Attachment 6 - Stormwater Management Plan and Engineering Response. · MCU23/0078

3.2   PROPOSED STORMWATER DRAINAGE

      The following points outline the proposed stormwater infrastructure for the development site:

          −     Minor events from within Lot 3 will be conveyed by an internal stormwater piped network to the existing
                1200mm x 600mm box culvert at the south-west corner of the development site;

          −     Minor events from within Lot 7 will be conveyed by an internal stormwater piped network to the existing gully
                pits along the private access roadway along the western frontage of the development site;

          −     Major events will be conveyed via overland flow towards and onto the surrounding private access roadways and
                environmental zones as per existing conditions;

          −     Run-off from the remaining un-developed areas of Lot 7 will be conveyed via catch drains around the proposed
                development works area to the private access roadway as per existing conditions;

          −     As there will be an increase to the impervious area due to the proposed development, there will be an increase to
                peak stormwater discharge rates from the development site. Based on this, stormwater detention measures are
                required, and four below-ground stormwater detention tanks are proposed across both Lot 3 and Lot 7; and

          −     Considering the development works area is greater than 2500m2, stormwater quality treatment measures are
                required for the proposed development as per the State Planning Policy and Noosa Shire Council Planning
                Scheme. Treatment is proposed within multiple bio-retention basins across both development sites.


      Refer to the engineering drawings in Appendix B for further information.


4. STORMWATER QUANTITY ANALYSIS

      Under the Noosa Shire Council Guidelines, several design elements were identified in relation to stormwater management
      issues. Noosa Shire Council must be satisfied that:


          −     The proposed development can be drained;

          −     The stormwater management system can follow the features and functions of the natural drainage system;

          −     The volume, timing, velocity and pollutant load of stormwater has no adverse effect on any surrounding
                properties or receiving waters; and

          −     Suitable provision is made in the design layout to accommodate for major drainage.


      All stormwater discharge will comply with Noosa Shire Council’s Stormwater Management Code with quantity calculations
      completed in accordance with the requirements and methods outlined in the Queensland Urban Drainage Manual
      (QUDM).


      A detailed hydrologic and hydraulic analysis using XP Storm software has been undertaken to model the stormwater flow
      characteristics of the site more accurately, specifically regarding water levels within the adjoining natural drainage channel.


      XP Storm is a hydraulic modelling software tool that utilises detailed hydrograph flow analysis to provide an effective
      representation of urban stormwater systems. Hydrographs are calculated using the Laurenson Method for runoff routing
      in conjunction with the Uniform Loss model for determining catchment losses. XP Storm has been used to demonstrate
      non-worsening on downstream properties through comparison of the results obtained for existing and developed
      mitigated scenarios.




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