RFI - updated flood investigation report

              Results
      The TUFLOW hydraulic model been assessed for a range of flood events including the 63.2%,
      39%, 10%, 1%, 1% (2100) AEP storms and the Probable Maximum Flood (PMF) event. The
      assessment has considered a range of storm durations to ensure that the critical storm is
      defined. The results of the hydraulic assessment are presented as a series of flood maps to
      illustrate the flooding characteristics of the overland flow path and the resulting flood
      behaviours including flooded depth, height, velocities, hazard and afflux.
      The flood maps are presented in Appendix F and Appendix I.

      4.3.1. Existing Scenario

      The existing scenario flood maps are shown in Appendix F. This details the existing flood
      extent and behaviour in relation to the development site. The results of this have been used
      as the basis for assessing the potential impacts that may result from the development.

      4.3.2. Proposed Scenario

      The proposed scenario flood maps are shown in Appendix G. This details the proposed flood
      extent and behaviour in relation to the development site. An indicative lot plan is overlaid on
      this map set to highlight how the development relates to the flood plain.

      The PMF event was run for the proposed scenario to identify flood behaviour in this extreme
      event and how residents would be able to safely manage this event from their property. The
      PMF level across the site ranges from 13.48 to 13.51m AHD, which is approx. 0.4m and 1.0m
      higher at the upstream and downstream boundary of the site, respectively.

      Each building envelope will require fill to achieve appropriate freeboard, however, all filling is
      generally above the 1% (2100) AEP flood level. Minor filling of Lot 1 is required and some
      minor shaping of the land beside the Lot 1 building envelope to manage afflux cause by
      flooding backing up behind the building envelope.

      4.3.2.1. Peak Discharge

      The assessment found the peak discharge at the site boundary are predicted to decrease
      across all modelled events. This is shown in Table 4-3 which compares the Existing and
      Proposed peak discharge at the site boundary for each event. As such it is considered that
      a formal detention is not required to service this development as the design has achieved
      the required non-worsening outcome.
      Table 4-3: Existing and Proposed Peak discharge at the Site Boundary.

        Design            Critical Duration       Existing Peak          Proposed Peak      Difference (m3/s)
        Event (AEP)       (min) / Temporal        Discharge (m3/s)       Discharge (m3/s)
                          Pattern
        1% (2100)              120 (TP3)                  49.00                48.81              -0.19
        1%                     120 (TP6)                  38.83                38.70              -0.13
        10%                    180 (TP8)                  22.93                22.45              -0.48
        39%                    540 (TP5)                  8.94                 8.89               -0.05
        63.2%                  540 (TP5)                  6.19                 6.16               -0.02




21-0288-FS001-V3 | Flood Investigation | 60 Glenridge Drive, Cooroibah                                          | 12