Engineering Hydrology Plans (IWMP)

 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.

 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 18.

 Table 18: 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                 250                 0.99



 A comparison of the peak flows between the existing, developed and mitigated
 scenarios are presented below in Table 19. It can be seen that non-worsening of
 peak flows are achieved at each outlet to the site.

 Table 19: 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               0.73               0.69                0.69              -0.04
      10               1.02               0.95                0.95              -0.07
     100               1.40               1.31                1.31              -0.09
Outlet 2
       2               0.14               0.19                0.14              0.00
      10               0.19               0.26                0.19              0.00
     100               0.26               0.35                0.25              -0.02
Outlet 3
       2               0.07               0.13                0.07              0.00
      10               0.10               0.18                0.10              -0.01
     100               0.14               0.25                0.13              -0.01




                                                                                          20