Attachment 5 - Stormwater Managment Plan · 132007.12.8

3.1.2 Developed Scenario
The proposed development has been incorporated into the WBNM model by adjusting
the site fractions impervious values. Area and fraction impervious values for the
developed scenario are provided in Table 9 below. The impervious percentage of this
scenario are reflective of the development being proposed.
Table 9: Sub-Catchment Area and Fraction Impervious Values – Developed Scenario
                                                              Developed Scenario
   WBNM Sub-Catchment                 Area (ha)
                                                                Impervious (%)
           SUB1                            0.262                     56
           SUB2                            0.202                     90
           TOTAL                           0.464

As the development, will direct all flows from the site to the outlet reported as
‘Easement’ in Figure 4 and Table 8, peak flows in the developed scenario will be
compared to flows at this outlet. A comparison of the existing and developed is
presented below in Table 13.
Table 10: Developed Scenario Outlet Peak Flow Comparison – Easement Outlet
                      Existing Peak Flow      Developed Peak Flow   Change in Peak Flow
                             (m3/s)                  (m3/s)               (m3/s)
       1 year               0.101                    0.107                   0.006
       2 year               0.142                    0.146                   0.004
       5 year               0.192                    0.195                   0.003
       10 year              0.220                    0.223                   0.003
       20 year              0.259                    0.261                   0.002
       50 year              0.279                    0.280                   0.001
      100 year              0.314                    0.315                   0.001

It can be seen that the peak flows have increased slightly to the catchment outlet.
This is a result of the development site increasing the impervious area. Mitigation
within the catchment will be required. The developed catchment is the same as the
existing catchment shown in Figure 4.




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