Engineering Hydrology Plans (IWMP)

The WBNM model has been calibrated by adjusting rainfall loss rates and by
adjusting the WBNM catchment lag parameter. The value of the catchment lag
parameter adopted for the investigation is 1.8, which is within the accepted range of
values provided in the WBNM technical reference manual. As the existing site and
developed site are largely impervious (carpark, road and roof) and hard pervious
(bowling green) the rainfall loss rates adopted for this investigation are an initial and
continuing loss of 5mm and 0mm for pervious, and 1mm and 0mm for impervious.

WBNM modelling has been carried out for the range of standard design ARI events
ranging from 2 years up to 100 years. Modelling has also been carried out for the
full range of standard design event storm durations ranging from 15 minutes up to 3
hours. Peak flows tabulated in this report represent the peak values extracted from
this range of storm events.

Table 13: Existing Scenario WBNM Outlet Peak Flows

                                   2 year ARI       10 year ARI   100 year ARI
                  Catchment
                                     (m3/s)           (m3/s)         (m3/s)
                   Outlet 1           0.73             1.02           1.40
                   Outlet 2          0.14               0.19         0.26
                   Outlet 3          0.07               0.10         0.14



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 drainage reserve to the
west of the site (WBNM catchment node ‘Outlet 1’). Calibration has been achieved
by adjusting rainfall losses and 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 14 and
additional details of the Rational Method calculation are provided in Appendix 1 of
this report.

Table 14: Rational Method Calibration Summary (Total Catchment)

      Parameter                                                          Value
      Time of Concentration (tc)                                       6 minutes
      Catchment Area                                                 2.01 hectares
      C10 Value                                                          0.87
      100 year Rainfall Intensity (6 minute duration)                 302 mm/hr



A comparison of peak Rational Method flow rates compared with peak WBNM
model flow rates are provide in Table 15 below.



                                                                                            15