Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 33
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COMPUTER FLOOD MODELS
3.1
General
Computer models are the most common method of simulating flood behaviour through a
particular area of interest. They can be used to predict flood characteristics such as peak
discharges, flood level and flow velocity.
Two computer models were developed to simulate flood behaviour across the Narellan
Creek catchment:
A hydrologic model was developed to simulate the transformation of rainfall into runoff
across the catchment. The hydrologic model was developed using the WBNM software;
A hydraulic model was developed to simulate how the runoff from the hydrologic
model would be distributed/move across the catchment. The hydraulic model was
developed using the TUFLOW software.
The following sections describe the development process for the hydrologic and hydraulic
models.
3.2 WBNM Model Development
3.2.1
Subcatchment Delineation
The Narellan Creek catchment was subdivided into a number of smaller subcatchments to
provide a detailed description of the spatial variation in hydrologic characteristics across the
catchment. The subcatchments were delineated with the assistance of the CatchmentSIM
software (Catchment Simulation Solutions, 2018) using a 2 metre Digital Elevation Model
(DEM) that was developed based on the 2023 LiDAR dataset described in Section 2.4.1.
A major goal of the study was to define overland flood behaviour. Therefore, it was
necessary for the subcatchment delineation to be sufficiently detailed to ensure all potential
overland flow paths were identified. Based on experience in similar overland flood studies
in the Sydney metropolitan area, significant overland water depths (i.e., >0.1m) are
frequently encountered once the contributing catchment area reaches about 2 hectares.
Therefore, the subcatchment delineation targeted incorporating subcatchment outlets once
the contributing catchment area reached approximately 1 to 2 hectares to ensure all
potential overland flow paths were captured.
A total of 1,538 subcatchments were ultimately delineated across the catchment. The final
subcatchment layout is presented in Figure 6.
3.2.2
Subcatchment Parameterisation
Key hydrologic properties including area were calculated automatically for each
subcatchment using CatchmentSIM.
Estimates of impervious areas within each
subcatchment were derived from the remote sensing land use information shown in
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