21-0288-FS001-V4_GlenridgeDr_FS-reduced

              Response to Further Advice
      A Further Advice Request was received on 14 March, 2022 as part of Reconfiguration of Lot
      Application No: RAL21/0033. Items 1 to 3 relate to Flooding, with response to each item
      detailed below:
      Issue
      Blockage of hydraulic structures needs to be accounted for in the design of the new internal
      roadway and assessment of impacts of the development.
      Information Request:
      1.     Provide an updated Flood Investigation report which documents the blockage
      assumptions for each of the culverts represented in the ‘existing’ and ‘proposed’ model
      scenarios and demonstrate the assumptions are consistent with the guidance of ARR2019
      Book 6 Chapter 6.
      Response:
      A blockage assessment was completed and identified a 20% blockage should be applied to
      the culverts within the model. With the inclusion of 20% blockage on the new culvert
      crossing, an additional cell is required (9 x 1200 (w) x 600 (h)).
      Issue
      Building envelopes need to be entirely flood free.
      Information Request:
      2.      Update the TOD subdivision plan such that the entirety of the building envelopes
      achieves the minimum levels specified in Table 4-5 of the updated ‘Flood Investigation’
      report.
      Response:
      Site plan has been updated to show building envelope extents being filled.
      Issue
      Lot 1 building envelope flood immunity for the 1% (2100) AEP is required to be achieved by
      filling. The access of the building envelope to Glenridge Drive is inundated in the 39%AEP
      event. These outcomes are unacceptable and contrary to the Flood Hazard Overlay Code.
      Information Request:
      3.       Update the TOD subdivision plan to Delete Lot 1 and amalgamate with proposed Lot
      Response:
      Inundation shown to occur on Lot 1 relates to the small catchment (3.6 ha) that extends to
      the north of Lot 1. The inundation would be more classified as overland flow and sheet flow
      and does not relate to the flood plain associated with the main waterways which traverse
      the site.
      In the image below (refer Figure 1-3), spot flood levels in the existing 1% (2100) AEP event are
      noted within Lot 1 and within the main waterway. It can be seen that the levels are approx.
      0.5m higher in Lot 1, which highlights they are from different flood sources.
      It is considered that given the small catchment contributing to Lot 1 and the shallow nature
      of the sheet flow that filling of this building envelope is not contrary to the intended outcome
      of the Flood Hazard Overlay Code regarding altering the landform within a flood plain.




21-0288-FS001-V4 | Flood Investigation | 60 Glenridge Drive, Cooroibah                                   |3