RFI - updated flood investigation report

      4.3.2.2. Afflux

      Afflux mapping has been undertaken for the 63.2%, 39%, 10% and 1% AEP to identify whether
      there are any impacts resulting, and if so, whether an alternative development arrangement
      needs to be investigated. Afflux maps have been provided in Appendix H.
      The afflux mapping shows that there are no external impacts resulting to any adjoining
      properties as a result of the development. An increase in flood levels of up to 330mm is
      estimated upstream of the proposed Internal Road extension in the 1% (2100) AEP, however,
      this is contained within the site boundary.

      4.3.2.3. Flood Storage

      A flood storage assessment has been completed over the site to ensure there is no loss of
      flood plain storage as a result of the development. For the existing flood storage calculation,
      the existing peak 1% (2100) AEP flood surface was compared to the existing model terrain.
      Similarly with the proposed flood storage calculation, the proposed peak 1% (2100) AEP flood
      surface was compared to the proposed model terrain. Details of the flood plain storage in
      the existing and proposed cases is summarised below in Table 4-4, where it is shown there is
      an increase in the amount of flood storage as a result of the development, which is
      considered positive outcome.

      Table 4-4: 1% (2100) AEP Flood Storage Comparison.

        Scenario                              Flood Plain Storage (m3)
        Existing                                                         36,831
        Proposed                                                         45,932
        Difference                                                       +9,102

      4.3.2.4. Internal Road

      The Internal Road extension has been designed achieve flood immunity in the 10% AEP
      event, whilst allowing some overtopping in the 1% AEP (major event). The high point of the
      road (which acts as a weir) is at a minimum 12.81m AHD (refer Figure 4-1). This results in a
      flow depth of less 0.2m occurring over the road pavement in the 1% AEP, with flow velocities
      less than 1.6 m/s and flow hazard less than 0.3m2/s. These parameters are below the
      trafficability requirements of QUDM, which require a maximum depth of 0.2m and hazard less
      than 0.3m2/s in the 1% AEP event.

      The new culverts have a flow velocity of 2.1m/s. Outlet scour protection will be required to
      be provided with D50=200mm rock provided and extend 3.0m past apron.

      It is recommended that appropriate depth markers and flood warning signs be placed on
      either side of the internal road flood extents to warn residents of flood risk and depth of water
      over road should it overtop.




21-0288-FS001-V3 | Flood Investigation | 60 Glenridge Drive, Cooroibah                                    | 13