5179Flood Report_REVISION_2_7Feb2021

                                                                                               Callaghan & Toth




4.0       SUMMARY
The proposed residential lots fronting the creek’s floodway will have at least metre of a freeboard to the
Year2100 1%AEP flood level 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 of 398 cubic metres can be achieved, within the
site’s constraints, by two facilities – an open basin with 384 cubic metres of storage above the extended
bio-retention level and an oversized stormwater pipe with 22 cubic metres of nett detention capacity. 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 398 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 open basin will have a 100 year water depth of 1.25 metres, or 1.05 metres above the extended bio-
retention level. The freeboard of 0.25 metres to the embankment crest will be sufficient to contain the
inflows from a greenhouse-induced 20 percent increase in storm rainfall intensities.




                                                                                                       11
2035 2 Pearsons Road – Flood Study – Revision 2                                     Feburary 2021