Engineering_Integrated Water Management Plan

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 2007 version has been used for this investigation. A peak flow
rational method calibration has also been carried out.

Modelling has been carried out for the 2, 10 and 100 year ARI storm events. The
following scenarios have been investigated:

       1.      Existing Scenario
       2.      Developed Scenario
       3.      Mitigated Scenario

3.1    Existing Scenario
Sub-catchments delineated for the WBNM model have been based on detailed
survey and existing drainage details provided for the site. The existing scenario
WBNM sub-catchment layout is shown in Figure 7 of this report. The WBNM model
has been split into 5 sub-catchments with a total combined catchment area of 2.52
ha.

Sub-catchment fraction impervious values have been calculated based on aerial
imagery data. The existing scenario sub-catchment areas and fraction impervious
values for each sub-catchment are tabulated below in Table 13.

Table 13: WBNM Sub-Catchment Fraction Impervious Values – Existing Scenario

                                                               Existing Scenario
  WBNM Sub-Catchment                  Area (ha)
                                                                Impervious (%)
            SUBA                        1.24                           58
            SUBB                        0.76                          89
            SUBC                        0.34                          90
            SUBD                        0.07                          90
            SUBE                        0.12                          90
            TOTAL                       2.52                           -

The WBNM model has been calibrated to Rational Method derived peak flows at the
outlet of Catchments A and B which flow to drainage reserve to the west of the site
(WBNM catchment node ‘Outlet1’). Details of the Rational Method calibration are
provided in Section 3.2.



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