RFI - stormwater report

3.1.2 Site Rational Method Validation (A01)
The WBNM model has been validated to the Rational Method derived peak flows at the
outlet of the catchment (WBNM catchment node ‘Outlet’). Validation has been achieved
by the catchment lag parameter.
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 2 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 5% impervious with I10 rainfall intensity between 60-64
mm/hr.
Table 3-2: Site Rational Method Validation Summary (A01)

              Parameter                                                Value
              Time of Concentration (tc)                             6 minutes
              Catchment Area                                       1.256 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 3-3: Site Existing Rational Method Peak Flows Versus WBNM Peak Flows (A01)

                    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          0.173     0.207        0.311     0.383      0.459    0.583        0.670
    WBNM            0.198      0.228        0.319     0.382      0.441       0.513     0.569
 % Difference       14.4%      10.3%        2.5%      -0.3%      -3.8%      -11.9%    -15.1%

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.

3.1.3 Site 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 is represent by 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 48% impervious for catchment
A01.
Table 3-4: WBNM Sub-Catchment Fraction Impervious Values – Site Developed Scenario
                                                      Existing Scenario    Developed Scenario
  WBNM Sub-Catchment               Area (ha)
                                                       Impervious (%)        Impervious (%)
              A01                       1.256                 5                   48
          TOTAL                         1.256



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