Stormwater Management Plan 12_05_26

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4.5.2 Rational Method
Stormwater calculations utilising the Rational Method were undertaken to validate the DRAINS
modelling. The following criteria was applied:
    −    The catchment area of the lot is 0.3161 ha;
    −    Existing case of 0% fraction impervious (fi), based on detail survey;
    −    Developed case of 69% fi, based on detailed design drawings;
    −    Time of concentration of 11 and 5 minutes were adopted for the existing and developed cases;
Rational method calculation was undertaken, peak flow summary for the site is shown in the following
Table 1:
TABLE 1: RATIONAL METHOD PEAK FLOWS

    Storm Event         Existing Case      Developed Case            Difference               Difference
       (AEP)                (m3/s)             (m3/s)                  (m3/s)                     (%)
         63%                 0.046               0.071                 0.024                      52.3%
         39%                 0.062               0.094                 0.032                      52.1%
         18%                 0.085               0.131                 0.046                      53.6%
         10%                 0.103               0.158                 0.055                      54.1%
          5%                 0.122               0.189                 0.067                      54.7%
          2%                 0.154               0.242                 0.088                      56.9%
          1%                 0.177               0.277                 0.101                      57.0%
        1% (CC)              0.212               0.333                 0.121                      57.0%
Runoff flows are shown to increase requiring mitigation measures. Detention is considered in the
following sections to mitigate flow increases.
4.5.3 Modelling
Hydrological analysis of the Existing, Developed, and Mitigated cases was modelled using DRAINS.
The parameters adopted were generally based on recommendations by Australian Rainfall & Runoff
(AR&R) and QUDM. The model node parameters are summarised in Table 2.
TABLE 2: MODEL CATCHMENT NODES – SUBJECT SITE

              Node                   Area (ha)           Impervious (%)                     Scenario
 Cat Existing                         0.3161                    0                            Existing
 Cat Developed                        0.3161                    69                         Developed
 Cat Roof                             0.1803                   100                          Mitigated
 Cat Road                             0.0135                   100                          Mitigated
 Cat Landscape                        0.1223                    20                          Mitigated
Initial Loss (IL) and Continuing Loss (CL) information have been adopted for the DRAINS model, and
the parameters are outlined in Table 3. The site-specific ARR data hub information is also included for
reference; however, the initial loss value is not to be adopted directly for urban development and
therefore reduced according to AR&R 2019 guidance.




12 May 2026