Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 7
Narellan Creek Catchment
Flood Study
The responses to the questionnaire showed that around 30% of the respondents had been
impacted by flooding. The most common reported flood impacts were roadways being cut
by floodwaters as well as inundation of front and back yards.
Computer Flood Models
Flood behaviour across the catchment was defined using two computer models that were
developed for the study:
A hydrologic model of the catchment was developed using the WBNM software. The
hydrologic model was used to simulate the transformation of rainfall into runoff and
generate discharge (flow) hydrographs at various locations across the catchment.
A hydraulic computer model of the catchment (including the creek and stormwater
system) was developed using the TUFLOW software. The hydraulic model takes the
discharge hydrographs produced by the hydrologic model and simulates how that flow
would move and be distributed across the catchment.
The WBNM and TUFLOW models were calibrated using historic rainfall and flooding
reports/photos that were provided by the community. The floods that were selected for
calibration include events that occurred in 2016 as well as March and July 2022. The
outcomes of the calibration showed that the computer models were producing reliable
reproductions of each historic flood.
Design Flood Simulations
The calibrated models were used to simulate the design 50%, 20% AEP, 10% AEP, 5% AEP,
2% AEP, 1% AEP, 0.5% AEP and 0.2% AEP floods. The Probable Maximum Flood (PMF) was
also simulated (i.e., the largest flood that could occur). The results of each design flood
were extracted, and figures were prepared to display the results. The figures are provided in
Volume 2 and include:
Floodwater Depths: Figures 14 to 22
Floodwater Velocity: Figures 32 to 40
Flood Hazard: Figures 42 to 46 (flood hazard mapping shows the potential impact that
floodwaters are likely to have on people, vehicles and buildings in the catchment)
Hydraulic Category: Figures 53 to 56 (hydraulic category mapping shows areas that
should preserved for the conveyance and storage of floodwaters)
Analysis of Results
The flood mapping shows that during more frequent floods (e.g., up to and including the
10% AEP event), overland flooding is characterised by shallow water depths that are mostly
contained to roads.
During moderate to larger floods (i.e., up to and including the 1% AEP event), water depths
along roads become more significant. A significant number of roads (i.e., 165 road
segments) are predicted to experience water depths of at least 0.3 metres at the peak of
the1% AEP flood, which would typically be sufficient to prevent vehicular movement (i.e.,
many roads would be cut by water).
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