Appendix D- Stormwater Management Plan · MCU23/0078

4.1.5   Detention Basin Design


        For the developed mitigated scenario, a storage node representing the proposed detention basins were included in the
        model. Flows from all three development catchments were routed to the assigned detention tank.


        The storage node inputs were sourced based on an in-situ tank design for the Site A catchment, and a pre-cast tank
        system for the two Site B catchments and tanks. The three detention tanks modelled for each of the three sub-catchments
        are outlined in Table 9 below.


                                         Table 9: XP Storm Developed Model Hydraulic Routing
                Development Sub-           Invert Elevation    Internal Height   Surface Area            Approximate Volume
                   Catchment                       (RL)                 (m)                  (m2)                (m3)
                     Site A                        5.50                  2.0                 1,120               2,240
                  Site B (North)                   7.00                  1.0                 160                  160
                  Site B (South)                   7.30                  1.0                 270                  270


        The outlets to each of the three detention tanks were modelled as per the below:


            •     Site A Catchment Tank:
                      o    Low-flow outlet: 150mm diameter orifice with invert at RL 5.50;
                      o    Mid-flow outlet: 300mm diameter orifice with invert at RL 6.30;
                      o    High-flow outlet: 600mm diameter orifice with invert at R: 6.50; and
                      o    High-flow weir: 20m long weir with crest height at RL 7.30.
            •     Site B (North) Catchment Tank:
                      o    Low-flow outlet: 90mm diameter orifice with invert at RL 7.00;
                      o    Mid-flow outlet: 225mm diameter orifice with invert at RL 7.40; and
                      o    High-flow weir: 2m long weir with crest height at RL 7.85.
            •     Site B (South) Catchment Tank:
                      o    Low-flow outlet: 100mm diameter orifice with invert at RL 7.30
                      o    Mid-flow outlet: 275mm diameter orifice with invert at RL 7.70; and
                      o    High-flow weir: 1.6m long weir with crest height at RL 8.13.


        See Engineering Drawings in Appendix B for further information.


4.1.6   Results for Developed Scenario


        Tables 10, 11 and 12 indicates the developed total peak discharge rate from the development sites as outputted by XP
        storm. The peak flow rate indicated is the combined flow of the site and external catchments. To determine the critical
        storm event for the developed mitigated case, the 1-to-100-year ARI storm events for 20, 25, 30, 45, 60, 90, 120 and 180-
        minute durations were simulated in the model.


        From these simulations it was determined that for both catchments, the critical duration for the 100-year and 20-year ARI
        storm events was the 25-minute storm and the critical duration for the 1-to-10-year and 50-year ARI storm events was the
        60-minute storm. The 25-minute and 60-minute storm duration results will therefore be compared against the existing
        scenario discharges for both catchments, as identified in Section 4.1.7.




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