5179Flood Report_REVISION_2_7Feb2021

                                                                                               Callaghan & Toth




Figure 3.2 URBS Model of The Post-Development Catchment Condition

The models’ operating parameters comprised the commonly-used default value of 1.2 for the alpha
parameter and an m (non-linearity) exponent of 0.8. The initial rainfall loss was set at 20 mm and the
continuing loss rate was set at 2 mm/hr, for all of the modelled ARI’s from 1 year to 100 year.

The models’ predictions of the peak outflow rates from the system, at the location of the outflow from the
proposed detention basin, are summarised in Table 1 hereafter. That tabulation also includes Rational
Method calculations of the peak outflow rates, to provide a sanity check on the models’ performance.


Table 1:        Predicted Peak Outflow Rates (m3/sec)

     ARI                    Pre-Development               Post-Development
   (years)         URBS           Rational Method   URBS        Rational Method
     100            0.93                1.08         1.06             1.27
      20            0.62                0.59         0.74             0.73
       5            0.43                0.40         0.53             0.48
       1            0.16                0.21         0.25             0.25




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2035 2 Pearsons Road – Flood Study – Revision 2                                     Feburary 2021