Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 36
Narellan Creek Catchment
Flood Study
3.2.5
Detention Basins
The catchment includes a number of water bodies, dams and formal detention basins.
These features have the potential to attenuate downstream flows from the catchment by
temporarily storing runoff.
The representation of detention basins in WBNM requires the storage and basin outlet
characteristics of the basin to be defined. The storage characteristics were defined using a
water level versus storage volume relationship that was extracted from CatchmentSIM
based upon the LiDAR DEM. For detention areas with a permanent water body (e.g., Annan
Lake), it was assumed that only storage volume was provided above the permanent water
level.
The outflow characteristics were specified by defining the elevation and dimensions of the
overflow weir as well as the details of any outlet culverts/pipes (e.g., invert elevation, size
and number of conduits).
A total of 33 detention areas were included in the WBNM model. The location of all
detention areas that were represented is shown in Figure 6.
3.3
TUFLOW Model Development
3.3.1
Model Extent
A 2-dimensional hydraulic computer model of the catchment was developed using the
TUFLOW Highly Parallelised Computer (HPC) software (version 2023-03-AA). TUFLOW is a
fully dynamic, 1D/2D finite difference model developed by BMT WBM (2023). It was
selected for the project as it provides the following advantages:
The software was developed through collaborative efforts with universities and
research organisations to ensure it is scientifically of a high standard and is thoroughly
benchmarked (BMT, 2023).
The software is used extensively across Australia and overseas to simulate mainstream
and overland flow behaviour and, therefore, has a strong pedigree of proven
performance in similar studies.
The HPC version of the software takes advantage of the additional processing power of
graphics cards to provide expedited simulation times. This allows the model
simulations to be completed within short time frames relative to previous versions of
the software.
It is fully dynamic. That is, it simulates the time variation in flood hydraulics along the
river and across the floodplain. This is particularly important for evaluation of timerelated flood behaviour with implications for emergency management, such as the
amount of time before evacuation routes are cut and how long they would remain
submerged.
It allows representation of the stormwater system as well as the movement of overland
flow once the capacity of the stormwater system is exceeded.
The extent of the TUFLOW model area is shown in Figure 7. As shown in Figure 7, the
TUFLOW model extends across the full extent of the catchment extending downstream to
the Nepean River and covers a total area of 36 km2.
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