Response to Stormwater Management Approval Conditions Letter

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Response

   As indicated in Section 4.5 in SWMP Report No. 16-001014-01A, proposed stormwater management devices are
   adequate to treat stormwater runoff to meet Water Quality Objectives (WQOs) specified under the Healthy
   Waterways WSUD Technical Guidelines for SEQ (2006) and the SPP (2014) prior to release off site.

   Table 4-7 from SWMP Report No. 16-001014-01A has been provided below demonstrating that WQOs will be
   achieved.

   Table 4-7: MUSIC Modelling Results (Extract from SWMP Report No. 16-001014-01A)

                                                     TSS              TP             TN         Gross Pollutants
                  Description
                                                    (kg/yr)         (kg/yr)        (kg/yr)          (kg/yr)

    Developed Site Only (Without
                                                    29,700            65             425              4,270
    Mitigation)
    WQOs (Percentage Reduction)                      80%             60%            45%                90%

    Load Retention Target                           23,760            39             191              3,843

    Developed Site plus External
                                                    36,500            77             480              4,630
    Catchment (Without Mitigation)
    Developed Site plus External
                                                    12,700            33             289               488
    Catchment (With Mitigation)
    Load Retention Achieved                         23,800            44             191              4,142

    Load Retention Achieved as % of
                                                     80%             68%            45%                97%
    Developed Site Load

    Objective Achieved?                              YES             YES            YES                YES


   Two models have been provided to Council, the “Developed Site Only” model and the “Developed Site with External
   - Mitigated” model. The methodology for demonstrating that WQOs for the site have been met is summarised below.
       The Load Retention Target (in kg/yr) for the site was determined using the “Developed Site Only” model,
        estimating the pollutants generated from the developed site only.
       The Load Retention Achieved (in kg/yr) was determined using the “Developed Site with External - Mitigated”
        model, estimating the amount of pollutants removed from site and External Catchment B1 and C1 as a result of
        the stormwater quality improvement devices.
   A comparison between the targets and pollutant retention achieved demonstrate that WQO’s for the site have been
   met. This approach is consistent with past DA approvals (for the interim layout) and earlier discussions with
   Council.

   Catchment B1 and C1 as shown on Drawing No. B14158.W-SK102 are external and are not part of the site area.
   While External Catchment B1 and C1 have been included in the stormwater quality analysis (as runoff from these
   areas enter the site and into proposed stormwater quality treatment devices), these areas are not considered a “part
   of the development” or identified as ‘site area’. As such, determining the amount pollutant reduction required to meet
   objectives for the site (i.e. Load Retention Target) does include the pollutants generated from the external
   catchment areas. The Load Retention Achieved has been determined by the total pollutants removed by the
   proposed SQID’s prior to discharge from the site.

   The approach and methodology outlined above is in line with past approved documentation and is therefore
   considered to be appropriate as it demonstrates that WQO’s for the site will be achieved through the proposed
   stormwater quality improvement devices.