17 Bottlebrush Avenue, · MCU26/0001

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

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

         63.2%                           0.010                     0.011                         -6%

          50%                            0.012                     0.013                         -6%

           20%                           0.018                     0.020                         -6%

           10%                           0.022                     0.024                         -5%

           5%                            0.026                     0.028                         -4%

           2%                            0.033                    0.034                          -2%

            1%                           0.037                     0.038                          -1%



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
         assessment of catchment behaviour. Therefore, DRAINS model results have been adopted for final design.

    •    Minor discrepancies, particularly for higher AEP events (20-year and above), indicate that the Rational
         Method provides a reasonable approximation for design verification.

    •    The largest discrepancies are typically observed in low-frequency events (1 to 5 year), where DRAINS
         captures additional hydrological complexities such as temporal rainfall variation and surface storage,
         which are not represented in the Rational Method.




 17 BOTTLEBRUSH AVENUE, NOOSA HEADS
                                              7 January 2026                                       P a g e | 10
 Stormwater Management Plan
 Client Reference: P0433-25-001
 Prepared for Made Capital PTY LTD