RFI - Stormwater management (flood report)

                 0.97                                395
       Pipe Outlet                1 x 450mm RCP at RL0.0m
       Weir Outlet                4m wide at RL.0.60m
       Crest Level                RL 0.97m
       Q100 WSL                   RL0.80m
       Freeboard                  0.17m


      Peak flows from the mitigated scenario modelling are presented in Table 2-7 with a
      comparison against the existing peak flows. 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 2% 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 (refer
      Figure 1-2), 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.
      Table 2-7: Comparison of Peak Flows Site A (Existing Vs. Mitigated)

       Event AEP                  63.2%      50%           20%     10%      5%      2%     1%     1%CC

       Existing (m3/s)             0.34      0.42          0.64    0.80     0.95    1.12   1.27   1.59
       Mitigated (m3/s)            0.32      0.36          0.60    0.76     0.92    1.15   1.32   1.65
       Change (m3/s)               -0.03     -0.05         -0.04   -0.03    -0.02   0.03   0.04   0.06


      Table 2-6 provides a summary of the basin configuration and it is noted there is 0.17m
      freeboard from the 1% AEP flood level to the basin crest. A sensitivity analysis was undertaken
      to assess the impact of 100% blockage of both basin outlet configurations. Under this
      scenario the flood level rose to RL0.92m and demonstrates the spillway is able to convey the
      1% (2100) AEP flows under fully blocked conditions without overtopping the embankment
      crest. Given this and the fact that the land downstream of the detention basin will remain
      undeveloped it is considered that the 0.13m from the 1% (2100) AEP flood level (and 0.4m
      from spillway level) to dam crest level is acceptable.
      It is noted that with the inclusion of 0.3m of Extended Detention Depth for the co-located
      bioretention basin, the total Q20 depth for the basin is expected to be less than the 1.2m
      recommended by QUDM for safety concerns.
      Refer to Appendix B for summary of WBNM output.




21-0166SMP01-V1 | Stormwater Management Plan | 25 Pavilion Street, Pomona                                | 10