5179Flood Report_REV_3

                                                                                                Callaghan & Toth




4.0       SUMMARY
The proposed residential lots fronting the creek’s floodway will have at least a metre of a freeboard to the
Year2100 1%AEP flood level in the adjoining floodway and the proposed development will therefore be
adequately flood-immune, even with a 20% impost on the design rainfalls for future climate change.

The proposed stormwater detention facilities will be located outside the Year 2100 1% AEP flood
boundary and will therefore have no impact on the storage-routing capacity of the floodway.

The required 100-year stormwater detention volume is 334 cubic metres. This can be achieved by an
open offline basin linked to an elevated backwater profile in the minor waterway swale, with the required
volume being achieved over-and-above the normal (un-backwatered) flow volume in the swale. Lateral
flows from the swale to the basin will commence when the water depth in the swale reaches 0.3 metres.
A more complex modelling procedure at the Operational Works stage, using the Bureau of Meteorology’s
2016 design rainfall data, could result in a slightly smaller detention volume.

The 334 cubic metre volume includes 128 cubic metres for no-worsening of the 1-year peak discharge to
the downstream waterway, as required for downstream waterway stability.

The 100-year water depth is expected to be 1.15 metres above the invert of the low-flow outlet pipe and
0.8 metres above the floor of the open basin. A freeboard of 0.25 metres to the basin’s embankment
crest will be sufficient to contain the inflows from a greenhouse-induced 20 percent increase in storm
rainfall intensities.




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2035 2 Pearsons Road – Flood Study – Revision 3                                         April 2021