Updated SWMP

2.1.4.      Mitigation Scenario
When comparing the peak flows in the existing and developed scenarios, increases in the peak discharge
are predicted at the site outlet. A detention basin is proposed to attenuate peak flows discharging from the
site to demonstrate non-worsening and meet the legal point of discharge requirements.

All modelling parameters remain consistent between the developed and mitigated scenarios, with the
addition of detention basins. The proposed basin structure details are summarised in Table 2-7 with
Concept Engineering Drawings included in Appendix A.

Table 2-7: Summary of Basin Structure Details
   Sub-catchment                    Existing Basin         Upgraded Basin
     Elevation (RL)                                          Storage (m3)
            0                               0                      0
           0.1                             26                     35
           0.2                             55                     74
           0.3                             87                     117
           0.4                            123                     163
           0.5                            162                     214
           0.6                            205                     268
           0.7                            251                     327
           0.8                            301                     391
           0.9                            338                     458
          0.97                              -                     508
      Pipe Outlet            1 x 450mm RCP at RL0.0m   1 x 450mm RCP at RL0.0m
 Bioretention Outlet                Not modelled       1 x 450mm RCP at RL-0.9m
      Weir Outlet               4m wide at RL.0.60m      8m wide at RL.0.60m
      Crest Level                      RL 0.97m                RL 0.97m
     Q100CC WSL                        RL 0.83m                RL0.78m
 Q100CC Freeboard                        0.14m                  0.19m



Peak flows from the mitigated scenario modelling are presented in Table 2-8 with a comparison against the
existing peak flows. The results show that there is non-worsening for events up the 1% AEP. The intent of
sizing the detention basin is to mitigate flows up to the 10% AEP storm event to ensure non-worsening of
discharges into the downstream flow path. In larger events, flows from the site will start to sheet flow
across the rear of the site to Cooroora Creek. As the discharge point for the site becomes inundated from
Cooroora Creek flooding in the major storm event (refer Figure 1-2), it is considered that the non-worsening
has been achieved.

Table 2-8 contains a summary of peak mitigated flows for the 1% AEP event across all storm durations. The
analysis demonstrates non-worsening has been achieved across a range of durations.




23-0167SMP01-V3 | Stormwater Management Plan | 33 Pavilion Street, Pomona                           Page 23