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

4.2.3   XP Storm and Rational Method Comparison

        Before utilising data from XP storm, a comparison of results from XP Storm and the Rational Method was undertaken to
        ensure that the XP Storm model was functioning correctly. Peak discharge rates from the site in the existing case were
        compared (“site” catchments defined in Table 6). The comparison was undertaken for the Lot 3 catchment only outlined in
        Table 10. Detailed Rational Method calculations can be reviewed in Appendix D.

                   Table 10: Comparison of Rational Method & XP Storm Existing Peak Flows – Lot 3 (m3/sec)
                AEP (%)      ARI (Years)   Rational Method Result    XP Storm Result          % Difference
                 63.21              1                    0.152                     0.104                   -31.38
                 39.35              2                    0.184                     0.139                   -24.38
                 18.13              5                    0.286                     0.255                   -10.74
                   10              10                    0.358                     0.341                    -4.87
                    5              20                    0.436                     0.433                    -0.70
                    2              50                    0.565                     0.548                    -3.01
                    1             100                    0.660                     0.648                    -1.82


        As indicated, the XP Storm peak discharge results are of the same magnitude as the results initially estimated using the
        Rational Method. It was therefore concluded that the existing XP Storm model was functioning correctly.


4.2.4   Developed Scenario Model


        A developed scenario model was then created with internal catchments and parameters modified from the existing model
        to suit the proposed development as outline in Table 11, 12 and 13.


                                        Table 11: XP Storm Developed Model Parameters – Lot 3
                                         Parameter                    Lot 3 Catchment
                                                                 Pervious Data      Impervious Data
                                Area (ha)                            0.209                 2.770
                                Slope (%)                             2.0                   2.0
                                Mannings ‘n’                         0.03                  0.013
                                Initial Loss (mm)                     27                     0
                                Continuing Loss (mm/hr)               2.6                    0
                                Laurenson ‘n’                       -0.285                 -0.285


                                  Table 12: XP Storm Developed Model Parameters – Lot 7 (North)
                                       Parameter                Lot 7 Northern Catchment
                                                                 Pervious Data      Impervious Data
                                Area (ha)                            0.025                 0.224
                                Slope (%)                             2.5                   2.5
                                Mannings ‘n’                         0.03                  0.015
                                Initial Loss (mm)                     27                     0
                                Continuing Loss (mm/hr)               2.6                    0
                                Laurenson ‘n’                       -0.285                 -0.285




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