Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 52
5
DESIGN FLOOD ESTIMATION
5.1
General
Design floods are hypothetical floods that are commonly used for floodplain management
investigations. Design floods are based on statistical analysis of rainfall and flood records
and are typically defined by their probability of exceedance. This is often expressed as an
Annual Exceedance Probability (AEP).
The AEP of a flood flow or level or depth at a particular location is the probability that the
flood flow or level or depth will be equalled or exceeded in any one year. For example, a 1%
AEP flood is the best estimate of a flood that has a 1% chance of being equalled or exceeded
in any one year.
Design floods are typically estimated by applying design rainfall to the hydrologic model to
calculate the design rainfall excess. The hydraulic model is then used to route the flow
hydrographs produced by the hydrologic model across the catchment and calculate key
flood variables such as design flood levels, depths, and velocities.
The procedures employed in deriving the design flood estimates for the Narellan Creek
catchment are outlined in the following sections.
5.2
Hydrology
5.2.1
Rainfall
Point design rainfall depths were downloaded from the Bureau of Meteorology9s IFD
webpage. The rainfall was downloaded at the centroid of the catchment (-34.0445oS,
150.7456 oE). A copy of the point design rainfall depths is enclosed in Appendix H. Also
included in Appendix H is the corresponding ARR1987 rainfall. This shows that the ARR2019
rainfall is most commonly lower than the ARR1987 rainfall.
As part of the flood study, it was also necessary to define flood characteristics for the
Probable Maximum Flood (PMF). The PMF is the largest flood that could conceivably occur
across a particular catchment.
The PMF is estimated by routing the Probable Maximum Precipitation (PMP) through the
WBNM model. The PMP is defined as the greatest depth of rainfall that is meteorologically
possible at a specific location.
PMP depths were derived for a range of storm durations up to and including the 6-hour
event based on procedures set out in the Bureau of Meteorology's 8Generalised Short
Duration Method9 (GSDM) (Bureau of Meteorology, 2003). The GSDM PMP calculations are
included in Appendix H.
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