Reference Document - Flood & Stormwater Management Plan 28_8_2025

                 Item 21
                 Issue: Storage sizing does not appear to be in accordance with QUDM and ARR2019.

                 Information Request

                 21. Provide revised sizing/modelling and associated engineering drawings and reporting for the detention storage
                 proposed for the development site, which achieves the following:

                     a.   Is undertaken using the ensemble storms approach of ARR2019
                     b.   The detention storage volume of ~200m3/ha (of development) is considered low for this type of land use
                          change. 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
                     c.   Provide a copy of the WBNM model for review
                     d.   Update the basin design to ensure the freeboard requirements of QUDM are achieved for the 1%AEP
                          (2100) event. The proposed freeboard of 50mm is not acceptable as it presents an unacceptable risk of
                          embankment overtopping and failure
                     e.   Demonstrate 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.   Explanation is required for why the 1%AEP (2100) peak flow is predicted to decrease in Table 2-5 rather
                          than increase following development?
                     g.   Are Tables 2-1 and 2-2 meant to refer to Sub3 rather than Sub1 (the neighbouring site)?
                     h.   The SWMP makes a number of assumptions regarding contribution of flows from East of Pavilion St,
                          without justifying how this has been quantified. Calculations supporting the estimated conveyance
                          capacity of Pavilion St are requested.

                 Response:

                 21. Figure 2-3 in the report has been added to clarify proposed cut and fill areas in relation to the 1% AEP
                 (Climate change) flood level/

                     a.   iWBNM Lite (2019 version) ultilised, which applies ARR2019 with ensemble storms.
                     b.   IL revised to 35mm and CL revised to 2.4 mm/hr in line with ARR2019 datahub values.
                     c.   Files will be supplied to Council as requested.
                     d.   The updated basin achieves a freeboard of 0.19m. This improves upon the existing approved
                          freeboard (0.14m) currently provided within the basin for 25 Pavilion St, Pomona, as per
                          Stormwater Management Plan 21-066-SMP02-V1.
                          The augmented basin design has considered flood storage area within the flood overlay, and has
                          limited fill to achieve a balance between providing basin capacity, and achieving flood storage area.
                          As the basin has capacity to contain the 1% AEP (climate change) in the fully blocked case, it is
                          considered that the basin design satisfies the multiple objectives at the site.
                     e.   Refer Table 2-9 for a summary of peak mitigated flows for the 1% AEP event across all storm
                          durations. The analysis demonstrates non-worsening has been achieved across a range of
                          durations.
                     f.   Table updated.
                     g.   Tables updated.
                     h.   Table 2-4 has been provided to qualify the contribution of flows from catchments Ex1 and Ex2 east
                          of Pavilion St, demonstrating the portion of 10% AEP pipe flow diverted around the site and the
                          portion of overland flow that tops Pavilion St and enters the site.




                 23-0167SMP01-V4 | Stormwater Management Plan | 33 Pavilion Street, Pomona                                 Page 11

Document Set ID: 24092538
Version: 1, Version Date: 02/09/2025