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

        The storm durations listed in Table 5 were simulated in XP Storm for each catchment detailed within Table 4 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 through the site.


        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, for
        the developed scenario where stormwater quantity detention is provided for all development site sub-catchments.

                                        Table 5: Developed 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               6.953                        8.417                       9.239
                                                                                                                                 4.492m AHD
                South Catchment               0.885                        1.152                       1.345


        A comparison between Tables 3 & 5 shows that peak flow discharge and water levels across the Hoffman Drive culvert
        crossing are not increasing for both catchments in the development scenario. Thus, this flood assessment has determined
        that the proposed development will not worsen downstream water levels and flood extents across Hoffman Drive.


        The results of the developed 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. Developed flood levels are also shown
        in the Earthworks Sections on Drawing C-DA020 in Appendix B.


4.2     DETAILED HYDRAULIC MODELLING – XP STORM – DETENTION SIZING

4.2.1   Existing Scenario Model


        Initially the existing XP Storm model was built using catchments’ parameters outlined in Table 6. The initial and continuing
        loss properties were assumed from site investigations and recent experience with stormwater modelling in the subject
        area.
                                               Table 6: XP Storm Existing Model Parameters
                         Parameter                     Lot 3                       Lot 7 - North                   Lot 7 - South
                                            Pervious     Impervious         Pervious     Impervious            Pervious     Impervious
                                              Data             Data           Data            Data              Data            Data
                         Area (ha)            2.979              -            0.249             -               0.338             -
                         Slope (%)                      2.0                             2.4                               2.0
                         Mannings ‘n’         0.160              -            0.160             -               0.160             -
                         Initial Loss
                                               27                0             27               0                27               0
                         (mm)
                         Continuing
                                               2.6               0             2.6              0                2.6              0
                         Loss (mm/hr)
                         Laurenson ‘n’       -0.285            -0.285        -0.285           -0.285            -0.285          -0.285


4.2.2   Results for Existing Scenario


        Table 7, Table 8, and Table 9 indicates the existing total peak discharge rate for the site as outputted by XP storm for both
        Lot 3 and the two sub-catchments of Lot 7 (North and South). The critical storm duration for the existing catchment was
        determined by simulating all storm events from the 1 to 100-year ARI storm events for 20, 25, 30, 45, 60, 90, 120 and 180-
        minute durations. The critical storm events can be seen within tables below.




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