response to information request updated letter - pomona subdivision 19.11.2021

   8. Justify how the proposed discharge from the bioretention basin does not
      represent a concentration of flow onto the neighbouring lot and how
      discharge rights can be secured over the neighbouring lot in order to ensure
      that the outlet from the bioretention basin will not be impeded in the future.
      Is there any reason why the discharge from the bioretention basin cannot be
      directed onto the balance of proposed lot 10 instead, in order to avoid these
      issues with the neighbouring lot?

Preliminary Drawings have been prepared for the proposed bio-retention basin. The size
of the basin is based on parameters contain in the Stormwater Management Plan prepared
by Peak Urban. TODs plans demonstrate the required bio basin filter area (120m2) and
storage volume can be provided with the allocated space in the plan of development.

1 in 4 batters have been adopted throughout except for a 1 in 2 batter on the northern side
of the basin. There should be no need for any retaining walls in the bio basin. The outlet
from the basin is directed to the west towards the creek as well as discharging into the
balance area of lot 10 in a non-worsening manner.

   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.

Peak Urban have directly addressed this matter in their updated Stormwater Management
Plan dated 28 October 2021.

       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.

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