Further information re Request to Change Approval MCU 132003.221132.8

STORMWATER QUALITY MANAGEMENT DESIGN AND ASSESSMENTS                                                              2-2

             .     GP    -
                             90%.


             These load reduction targets relate to         a   comparison of the developed site without stormwater
             mitigation to the developed site with stormwater mitigated employed.

             Measures relating to quantity, i.e. frequent flow management and waterway stability are either being

             addressed by others, or are not applicable given that most of the site drains to tidal receiving waters.


2.1.1        Developed Site MUSIC Modelling

             Section 2.3.5 of the approved Doonella Noosa Storm water and Water Quality Management Plan

             (WBM,2005) provided modelling results for the proposed development.

             This existing site MUSIC model which has been continually refined and modified since the initial

             assessment phase back in the early 2000’s (in terms of representing          as   constructed works, and

             maintaining consistency with model releases). This model has again been checked to ensure that it
             represents proposed stormwater management controls for Stage C. Previous stages including A, B
             and D (which are approved) have been assessed and approved under the no-net-increase objective.


             For the MUSIC           modelling, the rainfall period utilised for the assessment was from 1/1/1973 to
             31/12/1982 which is consistent with the recommendations of the SEQ MUSIC Modelling Guidelines

             (WBD,2011). Selection of appropriate rainfall datasets and choice of a longer term modelling period
             for MUSIC assessments is generally considered best practice as it will provide for a more realistic

             estimate of mean annual loads. The model time-step selected for this 10 year period was 6 minute.


             The bio-retention nodes within the models were the "Version 3" nodes, which is consistent with all

             previous modelling work completed for the project, and are understood to provide relatively similar
             results to the most recent Version 5 bio-retention node, which SeRC may not have yet recommended

             for use.


             Stage C when assessed alone provided pollutant export results as included in Table 2-1.

                Table 2-1           Mean Annual Pollutant Loads Stage C    -

                                                                             Modelling Period 73-83, 6min,Cooroy
                                       Pollutant       Pollutant        Pollutant       Pollutant       Load Based
                                       Load (un-         Load             Load            Load           Reduction
               Parameter
                                     developed)       (developed       (developed      Reduction       Development
                                                     unmitigated)       mitigated)   (unmitvs mit)        Target
              Flow (ML/yr)                  46.1         69.9             56.2           19.5%               NA
              TSS (kg/yr)                   3,880       17,400            2,190          87.4%              80%
               TP (kg/yr)                   4.06         18.2              4.4           75.8%              60%
               TN (kg/yr)                   44.5         141               58            58.9%              45%
               GP (ky/yr)                    0          1,000             30.1           97.0%              90%


             The following can be seen from these results:

             .     When the Undeveloped Loads are compared to the Developed Mitigated Loads there is a

                   reduction in TSS loads and increases in TP, TN and GP loads;

             .     The increases in TP load predicted is around 8%. This is similar to what was originally included

                   in the approved Doonella Noosa Storm water and Water Quality Management Plan (WBM,

                   2005);



G:IADMIN’815573.G.BADIR.B15573.002.00.00C                                                       _+
                                                                                               !\,_   BMT WBM