RFI - P0478-26-001_45_Maple_Street_Cooroy_SWMP

4.6         Stormwater Mitigation Results

A detailed analysis of the mitigated peak flows, as summarised in Table 4-3, confirms that the use of the existing
detention tank successfully reduces post-development discharges across all assessed storm events, ensuring
compliance with QUDM requirements and mitigating potential downstream impacts.



Table 4-3: Existing Flow vs Mitigated Flow Comparison

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

  63.2% (1-Year ARI)                       0.048                   0.043                          -10%

   50% (2-Year ARI)                        0.059                   0.048                          -19%

   20% (5-Year ARI)                        0.093                    0.073                         -22%

  10% (10-Year ARI)                         0.112                   0.105                          -6%

   5% (20-Year ARI)                         0.133                   0.130                          -2%

   2% (50-Year ARI)                         0.159                   0.147                          -8%

  1% (100-Year ARI)                         0.179                   0.161                         -10%



Key Observations

The results of the hydrological modelling demonstrate that the proposed detention tank provides effective
attenuation of post-development peak flows, particularly for the more frequent rainfall events.

As summarised in Table 4-3, the following peak flow reductions were achieved:

     •    1 - Year AEP – approximately 10% reduction

     •    2 - Year AEP – approximately 19% reduction

     •    5 - Year AEP – approximately 22% reduction

     •    10 - Year AEP – approximately 6% reduction

     •    20 - Year AEP – approximately 2% reduction

     •    50 - Year AEP – approximately 8% reduction.

     •    100 - Year AEP – approximately 10% reduction.

These results indicate that the tank has been optimised to effectively manage frequent, smaller storm events,
which are most critical for protecting downstream properties and infrastructure, while allowing major flood events
to pass through the system without increased risk.

This outcome reflects the intent of the stormwater management strategy:

     •    To provide a significant reduction in nuisance flooding and frequent storm impacts

     •    To ensure safe conveyance of major flows consistent with pre-development drainage conditions and

     •    To maintain compliance with both QUDM and Noosa Shire Council stormwater management objectives.




 45 MAPLE STREET, COOROY
                                                 24 June 2026                                       P a g e | 15
 Stormwater Management Plan
 Client Reference: P0478-26-001
 Prepared for Myall Rise PTY LTD