easterly direction towards
the Cooks River in the neighbouring Local Government Area (LGA) of Strathfield.
It is important to note that the Punchbowl catchment does not represent the entire (hydrological)
catchment to Juno Parade (at the Bankstown LGA boundary). The entire stormwater catchment is
located in two LGAs; the western part of the catchment is situated in Bankstown City Council and the
eastern part is situated in Canterbury City Council, covering parts of the suburbs of Roselands, Wiley
Park and Lakemba. This report focuses on the study area only, although the model was developed
for the entire catchment.

For the remainder of this document and for clarity, the Punchbowl catchment in this report refers to
the study area and is wholly located within Bankstown LGA while the entire catchment will be referred
as the Punchbowl/Roselands catchment.

Outcomes from the Study

The main outcomes from this study include:
<ul>
<li>A database of all Council drainage assets within the study area;</li>
<li>Establishment of a computer model capable of assessing flood behaviour;</li>
<li>Information on flood behaviour under existing catchment conditions;</li>
<li>Indicative information on drainage asset performance for various storm events;</li>
<li>A property database including surveyed floor levels, flood levels and the resulting floor level
immunity, expressed in the magnitude of a storm event;</li>
<li>Predicted change in flood levels due to the potential impacts of climate change; and</li>
<li>A model that can be used to assess flood mitigation options and future development proposals.</li>
</ul>

Database of Drainage Assets

All drainage asset data collected by Council for the study has been incorporated into a GIS database.
This allows the data to be spatially represented across the study area and easily retrieved as
required. Information in the database includes data for some 510 stormwater pits and 610 stormwater
drainage pipes or culverts. The database also contains model results, including information for each
pipe on the design event at which it is running full.

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

Computer Modelling

A numerical computer model was developed for the catchment to simulate flood behaviour, using the
software TUFLOW. Surface flows are represented in the model through a two-dimensional grid
covering the entire study area. The stormwater pit and pipe network is included as one-dimensional
elements within this grid. The adopted grid size for this study is 2m, whereas the flood studies within
Bankstown undertaken prior to 2012 utilise a coarser grid size of 5m. The new grid size has been
adopted in line with ongoing improvements in computer technology (in particular speed, memory and
storage) to provide higher resolution of the floodplain whilst allowing for manageable model
simulation times and file sizes. Full details of the modelling approach, modelling parameters and
other assumptions are outlined in the Punchbowl Catchment Flood Modelling Report, which is
included as Appendix C.

Property Database

This flood study also includes the development of a GIS property database, utilising floor level data
collected by Council and comparing these to flood levels for various design flood events. This
comparison provides a dataset on floor level immunity, which is a very useful tool for assessing the
existing flood risk. Additional information such as property ground levels are also provided as part of
the property database. A summary of the property database is presented in Figure 5-1.

Existing Flood Behaviour

Design flood behaviour has been computed for a range of floods, ranging from relatively frequent
events to more extreme floods.

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 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.
Table ES- 1 summarises the number of properties at risk from above floor level flooding in the
Punchbowl stormwater catchment within Bankstown LGA.

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

It is understood that once the Flood Study has been adopted by Council, the GIS information
resulting from this Flood Study will be used for floodplain management purposes, including
assessment of flood-related development constraints on properties. Potential flood mitigation options
will be assessed in the Floodplain Risk Management Study phase of the floodplain risk management
process.

Tags: Greenacre, Mount Lewis, Punchbowl, hydraulic model
Organization: Canterbury-Bankstown Council