RFI - stormwater report

A summary of the Rational Method calculation is provided below in Table 3-2 and
additional details of the Rational Method calculation are provided in Appendix 3 of this
report. Time of concentration was obtained from Table 4.6.3 of QUDM (Recommended
roof drainage travel times). The C10 value was obtained from Table 4.5.3 of QUDM and
the site was determined to be 6.8% impervious with I10 rainfall intensity between 60-64
mm/hr.
Table 3-7: Regional Rational Method Validation Summary (C1 Outlet)

              Parameter                                                   Value
              Time of Concentration (tc)                                29 minutes
              Catchment Area                                          33.658 hectares
              C10 Value                                                      0.59

A comparison of peak Rational Method flow rates compared with peak WBNM model
flow rates are provided in Table 3-3 below.
Table 3-8: Regional Existing Rational Method Peak Flows Versus WBNM Peak Flows (C1 Outlet)

                   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)
   Rational         2.653     3.066         4.540     5.528          6.511    7.550            8.493
    WBNM           2.404       2.879       4.335      5.334          6.374      8.083         9.308
 % Difference      10.4%       6.5%         4.7%      3.6%           2.1%       -6.6%         -8.8%

The above results show that a reasonable peak flow validation has been achieved from
the WBNM model when compared to a Rational Method calculation and is considered
acceptable in assessing non-increasing in nuisance flooding from stormwater flows.

3.1.6 Regional Developed Scenario
The proposed development has been incorporated into the WBNM model by adjusting
the site sub-catchments and the fractions impervious values to represent the
development. The catchment was split into six sub-catchments in order to represent the
surface contours and the existing discharge locations. Fraction impervious values of the
existing and proposed scenarios are provided in Table 3-4 below. The impervious
percentage of the new development has been calculated from the layout and
represented as 5% impervious for catchment B2.
Table 3-9: WBNM Sub-Catchment Fraction Impervious Values – Regional Developed Scenario
                                                      Existing Scenario       Developed Scenario
  WBNM Sub-Catchment               Area (ha)
                                                       Impervious (%)           Impervious (%)
              A1                       4.371                  0                       0
              A2                       4.694                    5                       5
              A3                       4.298                    5                       5
              B1                       3.499                    0                       0
              B2                       4.991                    1                       5
              C1                       11.805                 15                        15
          TOTAL                        33.658                   -                       -

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