Appendix C - Stormwater Management Plan

 The results of the modelling against the range of design storms for the development under the existing and
proposed development conditions are shown in Table 3.1 Existing and proposed unmitigated site peak discharge
rates.

                              Table 3.1 Existing and proposed unmitigated site peak discharge rates

                                                                   Wyandra Street
                                                                                                    Qchange
                                            A.E.P           Qexisting   Qproposed      Qchange       (%)
                                                            (m³/s)      (m³/s)         (m³/s)
                                             1EY             0.044       0.048          0.004         9%
                                             50%            0.051          0.054       0.003          6%
                                           0.2 EY           0.080          0.091       0.011         14%
                                             10%            0.095          0.098       0.003          3%
                                              5%            0.112          0.111       -0.001         -1%
                                              2%            0.129          0.137       0.008          6%
                                              1%            0.150          0.154       0.004          3%



The increased impervious area resulting from the development will increase in overall peak site run-off rates.
Attenuation will therefore be required to mitigate the increased peak flows to ensure that there is no impact that
may result in nuisance to downstream properties and infrastructure. The site model was therefore updated to
include an attenuation storage through which run-off from the new roof area will be drained before being discharged
to   the     existing   pit.  The    modified    layout    to   assess    attenuation     is   shown     below    in
Figure 3.5 – DRAINS Post-development Layout




                                   Figure 3.5 – DRAINS Post-development Layout with Attenuation

An iterative process was used to determine a suitable combination of attenuation storage and low-flow and high-
flow outlets that would restrict the developed flows to at or below existing peak flow rates. It was found that a 3KL
attenuation volume (e.g. Aquaslim Square 3008 or similar) would provide sufficient capacity with a 50mm low flow
orifice place at its base outlet and a 150mm overflow at the top of the tank. This configuration of attenuation would
sufficiently reduce peak flows to below existing flow rates, as shown in Table 3.2 Existing and proposed mitigated
site peak discharge rates.




Project number 20220213
File 20220213 Reserve St SWMP.docx, 2022-09-19 Revision 1                                                       Page 9 of 16

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