Stormwater Management Plan 12_05_26

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The modelled discharges were compared with the Rational Method calculations for the existing and
developed cases to validate the DRAINS model, refer Table 5 and Table 6. The modelled discharges
compare well with the Rational Method calculations.
TABLE 5: EXISTING FLOWS COMPARISON (RATIONAL METHOD VS DRAINS)

    Storm Event           Rational             DRAINS             Difference                 Difference
       (AEP)            Method (m3/s)           (m3/s)              (m3/s)                       (%)
        63%                  0.046               0.041               -0.005                     -11.5%
        39%                  0.062               0.063               0.001                        2.1%
        18%                  0.085               0.092               0.007                        8.0%
        10%                  0.103               0.110               0.007                        7.2%
         5%                  0.122               0.134               0.012                        9.6%
         2%                  0.154               0.154               0.000                        0.0%
         1%                  0.177               0.174               -0.003                      -1.4%
      1% (CC)                0.212               0.218               0.006                        2.9%
                                           Average (mean)            0.003                        2.1%


TABLE 6: DEVELOPED FLOWS COMPARISON (RATIONAL METHOD VS DRAINS)

    Storm Event           Rational             DRAINS             Difference                 Difference
       (AEP)            Method (m3/s)           (m3/s)              (m3/s)                       (%)
        63%                  0.071               0.073               0.002                        3.4%
        39%                  0.094               0.091               -0.003                      -3.0%
        18%                  0.131               0.119               -0.012                      -9.1%
        10%                  0.158               0.139               -0.019                     -12.1%
         5%                  0.189               0.171               -0.018                      -9.6%
         2%                  0.242               0.199               -0.043                     -17.7%
         1%                  0.277               0.227               -0.050                     -18.1%
      1% (CC)                0.333               0.283               -0.050                     -14.9%
                                           Average (mean)            -0.024                     -10.1%
4.5.5 Mitigated Case
Peak flows from the developed case are shown to be mitigated using OSD. A summary of the existing
and mitigated peak flows is shown below in Table 7. Roof water is directed to the OSD, prior to the
connection to the existing public network. The landscape surface flows bypass the tank whilst achieving
mitigation.




12 May 2026