Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 80
Narellan Creek Catchment
Flood Study
It was considered important to provide an assessment of the potential impact that climate
change induced rainfall intensity increases may have on the current flood risk across the
study area. In this regard, the results of the 0.5% AEP and 0.2% AEP flood were compared to
the results from the 1% AEP flood to gain an appreciation of the impacts of the rainfall
increases. The 0.5% AEP rainfall reflects a 9% average increase relative to current 1% AEP
rainfall intensities, while the 0.2% AEP rainfall reflects a 23% increase relative to current 1%
AEP rainfall intensities. Based on information contained in the NSW Government9s Flood
Risk Mangement Guideline: Understanding and Managing Flood Risk (2023), this is a little
lower than the RCP4.5 2090 projection (12.7% increase in rainfall) and the RCP8.5 2090
rainfall projection (26% increase in rainfall).
However, it provides an indicative
understanding of the sensitivity of existing 1% AEP results to rainfall increases.
Flood level difference mapping was prepared to quantify the impacts that a 9% and 23%
increase in rainfall would have on current 1% AEP flood level estimates. The difference
mapping was prepared by subtracting the peak 1% AEP flood levels from the 0.5% and 0.2%
AEP flood levels. The difference mapping is presented in Plate 13 and Plate 14.
The difference maps in Plate 13 and Plate 14 show that the rainfall increases will increase
current 1% AEP flood level estimates across most of the catchment. A 8% increase in rainfall
is predicted to increase 1% AEP flood levels by at least 0.1 metres along most watercourses.
The flood level increases across urban overland flowpaths are most commonly less than
0.05 metres.
Clause 5.21 of the Camden Local Environmental Plan 2010 includes the need to assess the
impacts of climate change. In this regard, the results of the climate change assessment
should be considered by Council when assessing development applications, particularly in
areas located near to major watercourses. If the projected climate change water level
increases exceed the nominal freeboard that Council applies, a freeboard can be considered
(e.g., adopting a 0.5 metres freeboard in overland flow areas rather than a 0.3 metre
freeboard).
The 23% increase in rainfall is predicted to increase existing 1% AEP flood levels by more
than 0.1 metres along main watercourse. Flood level increases along urban flow paths are
typically contained below 0.1 metres (along roads, the differences are generally less than
0.05 metres).
Accordingly, the outcomes of the climate change assessment show that increases in rainfall
associated with climate change have the potential to produce a notable increase in the
severity of flooding across the catchment. However, across most of the urbanise sections of
the catchment, the impact is less significant.
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