Callaghan and Toth IWMP Report revised

                                                                                Callaghan & Toth


6        STORMWATER DETENTION AND FLOODING REPORTS
6.1      Existing Conditions

This site is subject to ponding of floodwaters from the Q100 year ARI flood levels. The habitable
floor areas of the new buildings and new carpark are above the Q100 year ARI flood levels.


6.2      Proposed Drainage System

The additional carpark surface level is proposed to be drained to an existing underground
drainage system capable of taking a Q5 flow. The overflow will discharge to the existing storm
water drains and surrounding grass verge.

The habitable minimum floor level for the proposed centre is to be set at RL10.5. The minimum
carpark level is RL8.25, which is approximately at the Q100 flood level.

The proposed biopods filter media level will be at a minimum of approximately RL8.50,
providing outlet pipe level of approximately RL7.50. This matches existing outlet levels.

Allowance has been made to collect the runoff from the proposed new buildings, carparks, and
hard stand areas. The roofwater is collected in above ground rainwater tanks and will be used
for toilet flushing and landscape irrigation. The overflow from roofs and landscaped areas will
be piped to underground ECO AID detention cells which will attenuate the flow back to
predevelopment levels. Runoff will then be directed to the biopods which will then treat the
water to remove pollutants before releasing the stormwater into the existing gully riparian zone.

The Bioretention basin, rainwater tanks and infiltration/ detention basins have been modelled to
act as a detention basin. The storage size is sufficient to reduce the increase in runoff from the
additional hard stand area and buildings back to predevelopment levels as summarised in
preliminary design of stormwater detention volume Appendix C.


6.3      Flood Modelling

Q10, Q100 and Q500 flood levels based on amended Cardno flood study is shown on Middap
Architectural plan 1717 MCU01-B.




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1714-Cartwrigth Village Extension_IWMP_Revision 3.doc                           April 2019