P0198-25-001_12_Gympie_Street_Tewantin_SWMP

The pre-development flow rates for various AEP were calculated using DRAINS. The results are summarized in
Table 2-3.



Table 2-3: DRAINS results for existing catchment

 AEP %                                                 63.2%         50%     20%         10%         5%    2%            1%

 Pre-Development Flow (m³/s)                            0.050        0.066   0.115      0.148    0.177    0.213        0.242



A comparative assessment was conducted to evaluate the variance between DRAINS results and the Rational
Method calculations. The differences between the two methodologies are presented in Table 2-4.



Table 2-4: Comparison between DRAINS and the Rational Method

      AEP %                       Rational Method (m³/s)                             DRAINS (m³/s)         Difference %

      63.2%                                   0.065                                     0.048                     -15%

         50%                                  0.079                                     0.064                     -10%

         20%                                  0.119                                      0.113                    -2%

         10%                                  0.146                                      0.146                    0%

         5%                                   0.175                                      0.176                    0%

         2%                                   0.220                                      0.212                    -2%

         1%                                   0.252                                      0.242                    -2%



The results obtained from the Rational Method and the DRAINS model show differences that are within acceptable
engineering tolerances and align with expectations given the inherent methodological differences between the
two approaches.

     •    The Rational Method is a simplified empirical approach that assumes uniform rainfall intensity across the
          entire catchment for the duration of the storm event. It does not account for hydraulic routing, storage
          effects, or time-variant rainfall intensities, which can lead to either under- or over-estimations of peak
          flows depending on local site characteristics.

     •    DRAINS, in contrast, is a hydrologic and hydraulic modelling tool that provides a more detailed, event-
          based assessment incorporating:



               o     Rainfall losses (initial and continuing losses)

               o     Time-dependent storm profiles.

               o     Pipe and channel storage effects and

               o     Flow routing dynamics.



     •    The Rational Method remains a valid tool for preliminary estimation and cross-checking of peak flows;
          however, the DRAINS model, utilising the ILSAX method, provides a more detailed and accurate



 12 GYMPIE STREET, TEWANTIN
 Stormwater Management Plan                                                                                                   10
 Client Reference: P0198-25-001                    4 November 2025
 Prepared for Novato Group