Swmp · MCU22/0058

As expected, the site has increased the total peak flows from the site for a number of storm events. Attenuation will
be required to mitigate the outflows to ensure that the development does not exceed 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 4,000L for the proposed roof area with the maximum possible roof area to be plumbed to the tank. A low
flow outlet consisting of a 53mm orifice plate is to be provided the base of the tank, and a 100mm high flow outlet
at the head of the tank will provide discharge for larger storm events.

Using this mitigated development model, the proposed developed flows were again compared to the existing state
as shown below in Table 3.2 Existing and proposed mitigated site peak discharge rates.

                                Table 3.2 Existing and proposed mitigated site peak discharge rates

                                                                        Wyandra Street
                                                                                                         Qchange
                                           A.E.P                Qexisting      Qmitigated   Qchange       (%)
                                                                (m³/s)         (m³/s)       (m³/s)
                                             1EY                 0.016          0.015       -0.001         -6%
                                            50%                  0.019          0.017       -0.002        -11%
                                           0.2 EY                0.028          0.025       -0.003        -11%
                                            10%                  0.036           0.03       -0.006        -17%
                                             5%                  0.041          0.037       -0.004        -10%
                                             2%                  0.047          0.045       -0.002         -4%
                                             1%                  0.051          0.051          0           0%



As the total peak flows are equal to or below the existing site conditions for all storm events, the proposed OSD area
will provide the mitigation required for the peak flow storm events from the development.




Project number 20220078
File RECOR Preliminary Stormwater Management Report - 20220078.docx, 2022-06-24                                      Page 9 of 15
Revision 1


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