Flood Investigation & Stormwater Quantity Management Plan

Flood Investigation & Stormwater Quantity Management Plan
Proposed Residential Development at Doonella - Stage C

The developed scenario model results indicate that significant increases in peak flow to the site boundary occur for all
events modelled. Mitigation of the peak flow increases is required in order to ensure no adverse impact will occur on
downstream properties.

3.4      Management Strategy
Two detention storages are proposed to mitigate the increase in peak flows from the site. The two detention basins
are the following:
The Dodonaea Close Basin
The construction of the road embankment across the environmental corridor at Dodonaea Close will effectively detain
flows from the upstream catchment. Flows through an optimally sized culvert underneath the crossing will control
outflows downstream. Design of the crossing is to also ensure overtopping of the road embankment occurs during
high ARI events only, with the depth and velocity of overtopping flow limited to ensure the crossing is still trafficable.
Twigrush Court Bio-Retention Basin
The bio-retention basin at the lower end of Twigrush Court will also capture and detain flows from upstream sub-
catchments C1 and C2. Outflows from the bio-retention basin will be controlled by the bio-retention basin filter and
stormwater drainage field inlet located above the invert of the basin.
Refer to the concept basin designs on Drawing No. N09004-SK26 in Appendix D for details of the proposed detention
storages.

3.5      Mitigated Scenario
The two proposed detention storages have been entered into the WBNM as Outlet Structures. Stage-Storage-
Discharge relationships have been used to model the change in water surface level and outflow from each detention
storage. The configuration for each detention storage is summarised below in Table 3.9. Refer to Appendix B for
detailed calculations used to generate the Stage-Storage-Discharge relationships.
                                  Table 3.9: Detention Storage Configuration Details
        Details                      Dodonaea Close Basin             Twigrush Bio-Basin
        Basin Invert RL (m.AHD)      9                                7.5
        Basin Top RL (m.AHD)         12.25                            9.6
        Storage (m3)                 2,520                            1,210
                                     1 x 450mm dia RCP, U/S IL        1 x 525mm dia RCP, U/S IL 6.9m.AHD,
                                     9.3m.AHD, D/S IL                 D/S IL 6.50m.AHD. RCP fed by 900mm x
        Outlet Structure             9.15m.AHD                        900mm field inlet at RL 9.00m.AHD.
                                                                      2 x 225mm dia RCP, U/S IL 7.8m.AHD.
                                                                      RCP’s within D/S wall of Bio-Basin.
                                     Road Crown, minimum level        5m Wide Weir at RL 9.3m.AHD
        Overflow Weir
                                     11.93m.AHD

The conceptual detention Stage-Storage-Discharge relationships were entered into the WBNM and run to determine
their effectiveness in mitigating the increase in peak flows resulting from the development. The results from the
mitigated scenario model are presented below in Table 3.10.




B09182.W-02A                                                                                                         Page 7