Engineering Hydrology Plans (IWMP)

3.3     Developed Scenario
The proposed development has been incorporated into the WBNM model by
adjusting the site sub-catchments and the fractions impervious values. Fraction
impervious values of the existing and proposed scenarios are provided in Table 16
below.

Table 16: WBNM Sub-Catchment Fraction Impervious Values – Developed Scenario

      WBNM Sub-                          Developed Scenario      Existing Scenario
                        Area (ha)
      Catchment                            Impervious (%)         Impervious (%)
        SUBA              1.11                  90                       58
        SUBB              0.65                  90                      89
        SUBC              0.46                  90                      90
        SUBD              0.31                  90                      90
        TOTAL             2.52                   -                       -

Peak flows tabulated in this report represent the peak values extracted from the
modelled range of storm events.

Table 17: Developed Scenario WBNM Outlet Peak Flows

                    Existing Peak Flow     Developed Peak Flow   Change in Peak Flow
                           (m3/s)                 (m3/s)               (m3/s)
Outlet 1
       2 year              0.73                      0.69                -0.04
       10 year             1.02                      0.95                -0.07
      100 year             1.40                      1.31                -0.09
Outlet 2
       2 year              0.14                      0.19                    0.05
       10 year             0.19                      0.26                    0.06
      100 year             0.26                      0.35                    0.09
Outlet 3
       2 year              0.07                      0.13                    0.06
       10 year             0.10                      0.18                    0.08
      100 year             0.14                      0.25                    0.11



The catchment boundaries have changed slightly between the existing and
developed scenarios. It can be seen that the peak flows have increased to Outlets 2
and 3. This is a result of the catchment areas increasing. Peak flows from Outlet 1
have decreased as the catchment area has reduced by 0.25ha. Mitigation within the
catchment draining to Outlets 2 and 3 will be required. The developed scenario
WBNM model layout is shown in Figure 8 of this report.




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