MCU14/0056 Stormwater Quantity Management Plan Att D Information response

4.2    Developed Model
The existing URBS model was modified to represent development of the site. Flow
directions, sub-catchment use and fraction imperviousness were updated using the
development plan which is presented in Figure 5, Appendix A. A comparison between the
existing and developed URBS model results is presented in Table 4.3 below.

Table 4.3 –URBS Model Results
       ARI                Existing URBS         Developed URBS           Difference
       years                   m3/s                   m3/s                  m3/s
         1                     8.55                    8.57                  0.02
         2                     11.92                  11.90                 -0.02
         5                     17.08                  17.10                  0.01
         10                    19.78                  19.81                  0.03
         20                    24.75                  24.68                 -0.08
         50                    35.12                  35.10                 -0.02
        100                    44.04                  43.98                 -0.06

The URBS model results show a minor increase in peak discharges during a Q1, Q5 and Q10
storm event; however reductions are seen during a Q20, Q50 and Q100 storm event. The URBS
results show a net marginal benefit. The nominal storage in the bio retention will attenuate
the frequent low flows. Therefore it is considered that a detention basin is not required.
Detailed URBS input data for the Developed Scenario is presented in Appendix C.




                               J4279 Report 1.0.doc
                                 14 January 2015                                      Page 9