Engineering_Integrated Water Management Plan

 3.4    Mitigated Scenario
 When comparing the peak flows in the existing and developed scenario, increases in
 the peak discharge occur at Outlets 2 and 3. As a result a detention system will be
 required to be implemented in order to demonstrate non-worsening from the site.

 Peak flows at Outlet 1 have decreased slightly due to catchment boundaries
 changing, resulting from a reduction in total catchment. While flows draining to
 Outlet 4 remain the same as the percent impervious remain the same in the existing
 and development scenario.

 All modeling parameters remain consistent between the developed and mitigated
 scenarios, with the addition of a detention system within sub-catchments C and D.
 Due to site constraints the detention system has been modeled as a
 square/rectangular underground detention system. The storage details included
 within the model are provided in Table 19.

 Table 19: Storage and Outlet Details of Detention System

Sub-Catchment       Storage       Storage Height       Outlet Pipe
                                                                         Peak 100yr WSL (m)
   Location       Volume (m3)          (m)           Diameter (mm)
    SUBC              100              1.0                375                   0.87
    SUBD              100               1.0                 150                 0.99

 A comparison of the peak flows between the existing, developed and mitigated
 scenarios are presented below in Table 20. It can be seen that non-worsening of
 peak flows are achieved at each outlet to the site.
 Table 20: Comparison of Existing, Developed and Mitigated WBNM Outlet Peak Flows

                  Existing Peak     Developed Peak      Mitigated Peak
                                                                              Reduction
                   Flow (m3/s)        Flow (m3/s)        Flow (m3/s)
Outlet 1
       2               0.72               0.71                0.71              -0.02
      10               1.01               0.97                0.97              -0.04
     100               1.40               1.35                1.35              -0.05
Outlet 2
       2               0.14               0.18                0.14               0.00
      10               0.19               0.25                0.19              -0.01
     100               0.27               0.34                0.24              -0.03
Outlet 3
       2               0.03               0.07                0.03               0.00
      10               0.04               0.10                0.03              -0.01
     100               0.06               0.14                0.04              -0.01
Outlet 4
       2               0.05               0.05                0.05               0.00
      10               0.07               0.07                0.07               0.00
     100               0.10               0.10                0.10               0.00


                                                                                          20