RFI - P0478-26-001_45_Maple_Street_Cooroy_SWMP

4.3        Hydrological Modelling Results


Analysis of the unmitigated DRAINS model results compared to pre-existing conditions indicates a significant
increase in peak flows across a range of AEP storm events, attributable to the transition from a largely undeveloped
lot to a developed urban condition.
Key Observations

     1.   Increase in Flow Depths and Velocities

               •    The developed scenario shows higher water levels across all storm events, particularly at
                    constricted sections of the drainage network.

               •    Flow velocities have increased, indicating a higher runoff volume due to reduced permeability.

     2.   Peak Discharge Variations

               •    Peak discharge rates in the developed scenario have increased substantially, particularly for
                    higher return period storms (e.g., 1% AEP).

               •    The most significant changes occur at sections with concentrated overland flow paths,
                    suggesting increased runoff from new impervious surfaces.

     3.   Impact of Increased Impervious Area

               •    The development introduces more impervious surfaces (e.g., roads, driveways, rooftops),
                    reducing infiltration and increasing direct runoff.

               •    The loss of natural detention and permeability results in quicker times of concentration and higher
                    peak flows.



Table 4-1: Comparison of Pre-Development vs. Unmitigated Post-Development Peak Flows

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

  63.2% (1-Year ARI)                     0.048                               0.068                   42%

   50% (2-Year ARI)                      0.059                               0.080                   36%

   20% (5-Year ARI)                      0.093                               0.113                   22%

  10% (10-Year ARI)                       0.112                              0.138                   23%

   5% (20-Year ARI)                       0.133                              0.159                   20%

   2% (50-Year ARI)                       0.159                              0.186                   17%

  1% (100-Year ARI)                       0.179                              0.206                   15%




 45 MAPLE STREET, COOROY
                                              24 June 2026                                             P a g e | 13
 Stormwater Management Plan
 Client Reference: P0478-26-001
 Prepared for Myall Rise PTY LTD