RFI - P0478-26-001_45_Maple_Street_Cooroy_SWMP

   •    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

             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 return period events (e.g. 20-year ARI and greater) 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.




45 MAPLE STREET, COOROY
                                       24 June 2026                                               P a g e | 10
Stormwater Management Plan
Client Reference: P0478-26-001
Prepared for Myall Rise PTY LTD