Updated SWMP

Detention Basin
Detention Basin - storage sizing does not appear to be in accordance with QUDM and ARR2019.

Information Request

1)   Provide revised sizing/modeling and associated engineering drawings and reporting for the detention storage
     proposed for the development site, which addresses the following:

     a)   The response states that the initial losses have increased, however page 14 of the SWMP states that the
          losses are unchanged. This needs to be resolved.
     b)   Provide a copy of the WBNM model for review
     c)   The results provided in Appendix B show peak flows increasing for a number of event frequencies and
          durations. 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
     d)   The external catchment areas stated in the existing case (Table 2-1) and developed case (Table 2-5) are
          significantly different. There does not seem to be any valid reason for this. Sub3 and Sub4 also change
          between tables, though these are within the site and this change may be due to physical changes. Justify
          or resolve these differences and provide a ‘developed’ catchment plan where significant differences to the
          existing case do exist.
     e)   Provide calculations quantifying the underground pipe and surface flow capacity of Pavilion St, in order
          to demonstrate that the assumptions made in relation to the amount of external catchment flow which
          enters the site are valid.

Response:

     a)   This is a typographical error. Losses in Section 2.1.1 have been updated to reflect the existing
          scenario WBNM model initial and continuing loss of 35mm and 2.4mm/hr, in line with ARR2019
          datahub values.
     b)   Supplied with SMP-V3.
     c)   The basin design is based upon augmentation of an approved and constructed basin at 25 Pavilion
          St with outlet structure modelled as constructed. Effort has been taken to develop a design that
          mitigates peak flows back to pre-development flow rates whilst improving upon freeboard of the
          approved basin, as per Council’s RFI item 21 (d), dated 15 August 2023. The design achieves both
          objectives, demonstrating:
                   non-worsening has been achieved across the full range of durations for peak 1% AEP
                    event (Appendix B).
                   the critical peak flows for the mitigated scenario are below the peak existing scenario flow
                    rate across all storm events.

          The basin discharges to the local overland flow path which is significant in cross section due to the
          fact that it acts to also convey breakout flows from the regional system in events greater than a
          moderate event (approximately equivalent to at 10% AEP event). The use of this area within both
          sites is restricted by existing regional flood conditions across the sites.
          While it is noted that there are minor increases in some duration events (not the critical event) with
          a single AEP (as provided in Appendix B) the minor change in small, frequent and short duration
          events will be of no consequence and will be unlikely to create impact on downstream properties.
          The basin is located a sufficient distance from the downstream boundary that any minor change (in
          the order of 200 L/s) will be spread and flow across the downstream boundary with no observable
          impacts across the width of the significant flowpath.




23-0167SMP01-V3 | Stormwater Management Plan | 33 Pavilion Street, Pomona                                  Page 15