MCU21.0117 OPW21.0199-Att 4 Civil Eng · MCU21/0117

                       g) Accurately represents the existing catchment and flows reporting to the
                          proposed discharge point from the site, as the comparison for the
                          developed-site discharge; and
                       h) Provides existing and developed catchment plans, showing the above areas

            The drainage network and detention storage has been redesigned for the site to address
            the points raised. Pre and post development catchment plans have been prepared to
            represent the site catchments flows to each element of the drainage system, external
            catchments conveyed through the site and areas bypassing the site detention systems.

            As the pre-development site drainage flows into private property rather than the legal
            point of discharge, there is currently no predevelopment peak flow from the development
            into the legal drainage point for the site to achieve. Rather, the proposed development
            has been modelled to demonstrate that there is sufficient capacity within the internal
            drainage network at the discharge point and in the table drain along Pearl Street. This
            takes into account the upstream and downstream catchments draining to the table drain,
            to determine the maximum discharge rates from the development with minimal impacts
            to upstream or downstream properties for storms up to the 1% AEP. Please refer to the
            updated Stormwater Engineering Report for details.

                    6. Please provide MUSIC modelling and updated engineering plans which
                    demonstrate compliance with PO6 and AO6.2 of the Water Quality and Drainage
                    Code.

            The site and the treatment train proposed for the development has been modelled in
            MUSIC in accordance with the Water by Design MUSIC Modelling Guidelines for
            Southeast Queensland. The results demonstrate that the site achieves compliance with
            PO6 and AO6.2, being reductions in excess of 90% of gross pollutants, 80% of total
            suspended solids, 60% total phosphorous and 45% total nitrogen at the discharge point.
            Please refer to the updated Stormwater Engineering Report for further details.

                    7. Is the1200x600 grated inlet outside POD B a sag gully (this is unclear due to
                    limited design levels)? If it is a sag, demonstrate that the minimum 300mm
                    freeboard required by AS3500 to the adjacent building floor levels is achieved.

            This pit has been removed, however pits in the driveway area are on grade, and within the
            bioretention areas, a minimum of 300mm freeboard to the adjacent building floor levels is
            achieved.

                    8. Provide calculations/HGL analysis demonstrating that the surcharge pit outlet
                    from the
                    site will not surcharge the upstream pits during the design event due to the limited
                    grade.

            A hydraulic analysis of the proposed stormwater drainage design has been undertaken
            for the critical 1% AEP storm event, which demonstrates that there is no surcharge . Please
            refer to the updated Stormwater Engineering Report for details of the HGL analysis.




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