Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 76
Narellan Creek Catchment
Flood Study
modelling and were used to prepare flood level difference mapping which are presented in
Appendix K.
The results of the sensitivity analysis show that the TUFLOW model is relatively insensitive
to changes in continuing loss rates. More specifically, Appendix K shows that only small
(i.e., generally no greater than 0.01m) localised changes in 1% AEP flood levels are predicted
with the modified continuing loss rates.
Therefore, it can be concluded that any uncertainties associated with the adopted
continuing loss rates are unlikely to have a significant impact on the results generated by
the TUFLOW model.
6.2.3
Temporal Pattern
As discussed in Section 5.2.4, ten different temporal patterns were applied to each design
storm frequency and duration. A temporal pattern that provided a peak discharge roughly
in the middle of the range was adopted as part of the 8base9 design flood simulations.
However, it was considered important to gain an understanding of how variations in the
rainfall temporal pattern (i.e., how the rainfall is distributed over time) may impact on the
results generated by the model. Therefore, additional simulations were completed by
adopting a temporal pattern at the upper and lower end of the adopted critical durations.
Appendix J summarises the peak discharges at each subcatchment that are generated using
the temporal patterns listed above. The 8base9 peak discharges are also listed for
comparison.
The comparison presented in Appendix J shows that the temporal patterns that produce
lower peak discharges will generate peak design discharges that are 9% lower (on average)
respectively relative to the base peak discharges. Adopting a temporal pattern that
produces high peak discharges is predicted to increase peak design discharges by 16%, on
average.
The revised discharge hydrographs were then applied to the TUFLOW model and the
TUFLOW model was used to re-simulate the 5% AEP, 1% and 0.5% AEP floods with the
modified temporal patterns. Flood level difference mapping was prepared to display the
impacts of the temporal pattern modifications and are presented in Appendix K.
The difference mapping shows that adopting a lower temporal pattern can reduce 1% AEP
flood levels by up to 0.1 metres. However, the reductions were most commonly less than
0.05 metres.
Adopting the higher temporal patterns produced flood level increases of more than 0.2
metres in isolated locations, however, increases of up to 0.1 metres are more common. It
was noted that the higher temporal patterns did not increase flood levels at all locations,
with some areas exposed to small flood level reductions of up to 0.05 metres. This was
most evident in volume dependent areas such as Harrington Park Lakes (i.e., areas that are
more driven by the volume of runoff than the absolute peak discharge
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