RFI - 20220156 Kauri St SWMP Rev B

The development of the site will therefore increase the total peak flows from the site under all storm events.
Attenuation is required to mitigate the outflows to ensure that the development does not exceed the
predevelopment levels to minimise impacts to the downstream environment.

To determine the attenuation requirements, the developed site was duplicated an on-site detention (OSD) tank
receiving the developed catchment outflows. An iterative process was used to determine a suitable configuration of
storage volume, low-flow orifice and high flow overflow weir 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 54m³ to ensure adequate capacity to achieve attenuation. A low flow outlet consisting of a 132mm orifice
plate is to be provided the base of the OSD, and an overflow 0.3m wide overflow weir discharging to the outlet 300m
from the top of the tank for larger storm events.

Using this OSD combination for the 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                 0.021          0.02         -0.001         -5%
                                            50%                 0.026          0.023        -0.003        -12%
                                           0.2 EY               0.044            0.04       -0.004         -9%
                                            10%                 0.060          0.059        -0.001         -2%
                                             5%                 0.075          0.073        -0.002         -3%
                                             2%                 0.094          0.092        -0.002         -2%
                                             1%                 0.106          0.106           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 20220156                                                                                              Page 10 of 27
File RECOR Stormwater Management Report - 20220156.docx, 2022-10-28 Revision 1

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