Request to Amend Application and Response to DSDIP's Information Request · MCU14/0009

Table 7: Existing Scenario WBNM Outlet Peak Flows

                 1yr ARI     2yr ARI      5yr ARI   10yr ARI 20yr ARI 50yr ARI        100yr ARI
 Catchment                                (m3/s)                                       (m3/s)
                 (m3/s)      (m3/s)                  (m3/s)   (m3/s)   (m3/s)
   Outlet         0.048       0.062       0.078      0.088     0.101       0.112       0.125


3.2    Rational Method Calibration
The existing scenario WBNM model has been calibrated to Rational Method derived
peak flows at the outlet of the catchment draining to the Hill Street road reserve to
the south-east of the site (WBNM catchment node ‘Outlet’). Calibration has been
achieved by the catchment lag parameter. The adopted calibration parameters are
detailed in the Section 3.1.
A summary of the Rational Method calculation is provided below in Table 8 and
additional details of the Rational Method calculation are provided in Appendix 2 of
this report.
Table 8: Rational Method Calibration Summary (Total Catchment)

             Parameter                                               Value
             Time of Concentration (tc)                            6 minutes
             Catchment Area                                      0.164 hectares
             C10 Value                                                  0.88
             100 year Rainfall Intensity (6 minute duration)      296 mm/hr

A comparison of peak Rational Method flow rates compared with peak WBNM
model flow rates are provide in Table 9 below.
Table 9: Rational Method Peak Flows Versus WBNM Peak Flows

                                          5yr ARI     10yr      20yr        50yr
                 1yr ARI    2yr ARI                                                   100yr ARI
 Catchment                                (m3/s)      ARI       ARI         ARI
                 (m3/s)     (m3/s)                                                     (m3/s)
                                                     (m3/s)    (m3/s)      (m3/s)
  WBNM            0.033       0.045       0.063      0.074     0.089       0.114        0.133
  Rational        0.048       0.062       0.078      0.088     0.101       0.112        0.125
% Difference       30.9       27.3         19.1       15.5     11.5            -2.2     -6.2

The above results show that a reasonable peak flow calibration has been achieved
from the WBNM model when compared to a Rational Method calculation. The
WBNM model produces a higher peak in the smaller events, however, this would be
considered acceptable in assessing non-worsening of flows.




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