P0198-25-001_12_Gympie_Street_Tewantin_SWMP

3.3        Hydrological Modelling Results

Analysis of Unmitigated DRAINS Model Results and Required Mitigation Measures

A review of the unmitigated DRAINS model results indicates a significant increase in peak flows across various
AEP storm events due to the transition from an existing rural setting 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 3-1: Comparison of Pre-Development vs. Post-Development Peak Flows


 Storm Event (AEP)           Pre-Development Peak Flow                Post-Development Peak        Increase %
                                       (m³/s)                               Flow (m³/s)


          63.2%                           0.048                               0.163                   55%
          50%                             0.064                               0.190                   50%
          20%                             0.113                               0.273                   41%
          10%                             0.146                               0.328                   38%
           5%                             0.176                               0.381                   37%
           2%                             0.212                               0.449                   36%
           1%                             0.242                               0.500                   35%




 12 GYMPIE STREET, TEWANTIN
 Stormwater Management Plan                                                                                         14
 Client Reference: P0198-25-001               4 November 2025
 Prepared for Novato Group