Attachment 5 - Stormwater Managment Plan · 132007.12.8

3.1.3 Mitigated Scenario
When comparing the peak flows in the existing and developed scenarios, increases in
the peak discharge occur at the catchment outlet. As a result, a detention system will
be required to be implemented in order to demonstrate non-worsening from the site.
All modeling parameters remain consistent between the developed and mitigated
scenarios, with the addition of a detention basin downstream of SUB1 and SUB2. The
storage details included within the model are provided in Table 11.
The catchment for the mitigated scenario is the same as the existing scenario as shown
in Figure 4, however, the local detention system has been applied to SUB2.
Table 11: Storage and Outlet Details of Detention System
                                Basin 1
       Elevation             Storage (m3)                          Comment
          5.7                        0                Top of Bioretention Extended Detention

          6.1                        23                             Weir Level

       Pipe Outlet      450mm Dia at RL5.7m
      Weir Outlet       2.7m wide at RL6.1m
       Q100 WSL                 RL6.08m
     Q100 Volume                 22m3
        Q10 WSL                 RL5.98m
      Q10 Volume                 16m3

A comparison of the peak flows between the existing, developed and mitigated
scenarios are presented below in Table 12. It can be seen that non-worsening of peak
flows is achieved at the outlet to the site.
Table 12: Comparison of Existing, Developed and Mitigated Peak Flows at SUB1
                     Existing Peak        Developed Peak     Mitigated Peak       Change in Peak
                      Flow (m3/s)           Flow (m3/s)       Flow (m3/s)          Flow (m3/s)
     1 year              0.101                 0.107              0.100               -0.001
     2 year             0.142                 0.146                0.138               -0.004
     5 year             0.192                 0.195                0.185               -0.007
     10 year            0.220                 0.223                0.212               -0.008
     20 year            0.259                 0.261                0.249               -0.010
     50 year            0.279                 0.280                0.269               -0.010
    100 year            0.314                 0.315                0.302               -0.012

WBNM outputs are provided in Appendix 2 of this report.
Preliminary design plans of the system have been prepared by TOD Consulting and are
provided in Appendix 3.




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