RFI - Stormwater management (flood report)

      Issue: Stormwater Drainage
      Detention storage sizing does not appear to be in accordance with QUDM and ARR2019
      Information Request:
      9. Provide revised sizing/modelling and associated engineering drawings and reporting for
         the detention storage proposed for the development site, which achieves the following:
                a. Includes analysis for the 1%AEP (2100) event
                b. Is undertaken using the ensemble storms approach of ARR2019
                c. Clarify the continuing loss assumed for impervious site areas. The pervious
                    initial loss reported in the SWMP appears to be much lower than the
                    recommendations of either QUDM or ARR2019 and is likely to result in under-
                    sizing of the detention storage due to an over-estimate of the pre-
                    development peak flows. Justify the assumed initial loss or adopt a revised
                    value consistent with QUDM and ARR2019
                d. Update the basin design to ensure the freeboard requirements of QUDM are
                    achieved for the 1%AEP (2100) event.
                e. Demonstrates no increase in peak flows for all events up to and including the
                    1%AEP (2100) event, for the range of durations applicable to the downstream
                    land, not just the critical duration at the site boundary.
                f. Consider whether detention storage is necessary if discharges from the
                    bioretention basin are directed to the West into the balance lot 10, rather
                    than the neighbouring property


      Response:
          a. Updated to include the 1% (2100) AEP event.
          b. Modelling completed using ARR2019 ensemble storms.
          c. Sensitivity assessment of losses completed using 30mm initial loss (refer Section 2.1.5).
             Results achieve a similar outcome to the smaller initial loss of 10mm originally used.
          d. Discussion of freeboard in the 1% (2100) AEP is presented in Section 2.1.4.
          e. Peak flows for a range of durations are presented in Appendix B. This shows that the
             peak is critical with respect to non-worsening.
          f. As discussed in Section 2.1.4, an existing flow path direct runoff north and into an
             existing dam. This discharge point has been maintained so as to not impact the
             water catchment of these farm dams. Non-worsening has been achieved in the
             smaller events to this property. In the larger events, the small increase noted would
             be drowned out by flooding from Cooroora Creek.


      Issue: Stormwater Quality
      The MUSIC model does not appear to include all catchment areas which will drain to the
      bioretention basin. External lots 1RP189023 and 20SP316902 will contribute flows to the
      bioretention basin and impact its performance, although the pollutant load reduction
      targets do not need to be achieved for these external lots and should be accounted for in
      the reporting.
      Information Request:
      10. Provide an updated MUSIC model and SWMP which includes all catchment areas
          flowing to the bioretention basin in the model.
      Response: MUSIC modelling has been updated to reflect this request and detailed in Section
      3. Bioretention filter area has reduced to 80m2.




21-0166SMP01-V1 | Stormwater Management Plan | 25 Pavilion Street, Pomona                                |5