RFI - Stormwater management (flood report)

      6. CONCLUSION
      It can be seen from the results, presented in Table 3-7 that the treatment measures proposed
      for the development site achieve the performance criteria set by the NSC Planning Scheme
      (2020) and SPP (July 2017). Annual Load Reductions of Total Suspended Solids (TSS), Total
      Phosphorus, Total Nitrogen (TN) and Gross Pollutants (GP) between the Mitigated and
      Unmitigated Scenarios have been achieved.
      It has been demonstrated that non-worsening of peak flows has been achieved to the
      discharge point for events up to the 10% AEP event (refer Table 2-7). The results show that
      there is non-worsening for events up the 5% AEP, with a minor increase noted in the larger
      events. The intent of sizing the detention basin is to mitigate flows up to the 10% AEP storm
      event to ensure non-worsening of discharges into the downstream flow path. In larger
      events, flows from the site will start to sheet flow across the rear of the site to Cooroora Creek.
      As the discharge point for the site becomes inundated from Cooroora Creek flooding in the
      major storm event, it is considered that the very minor increases noted from the site become
      inconsequential as a result of downstream site being drowned out by creek flooding. The
      catchment size of Cooroora Creek reporting past the site is approximately 1,400 ha. The very
      minor increases in peak flows in the major event would be considered to not result in any
      quantifiable change to downstream flooding conditions.

      This plan demonstrates that effective treatment of stormwater at the proposed development
      can be achieved through the integration of Water Sensitive Urban Design (WSUD) principles.
      Through the incorporation of bioretention basins, the plan successfully demonstrates:

         Compliance with the requirements of the NSC Planning Scheme and SPPs Load Based
          Objectives;

         The end of line bioretention basin is appropriately sized for the proposed development and
          can be incorporated into the development; and

         Ecological sustainability in terms of the development’s impact upon receiving waters and
          the viability of the proposed site development.

         The development is located above the regional 1% AEP flood extent with 500mm
          freeboard recommended to future dwellings on the new allotments.

      We therefore request Council approval of the engineering components for the proposed
      development with reasonable and relevant conditions. Detailed design may result in
      changes to the proposed strategy; however, the design objectives will be maintained.




21-0166SMP01-V1 | Stormwater Management Plan | 25 Pavilion Street, Pomona                                   | 21