Information Request OPW25 0042 [As Published] · OPW25/0042

                        Provide a table identifying each property fronting Serenity Close for the section of
                        Serenity Close which grades to the sag and provide an explanation of how the direction
                        of drainage for each property has been determined (in particular, where it is assumed to
                        not fully drain to Serenity Close).
                 2.     Clarify what design blockage values have been adopted for the stormwater inlets in the
                        design case in the overland flow assessment.
                 3.     Clarify what climate change assumptions have bene adopted in the overland flow
                        assessment.
                        It is suggested that SPP2-4.5 and a planning horizon of 2100 be adopted consistent with
                        the current methodology of ARR.
                 4.     It is noted that 1% AEP (2100) immunity for the staff facilities building is achieved by the
                        use of a levee (block wall).
                        It would be preferable if the levee was only used to achieve freeboard rather than 1%
                        AEP (2100) immunity.
                 5.     Provide further contours or sections in the location of RIP 3B and RIP 5 to demonstrate
                        that surface levels or an overland flow path (which may be informal) will allow and
                        surcharge flows from these devices to flow to a lawful point of discharge without
                        impacting any of the development buildings or facilities.
                 6.     It is noted that a gabion rock wall is specified as the method of retaining along the edge
                        of the overland flow swale in places. Given the ability of water to seep into this type of
                        retaining (large voids), it is unclear if this method of retaining is suitable for the edge of
                        a drainage channel and if a more suitable method is available that provides better long-
                        term performance of the channel and adjoining areas.


                 Issue – Overland Flow Channel
                 The design of the Overland Flow Channel includes roughness assumptions consistent with
                 mown grass or rock at a maximum batter of 1:2. Council has concerns with the ability to
                 provide long-term stabilisation for a grass or rock 1:2 batter.
                 Information Required
                 7.     Provide additional detail demonstrating how long-term stabilisation will be provided to
                        the batters of the Overland Flow Channel. The additional detail must be prepared by a
                        suitably qualified person (RPEQ) and include treatment options consistent with the
                        current hydraulic design assumptions of the Overland Flow Channel (i.e., turf or rock).
                        Where long-term stabilisation of the proposed 1:2 turfed/rocked batter isn’t achievable,
                        provide a revised set of drawings amending the Overland Flow Channel to include a
                        reduced batter (i.e., a 1:3 vegetated batter).


                 Issue – Extent of Vegetation in Overland Flow Channel
                 The higher order development plans suggest that the trapezoidal channel will include
                 landscaping (beyond turf) in the Overland Flow Channel. This is Council’s preferred outcome.
                 However, the inclusion of vegetation within this channel is inconsistent with the current
                 hydraulic design assumptions and could lead to flow breaking out of the channel and cause
                 flooding.
                 Information Required
                 8.     Confirm the Overland Flow Channel will be vegetated to achieve a natural channel
                        design and meet the required hydraulic design assumptions.




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Document Set ID: 23692869
Version: 3, Version Date: 16/04/2025