5179SQMP_2 PearsonsRd_Feb2021_Rev2_

                                                                                Callaghan & Toth


5.0 FLOODING ASSESSMENT SUMMARY
Refer to Flood Assessment Report Revision 1 for detailed flood assessment. See also Plan
2035-01 Revision C showing approximate Q100 inundation line. Below summarises the
findings of the flood assessment.

5.1      Existing Conditions

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

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

5.2      Proposed Drainage System and Flood Implications

The proposed residential lots front the creek’s floodway will have at least 1.0 metre freeboard to
the Year 2100 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 small intrusion of the outer batter of the basin into the floodway will depreciate the Year
2100 1%AEP floodway storage by about 2 cubic metres, which should have practically no
impact on the Year 2100 1%AEP flood profile, however the area over the sewer adjacent the
bio basin will be scalloped out 3.4L x 3.0W x 0.2D to compensate.

The required 100 year stormwater detention volume of 398m3 can be achieved, within the site’s
constraints, by two facilities – a 384m3 open basin and a 22m3 enlargement of the main
stormwater pipe. 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 1.00 metres. 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.

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




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2035 2 Pearsons Road IWMP – Revision 2                                       February 2021