Appendix B - IWMP_2PearsonsRd_REV0

                                                                                 Callaghan & Toth


5.0 FLOODING ASSESSMENT SUMMARY
Plan 2035-01 Revision A shows approximate flood study Q100 inundation from an adjacent
2015 flood study.

5.1      Existing Conditions

The site is traversed by one tributary of Six Mile Creek; refer to plan 2035-01 Revision A.

The site is subject to floodwaters from the adjacent watercourse and the Q100 year ARI flood
levels (see plan 2035-01 Revision A).

5.2      Proposed Drainage System and Flood Implications

The proposed residential lots front the creek’s floodway will have at least 1.5 metre freeboard to
the 100 year flood level and the proposed development is therefore expected to be adequately
flood-immune, even with a 20% impost on the design rainfalls for future climate change.

The small intrusion of the outer batter of the basin into the floodway will depreciate the 100 year
floodway storage by about 9 cubic metres, which should have practically no impact on the 100
year flood profile.

The required 100 year stormwater detention volume of 398 cubic metres can be achieved,
within the site’s constraints, by two facilities – a 351 cubic metre open basin and a 47 cubic
metre EcoAid underground facility. 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 open basin will have a 100 year water depth of 1.05 metres. The freeboard of 0.2 metres to
the embankment crest will be sufficient to contain the inflows from a greenhouse-induced 20
percent increase in storm rainfall intensities.

See the Flood Study report for further details on flood modelling results.




                                                                                                 17
2035 2 Pearsons Road IWMP                                                    September 2020