Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 79
Narellan Creek Catchment
Flood Study
The difference mapping shows that the adopted Nepean River water level can have a
significant impact on flood extents and levels across the downstream sections of the study
area. This includes flood level impacts that extend as far upstream as Narellan
Road/Harrington Park Lakes. However, across the urban/overland sections of the
catchment, the flood level impacts are negligible.
Overall, it can be concluded that flood levels across the downstream sections of the
catchment are sensitive to changes in the adopted Nepean River level. However, flood level
impacts across the upstream sections of the catchment are predicted to be negligible.
6.4
Australian Rainfall & Runoff 1987
Flood studies across the Camden LGA over the last three decades have been prepared in
accordance with 8Australian Rainfall and Runoff – A Guide to Flood Estimation9 (Engineers
Australia, 1987) (ARR1987). The current study has been prepared in accordance with the
updated ARR2019, which reflects application of more thorough hydrologic procedures as
well as an additional 30 years of rainfall information. Nevertheless, it was considered
important to understand how results produced based upon ARR2019 may differ from those
generated using ARR1987. Therefore, an additional sensitivity assessment was completed
to confirm the impact that the revised hydrologic procedures may have on design flood
behaviour across the study area.
This involved re-running the 5% AEP and 1% AEP storms based on ARR1987 through the
WBNM model to confirm the critical storm durations and then running the critical storms
through the TUFLOW model. Difference mapping was then prepared by subtracting the
ARR2019 water level results from the ARR1987 water level results. The difference mapping
is presented in Appendix K.
The difference mapping presented in Appendix K also shows that ARR1987 is predicted to
produce higher flood levels relative to ARR2016. Along major watercourse, the ARR1987
levels are typically a minimum of 0.2 metres higher than ARR2019 levels. Localised
increases of more than 0.5 metres are predicted in the vicinity of major hydraulic controls
(e.g., roadway embankments/culverts) and detention basins. Across the urban overland
sections of the catchment, the flood level difference are most commonly less than
0.1 metres.
Accordingly, there are some notable differences between flood behaviour defined under
ARR1987 versus ARR2019. However, ARR2019 takes advantage of a greater amount of
historic rainfall information and employs that latest available research in defining the design
flood estimates. Therefore, it is considered that the flood estimates defined under ARR2019
are reasonable and improve upon the flood estimates provided by ARR1987.
6.5
Climate Change Analysis
Climate change refers to a significant and lasting change in weather patterns arising from
both natural and human induced processes. The former Office of Environment and
Heritage9s 'Practical Consideration of Climate Change' states that climate change is expected
to have adverse impacts on rainfall intensities in the future.
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