Appendix C - Flood Report_Sept2020

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Callaghan & Toth

 

   

\ 0 SCALE 50m

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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)

 

 

 

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

 

 

 

 

 

 

 

2035 2 Pearsons Road — Flood Study September 2020