12. APP G. Stormwater Management Plan

         STORMWATER QUANTITY, DETENTION &
         HYDRAULICS
         OBJECTIVES
         The stormwater discharge from the proposed development shall maintain the pre-developed
         flow rates for all storm events, from the 1 in 1 year (frequent flow) to 1 in 100 year (major storm
         events).

         The stormwater management strategy will also need to comply with the Noosa Council
         Stormwater Management Code.

         METHODOLOGY
         A hydrologic model was created using XP-RAFTS. The model was calibrated and verified via a
         comparison to the results of the Rational Method calculations (described in Section 4.5). The
         critical storm event for each standard ARI was determined via an investigation of storm
         durations ranging from 10mins to 720mins. The model was then used to route post-
         development runoff through detention basin structures to determine adequate detention basin
         size to comply with the objectives. The model was iteratively optimised to achieve the most
         efficient design dimensions.

         The required detention volume is proposed to be included as additional volume above the
         extended detention depth within the proposed bio-retention basin. The runoff routing model has
         therefore considered suitable dimensions to allow for the installation of filter material in the base
         of the basin. The weir width and outlet pipe size have also been individually designed according
         to the required discharge flow rates and receiving system.

5.1.1    MODEL CONDITIONS
         Three hydrologic conditions were modelled as shown below.

         Case 1 - Existing Scenario
         Case 2 - Developed Scenario
         Case 3 - Mitigated (Developed with Detention Scenario)

5.1.2    DETENTION BASIN DESIGN
         Table 5.1 presents the design details of the proposed detention basin which is required to
         achieve the waterway stability management objectives. The basin design has been iteratively
         optimised to ensure peak performance.

         It has been determined that due to the increase in impervious area, that the site requires
         stormwater quantity controls to mitigate flows. A detention basin has therefore been proposed to
         mitigate this consequent increase in peak discharge resulting from the development.

         The proposed lot layout incorporates the proposed basin area listed in Table 5.1.




                                                                                           Stormwater Management Plans
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