Attachment 5 - Stormwater Managment Plan · 132007.12.8

3 Stormwater Quantity
The objective of this component of the study is to investigate the potential impact
that the proposed development will have on peak flows discharging downstream of
the site. Where any increases of peak flows are found, potential mitigation options
will be investigated in the form of a detention system.
Hydrologic modelling of the catchment has been carried out using WBNM software
developed jointly by the University of Wollongong, Rienco Consulting and Balance
R&D. The January 2012 version has been used for this investigation.
Modelling has been carried out for the 1, 2, 5, 10, 20, 50 and 100 year ARI storm
events. The following scenarios have been investigated:
       1.     Existing Scenario
       2.     Developed Scenario
       3.     Mitigated Scenario
3.1.1 Existing Scenario
Sub-catchment fraction impervious values have been calculated based on aerial
imagery data and consistent to with existing Telstra site to the north and the existing
unsealed carpark. The existing carpark could be considered 100% impervious,
however, as there is likely to be slightly more runoff from a roofed surface, the existing
carpark has been assigned a fraction impervious of 80%. The existing scenario sub-
catchment areas and fraction impervious values for each sub-catchment are tabulated
below in Table 7. The existing catchment plan is shown below in Figure 4.
Table 7: Sub-Catchment Area and Fraction Impervious Values – Existing Scenario
                                                                    Existing Scenario
   WBNM Sub-Catchment                   Area (ha)
                                                                     Impervious (%)
             SUB1                         0.262                             56
             SUB2                         0.202                            80
             TOTAL                        0.464

In the existing scenario, there is one outlet, with all flows reporting to the bushland
park downstream of the site. Peak flows reporting to this outlet are presented
below in Table 8.

Table 8: Peak Flows at Each Outlet

                1yr ARI   2yr ARI    5yr ARI      10yr ARI 20yr ARI 50yr ARI      100yr ARI
 Catchment                           (m3/s)
                (m3/s)    (m3/s)                   (m3/s)   (m3/s)   (m3/s)        (m3/s)
   Outlet        0.101     0.142      0.192        0.220    0.259       0.279       0.314




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