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

        The following table highlights the peak flows for each catchment associated with the development site at the southern
        extents of the study area where the natural drainage channel crosses below Hofmann Drive via a series of box culverts. It is
        noted that Noosa Shire Council Flood Hazard Extents should be referred to for a holistic assessment of the flooding
        experienced downstream of the development site and Hofmann Drive culverts.


                                       Table 3: Existing Scenario Critical Peak Flow Runoff (m3/s)
                                                             Peak Flow (m3/s)                        Peak Water Level at
                   Catchment
                                        10 Year ARI            50 Year ARI          100 Year ARI    Hoffman Drive Culverts
              North Catchment               6.959                 8.418                9.243
                                                                                                             4.500m
              South Catchment               1.068                 1.300                1.477


        The results of the existing scenario flood assessment including water level depths, elevations, and velocities during the 1%
        AEP storm event (in the 2100 Year planning period) can be found in Appendix E.


4.1.2   Flood Assessment – Developed Scenario

        The hydrological modelling has been undertaken in XP Storm for the 100-year ARI storm event IFD parameters listed in
        Table 1.


        To establish the catchments contributing to runoff flowing through the natural drainage channel within the environmental
        zone in Covenant J under the developed scenario, DEM data was derived from the QLD Government’s Queensland LiDAR
        Data, in addition to detailed survey data. Catchment boundaries were then determined based on the LiDAR & survey
        contours, a review of the existing piped stormwater network of the existing shopping Centre, and surface levels of the
        proposed development modelled by Bornhorst & Ward. Refer to the Developed Catchment Plan DA-C041 in Appendix E
        for further information.


        Table 4 below outlines the input parameters for each catchment within the model. Stormwater detention is expected to be
        required under the developed scenario to suitably mitigate peak run-off volumes from the site due to an increase to
        impervious area when compared to existing conditions. As such, the impervious fractions adopted for each catchment in
        this developed scenario were not changed from the existing scenario, to replicate peak discharge run-off from the site
        when suitably mitigated to equal to or below that of existing.


        The two changes between the developed scenario and the existing scenario for this flood assessment are the changes to
        the shared boundary between the two catchments, and the land-use of the proposed development site limiting the
        maximum theoretical flood extents of the natural drainage channel where it bounds with Lot 3 development.


        For pervious areas within each catchment, an initial loss of 27mm and a continuing loss of 2.6mm/hr was adopted, as per
        information obtained from the Australian Rainfall and Runoff (ARR) Data Hub. No areal reduction factors have been
        applied to this local assessment.


                                     Table 4: XP Storm Developed Catchment Model Parameters
                                            Impervious                                 Initial Loss                Continuing
         Catchment          Area (ha)                       Slope (%)   Mannings ‘n’
                                                (%)                                       (mm)                    Loss (mm/hr)
            North              3.696                 0                                0.16            27                2.6
                                                                     1.3
          Catchment            8.812                100                              0.013             0                 0
            South              6.335                 0                               0.160            27                2.6
                                                                     1.8
          Catchment            1.038                100                              0.013             0                 0




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