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

        A flood assessment has been undertaken for the existing natural drainage channel within the environmental zone in
        Covenant J along the western boundary of Lot 3. The flood assessment provides the 1% AEP flood depths, water surface
        levels and velocities along the length of the natural drainage channel bounding Lot 3, for the existing and developed
        scenarios, to allow assessment of potential impacts resulting from the proposed development.


4.1     EXISTING NATURAL DRAINAGE CHANNEL FLOOD ASSESSMENT – XP STORM

4.1.1   Flood Assessment – Existing Scenario


        The hydrological modelling has been undertaken in XP Storm for the 100-year ARI (1% AEP) storm event using AR&R 87
        IFD. In line with Noosa Shire Council requirements, these rainfall intensities were increased by 20% to determine flood
        levels in the 2100 Year planning period, to account for predicted future increases in rainfall intensity and event severity.
        The following table outlines the IFD Parameters used for the analysed storm durations.

                                                          Table 1: IFD Parameters
                                                         Storm       Intensity (mm/hr)
                                                        Duration
                                                         (mins)         100 Year ARI

                                                           20               229
                                                           25               207
                                                           30               189
                                                           45               155
                                                           60               134
                                                           90               109
                                                          120               94
                                                          180               76

        To establish the catchments contributing to runoff flowing through the natural drainage channel within the environmental
        zone, 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, and a review of the existing piped
        stormwater network of the existing shopping Centre. Refer to the Existing Catchment Plan DA-C040 in Appendix E for
        further information.


        Table 2 below outlines the input parameters for each catchment within the model. The impervious fraction for each
        catchment has been determined based on review of aerial imagery. 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 2: XP Storm Existing Catchment Model Parameters
                                              Impervious                                    Initial Loss           Continuing
         Catchment             Area (ha)                       Slope (%)    Mannings ‘n’
                                                   (%)                                         (mm)               Loss (mm/hr)
            North                5.875             0                               0.160              27                2.6
                                                                    1.3
         Catchment               8.812            100                              0.013               0                 0
            South                4.156             0                               0.160              27                2.6
                                                                    1.8
         Catchment               1.039            100                              0.013               0                 0


        The storm durations listed in Table 3 were simulated in XP Storm for each catchment detailed within Table 2 to determine
        the critical storm duration for each catchment. Determining the critical storm for each catchment allows the maximum
        critical flow to be modelled in the hydraulics scenario, resulting in the maximum flow path along the drainage channel.




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