Attachment 1 - RECOR RFI Cover Letter Response_20210216

                         drainage isn’t compromised, while extending the chamber to the kerb inlet
                         opening. A concrete cover is to be provided above the existing pit.
                    •    The pit chamber and pipes will remain accessible for maintenance through the
                         proposed grated inlet pit.

            As the existing pipes aren’t impacted, the hydraulic intake efficiency of the pit is improved
            and the lintel is clear of the driveway so the proposed replacement would minimise any
            impact to the existing stormwater.


            Item 6. Stormwater Management Plan

            The original Stormwater Management Strategy undertaken by Cardno in 2008 (Ref:
            LJ8705/R2_V2) utilised the current rainfall data and assumed a post-development
            catchment area of 4.658ha and an impervious coefficient of 90%.

            A DRAINS model was set up using the catchment characteristics defined under the
            original stormwater management plan as well as the storage basin depth-volume
            characteristics. The post development peak run-offs as well as the developed peak run-
            offs with mitigation from the detention basin were validated against the reported values to
            confirm the configuration of the model was consistent with the existing. The validated
            results were relatively consistent with the previously modelled data. Refer to Attachment B
            for details of these calculations.

            The validated model was then rerun with the rainfall intensities increased by 20% to reflect
            the forecast average rainfall increase under 2100 climate conditions to determine the
            impact upon the effectiveness of the basin.

            A comparison of the basin mitigation effects under current conditions, as well as 2100
            climate conditions is shown below.

                                                Current Climate            2100 Climate (20% increase)
                        A.E.P        Qreduction (m³/s) Qreduction (%) Qreduction (m³/s)   Qreduction (%)
                         63.2% AEP              -0.797          -68%               -0.995              -71%
                              0.5 EY            -1.027          -70%               -1.282              -72%
                              0.2 EY            -1.340          -71%               -1.513              -66%
                           10% AEP              -1.527          -70%               -1.700              -64%
                            5% AEP              -1.605          -63%               -1.811              -59%
                            2% AEP              -1.799          -61%               -2.008              -57%
                            1% AEP              -1.842          -57%               -2.156              -56%

            As demonstrated above, the basin will continue to offer comparable reduction
            effectiveness compared to a non-mitigated development, despite the increased flows. As
            a result of the increased flow rates under the 2100 climate conditions however, there will
            be an increase in the basin depth during the peak storms.




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