P0198-25-001_12_Gympie_Street_Tewantin_SWMP

The basin outlet will discharge via a new stormwater pipe connection into Council’s dedicated reserve to the
north of the Butler Street Cul de sac.

Stormwater Mitigation Results

A detailed analysis of the mitigated peak flows, as summarised in Table 3-3, confirms that the proposed
bioretention basin successfully reduces post-development discharges across all assessed storm events, ensuring
compliance with QUDM requirements and mitigating potential downstream impacts.
Table 3-3: Mitigated Flow comparison to existing

 Storm Event                      Pre-Development Peak Flow          Post-Mitigation Peak Flow     Reduction %
                                            (m³/s)                             (m³/s)

 63% (1-Year AEP)                                  0.048                      0.033                    19%

 50% (2-Year AEP)                                  0.064                      0.037                    27%

 20% (5-Year AEP)                                  0.113                      0.097                     8%

 10% (10-Year AEP)                                 0.146                       0.119                   10%

 5% (20-Year AEP)                                  0.176                       0.151                    8%

 2% (50-Year AEP)                                  0.212                      0.194                     4%

 1% (100-Year AEP)                                 0.242                      0.231                     2%



Key Observations

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

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

     •    1 – Year AEP approximately 19% reduction

     •    2 - Year AEP – approximately 27% reduction

     •    5 - Year AEP – approximately 8% reduction

     •    10 - Year AEP – approximately 10% reduction

     •    20 - Year AEP – approximately 8% reduction

     •    50 - Year AEP – approximately 4% reduction.

     •    100 - Year AEP – approximately 2% reduction.

These results indicate that the basin 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 robust 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.




 12 GYMPIE STREET, TEWANTIN
 Stormwater Management Plan                                                                                    16
 Client Reference: P0198-25-001                    4 November 2025
 Prepared for Novato Group