20220214 12 Wyandra St SWMP

                                          Figure 3.3 – DRAINS Post-development catchments




                                       Figure 3.4 – DRAINS Split Post-development catchments


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                 0.021       0.022        0.001         5%
                                            50%                0.023        0.025       0.002          9%
                                           0.2 EY              0.035        0.039       0.004         11%
                                            10%                0.045        0.046       0.001          2%
                                             5%                0.052        0.052       0.000          0%
                                             2%                0.060        0.060       0.000          0%
                                             1%                0.065        0.066       0.001          2%



The results show that the site has increased the total peak flows from the site for a number of storm events.
Attenuation is required to mitigate the outflows to restrict developed peak outflows to below the predevelopment
levels to minimise impacts to the downstream environment.

The mitigated post-development model splits the catchment into a roof area, which drains to an on-site detention
(OSD) tank, and the remainder of the site which is not attenuated. An iterative process was used to determine a
suitable configuration of storage volume, low-flow, and high flow pipe outlets for the OSD to deliver the required
reduction of the flow rates from the site.

The result of the iterative sizing process determined that the OSD storage will need to provide an attenuation
capacity of 2,000L/unit for the proposed roof area with the maximum possible roof area to be
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Project number 20220214
File 20220214 12 Wyandra St SWMP.docx, 2022-09-14 Revision 1
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