Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 88
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CONCLUSION
This report documents the outcomes of investigations that were completed to quantify
mainstream and overland flood behaviour across the Narellan Creek catchment. It provides
information on design flood discharges, levels, depths and velocities as well as hydraulic and
flood hazard categories for a range of design floods.
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 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. TUFLOW is a two-dimensional
hydraulic software package that takes the discharges hydrographs produced by the
hydrologic model and simulates how that flow would move and be distributed across
the catchment. It can be used to produce a range of important flood information
including floodwater depths and velocities.
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.
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. The following conclusions can be drawn from the results of the
investigation:
Urban overland flooding typically occurs as a result of short, high intensity rainfall
bursts (i.e., storm durations of less than 2 hours). The critical mainstream/creek
flooding typically occurs in response to longer durations of rainfall (i.e., up to 6 hours).
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 or roads are
predicted to experience water depths of at least 0.3 metres, which would typically be
sufficient to prevent vehicular movement). Furthermore, several overland flow paths
are predicted to form, impacting on private property.
During extreme floods (i.e., the PMF), nearly 1,400 road segments would be cut. This
includes more than 300 roads inundated with more than 1 metre of water. This would
result in floodwater extending well beyond the road reserve in many areas resulting in
inundation of adjoining properties.
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