Sealed Judgement

3                Stormwater Quantity Management

3.1              Overview
A detailed hydrologic model the Site was developed using the Watershed Bounded Network Model
(WBNM) software package. The hydrologic assessment was undertaken in accordance with the 1987
edition of Australian Rainfall and Runoff, A Guide to Flood Estimation (ARR 1987) guidelines and
relevant Council technical guidelines. Details of the WBMN model setup are discussed in the following
sections.


3.2              Model Data

3.2.1            Rainfall Intensity Data
Design rainfall intensity frequency duration (IFD) data for the Site (Latitude -26.403 and Longitude
153.083) was extracted in accordance with the ARR 1987 guideline, using the parameters listed in
Table 3-1.
Table 3-1    IFD Calculation Parameters


    Design Event                                         Duration
       (ARI)                     1-hour                   12-hour                  72-hour

                                       Rainfall Intensities

         2 Year               52.48 mm/hr               10.17 mm/hr               3.44 mm/hr

        50 Year               94.41 mm/hr               23.20 mm/hr               8.51 mm/hr

                                     Geographic coefficients

            F2                                              4.40

          F50                                              17.65

        G (Skew)                                            0.10


3.2.2            Rainfall Losses
The hydrologic assessment used the initial loss-continuing loss model (IL-CL) approach for
simulating rainfall losses to calculate the excess (effective) rainfall depth as define below:
x   Initial loss = 20 mm for pervious surface and 0.0 mm for impervious surface.
x   Continuing loss = 2.5 mm/hr for pervious surface and 0.0 mm/hr for impervious surface.


3.2.3            Lag Parameter
A global value of 1.8 was adopted for the WBNM model Lag Parameter based on comparison to the
peak flow calculated using the Rational Method employed as a model verification exercise (as
discussed in the next section).




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