Sealed Judgement

3.3.2          Model Verification

To confirm that the peak flows predicted by the WBNM model under the pre-development scenario
were reasonable, a comparison was made to the peak flows calculated using the Rational Method.
The comparison was made at the outlet of sub-catchment A. The following parameters were adopted
for the Rational Method:
x     Pre-development area of sub-catchment A = 0.326 ha.

x     Time of concentration (TC) = TC was calculated in accordance with the Queensland Urban
      Drainage Manual (QUDM) using a combination of the Friend’s equation applicable for overland
      sheet flow and the Stream Velocity method applicable for concentrated flow. For the Friend’s
      equation, an overland sheet flow length of 50 m, a slope of 5.0% and Manning’s ‘n’ coefficient
      of 0.045 were used. For the Stream Velocity method, a stream length of 15 m and channel
      velocity of 0.7 m/s. Thus, combining the two methods, the TC was calculated to be 13.2
      minutes.

x     C10 = 0.70 and C100 = 0.84.
x     1% AEP (100 Year ARI) rainfall intensity corresponding to the TC = 223.2 mm/hr.

Based on the above parameters, 1% AEP (100 Year ARI) peak flow was calculated to be 0.170 m 3/s.
Table 3-3 presents a comparison between the Rational Method and WBNM peak flows for a selection
of design flood events. The results indicate that the WBNM model predicts peak flow rates that are
slightly greater than those of the Rational method. Notwithstanding, it was considered preferrable
to retain the conservative WBNM flow rates.

Overall, the pre-development WBNM model of the catchment was deemed to be fit for purpose.
Therefore, the verified model was utilized to assess the post-development conditions (Refer to the
next section for further details).
Table 3-3      WBNM Versus Rational Method Flows - Sub-catchment A

     Design Event (ARI)         Rational Method         WBNM          Difference
                                     (m3/s)             (m3/s)            (%)

     2 Year ARI (50% AEP)            0.061               0.068            + 11

    10 Year ARI (10% AEP)            0.098               0.126            + 29

    100 Year ARI (1% AEP)            0.170               0.197            + 16



3.4            Post-Development Model
The post-development properties were defined based on the proposed development layout and the
existing contours. Like the pre-development scenario, the drainage area was divided into three (3)
sub-catchments (namely, A, A1 and EXT as shown in Figure 3-2). The main lumped catchment
properties (area and impervious percentage) are listed in Table 3-4.
Table 3-4      Post-Development Lumped Catchment Properties

    Sub-catchment       Area          Impervious Value
                        (ha)                  (%)

          A             0.343                 53.7

          A1            0.111                 30.6

         EXT            0.099                 80.0




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