Appendix J - Civil Engineering Report and SBSMP

                                                                                   Noosa Residential Development
                                                                             Ref: 24553 C R002 REV03 10.08.21.docx
                                                                                                      August 2021

         Detailed design and documentation of the OSD will be carried out once the development application
         has been approved by Council. Refer to Appendix C for the conceptual stormwater drainage layout
         and details.

6.4      Peak Storm Results
         A comparison between the peak discharge values between each of the XP-STORM modelled
         scenarios is presented in Table 4.

                        Table 4 – XP-STORM Modelling Peak Storm Mitigation Results (Free Outlet)
               Design     Pre-Development    Post-Development      Post-Development      Flow change from
               Storm           Flows         Flows, Unmitigated     Flows, Mitigated     Pre-Development
               (AEP)           (m3/s)              (m3/s)                (m3/s)                (m3/s)
                39%             0.177               0.282                 0.170               -3.95%
                20%             0.275               0.359                 0.256               -6.91%
                10%             0.325               0.490                 0.326                0.31%
                 5%             0.402               0.578                 0.391               -2.74%
                 2%             0.707               0.712                 0.644               -8.91%
                 1%             0.807               0.812                 0.744               -7.81%

               Table 5 – XP-STORM Modelling Peak Storm Mitigation Results (Full Open Channel at LPD)
               Design     Pre-Development    Post-Development      Post-Development      Flow change from
               Storm           Flows         Flows, Unmitigated     Flows, Mitigated     Pre-Development
               (AEP)           (m3/s)              (m3/s)                (m3/s)                (m3/s)
                39%             0.177               0.282                 0.182                2.82%
                20%             0.275               0.359                 0.252               -8.36%
                10%             0.325               0.490                 0.331                1.85%
                 5%             0.402               0.578                 0.390               -2.99%
                 2%             0.707               0.712                 0.688               -2.69%
                 1%             0.807               0.812                 0.790               -2.11%


         As seen in Table 4 and Table 5, the proposed OSD successfully mitigates the post-development
         flows either to pre-development conditions or with a negligible increase over pre-development flows.
         As seen in the graphed flow-time graphs in Appendix G the timing of peak flows for each post-
         development storm event is closely matched to the pre-development case.

6.5      Recommendation
         As demonstrated in Table 4 and Table 5 above, there has been an increase in peak flow from the
         pre to post development unmitigated stage. To mitigate this increase in flow 14.0m3 of OSD is
         proposed downstream of Catchment B1, which is shown to mitigate total peak site discharge to pre-
         development levels. A conceptual arrangement of the proposed detention system can be seen on
         sketch plan SK02 in Appendix C.

         This arrangement may be subject to modification at the detailed design phase. Final levels across
         the site may facilitate provision of storage over the bioretention basin or necessitate reconfiguration
         of the OSD tank dimensions / outlet arrangement.

6.6      Onsite Detention Lifecycle Costs
         A lifecycle cost analysis is not a part of the scope of this report. All stormwater infrastructure
         associated with the development shall be maintained and serviced by the owners of the development
         at no cost to Council.




BRISBANE / SYDNEY / MELBOURNE / GOLD COAST / SUNSHINE COAST / DARWIN / PERTH                              Page 14