RFI - Stormwater management (flood report)

      2.1.5. Sensitivity Assessment

      A sensitivity assessment was completed on the WBNM assessment using a higher initial loss.
      The initial loss used in the main assessment, was applied to enable a verification of flows to
      the rational method. A higher initial loss of 30mm was applied (taken from QUDM Table 5.6.1-
      Healthy grass on loamy clay). Table 2-7 provides a comparison of the existing, developed
      and mitigated peak flows using the higher initial loss of 30mm. The results highlight there is a
      reduction in the peak flow rate (attributable to the higher loss) and reduction in peak flows is
      consistent with the results presented in Table 2-7. It is considered that the model is not overly
      sensitive to the increase in initial loss and the current basin configuration is acceptable.
      Table 2-8: Comparison of Peak Flows Site A (Existing Vs. Mitigated)

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

       Existing (m3/s)           0.21        0.26        0.46       0.57    0.70   0.89      1.05
       Developed (m3/s)          0.41        0.49        0.72       0.88    1.05   1.25      1.41
       Mitigated (m3/s)          0.20        0.24        0.41       0.56    0.73   0.90      1.07
       Change (m3/s)             -0.01       -0.02      -0.04       -0.01   0.03   0.02      0.02




              Flood Investigation
      The proposed development footprint is currently located outside of the NSC Flood Hazard
      Overlay Mapping; however, part of the Lot 10 building envelop is partially inundated in the
      1% (2100) AEP. The Noosa Plan Flood Hazard Overlay Code identifies that any development
      / filling below the 1% (2100) AEP flood level must demonstrate that no off-site impacts and no
      loss of flood plain storage is demonstrated.
      Council’s Pomona Flood Model was utilised to represent the development proposal and
      demonstrate the requirements of the Flood Hazard Overlay Code.

      2.2.1. Flood Model

      The NSC Pomona Flood Model was utilised as the basis for the flood assessment to represent
      flooding in the 1% (2100) AEP event. The model was setup and run for the Existing and
      Proposed Scenarios. The model was utilised in full, with the following updates made to
      represent these two scenarios:
      Existing (E03)
              Reduce model grid size from 5m to 2m.
              Update model terrain to utilise 2015 Lidar and site survey.
              Model was run using HPC and the latest TUFLOW engine (version 2020-10-AA).
              Represented 1D channels in 2D with smaller grid size.
      Proposed (P03a)
              Update model terrain to include preliminary engineering design (supplied by TOD
               Consulting).
              Represent drain in 3m easement (approx. 0.4m deep at 1% grade).
              Represent area of cut, 1,250m2 and 0.1m deep.




21-0166SMP01-V1 | Stormwater Management Plan | 25 Pavilion Street, Pomona                                 | 11