Appendix B - SWMP, Flood Cert & Form 15

            3. STORMWATER QUANTITY
            3. 1. P R E – P O S T C O M P A R I S O N

            The site has been modelled using the Autodesk Storm and Sanitary Analysis module in its
            existing and unmitigated proposed developed state. The catchment hydrology has been
            configured in accordance with the Bureau of Meteorology’s IFD curves for the locality, and
            a minimum time of concentration of 5 minutes for each catchment. All catchments for the
            proposed development and the internal drainage network and storage areas have been
            modelled as described in Figure 2.2 – Existing Catchments Plan and Figure 2.3 – Proposed
            Catchments 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

                                        53 Doonella Street                    55 Doonella Street
                                                                                                               Total
                     A.E.P       Qexisting     Qproposed        Qchange   Qexisting   Qproposed     Qchange   Qchange
                                 (m³/s)        (m³/s)           (m³/s)    (m³/s)      (m³/s)        (m³/s)    (m³/s)

                    63.20% 0.009               0.014           0.005      0.012       0.015        0.003      0.008
                    50%         0.010          0.016           0.006      0.013       0.017        0.004      0.010
                    20%         0.014          0.021           0.007      0.019       0.023        0.004      0.011
                    10%         0.017          0.025           0.008      0.022       0.028        0.006      0.014
                    5%          0.020          0.029           0.009      0.026       0.032        0.006      0.015
                    2%          0.024          0.033           0.009      0.030       0.037        0.007      0.016
                    1%          0.026          0.037           0.011      0.034       0.040        0.006      0.017


            As the site has increased the total peak flows from the site, attenuation storage will be
            required to mitigate the outflows to ensure that the development does not exceed the
            predevelopment levels.

            To determine the attenuation required, the model for the developed site was updated to
            include the proposed stormwater attenuation devices to be provided for the development.
            Rainwater attenuation tanks were modelled as storage nodes with 2 3kL tanks provided per
            development. An attenuation portion equal to half the capacity of the tank was modelled,
            with a low flow outlet of 25mm and high flow outlet of 100mm. The reuse portion of the tank
            was assumed to be full at the time of the storm so only the attenuation portion contributes
            to flow mitigation.

            Each bioretention area was modelled as a storage node consisting of the bioretention filter
            media and the extended detention depth. A porosity of 0.3 was considered as the water
            holding capacity for the filter media, with the low flow pipe consisting of the slotted
            drainage pipe and the high flow weir overflow for flows overtopping the pit.

            Using this mitigated development model, the proposed developed flows were again
            compared to the existing state as shown below in


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