RFI - Stormwater management (flood report)

      2.2.2. Existing Scenario (E03)

      The existing case model has been setup and run for the 1% (2100) AEP flood event. The
      results of this have been used as the basis for assessing the potential impacts that may result
      from the development.
      Appendix C details the existing flood extent in relation to the development site. It can be
      seen that there is a small amount of break flow occurring through the showgrounds (to south)
      that enters the building envelop of Lot 10. Flow depths in this area are quite shallow and up
      to 55mm deep (approx. 0.1m3/s).

      2.2.3. Proposed Scenario (P03a)

      The proposed case model has been setup and run for the 1% (2100) AEP flood event, which
      incorporates the proposed development site. Appendix D details the proposed flood extent
      in relation to the development site. An indicative site plan is overlaid on this figure to
      highlight how the development relates to the flood extent.

      2.2.3.1. Afflux

      Afflux mapping has been undertaken for the 1% (2100) AEP events to identify whether there
      are any impacts resulting, and if so, whether an alternative development arrangement
      needs to be investigated.
      Appendix E details the afflux map which compares the existing and proposed case for the
      1% (2100) AEP flood levels. It is noted that based on the current development arrangement
      there are no increases in flood levels noted either upstream or downstream as a result of the
      development site itself.

      2.2.3.2. Peak Flow

      The assessment found the peak discharge at the site boundary are predicted to decrease
      slightly. This is shown in Table 2-9 which compares the Existing and Proposed peak discharge
      at the site boundary. As such it is considered that non-worsening is achieved, and the
      development does not impact peak flow rates in the regional flood event.
      Table 2-9: Existing and Proposed Peak discharge at the Site Boundary.
       Design Event         Existing Peak Discharge          Proposed Peak Discharge   Difference
       (AEP)                (m3/s)                           (m3/s)                    (m3/s)
       1% (2100)                        304.00                              303.99          -0.01



      2.2.3.3. Flood Storage

      A flood storage assessment has been completed over the site to ensure there is no loss of
      flood plain storage has resulted from the development. The assessment found that there was
      some loss of flood plain storage due to the development, with some additional cut provided
      in the Lot 10 to offset the loss of flood plain storage.

      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 (no cut) peak 1% (2100) AEP flood surface was compared to the
      proposed model terrain. The proposed (with cut) scenario incorporates an area of free
      draining cut approximately 0.1m deep over 1,250m2 to achieve the flood storage balance.
      Details of the flood plain storage in the existing, proposed and mitigated cases is summarised
      below in Table 2-10.




21-0166SMP01-V1 | Stormwater Management Plan | 25 Pavilion Street, Pomona                               | 12