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5179Flood Report_REV_3
Callaghan & Toth
The Urbs model of the post-development catchment condition, covering the same overall catchment
area as the Urbs model of the pre-development condition, is shown in Figure 3.2. The model’s subarea
network for the external catchment in the adjoining development to the south is identical to that used for
the pre-development model.
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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) – Sanity Check
ARI Pre-Development Post-Development, Without Detention
(years) URBS Rational Method URBS Rational Method
100 0.89 1.07 1.00 1.25
20 0.60 0.58 0.67 0.72
5 0.41 0.39 0.48 0.48
1 0.15 0.20 0.24 0.25
8
2035 2 Pearsons Road – Flood Study – Revision 3 April 2021
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