P0198-25-001_12_Gympie_Street_Tewantin_SWMP

5          Conclusion
The Stormwater Management Plan (SWMP) for the proposed seven lot subdivision at 12 Gympie Street, Tewantin,
has been developed to ensure compliance with the requirements of the Noosa Shire Council, QUDM, and Water
by Design guidelines. Through comprehensive hydrological analysis and modelling, the proposed stormwater
management strategy effectively mitigates potential impacts arising from the development.



    1.   Hydrological and Hydraulic Considerations

The assessment of the existing site conditions confirmed that stormwater overland flow is primarily directed
towards the north following the natural grade of the site, discharging into the adjoining lot.

Pre-development peak flow rates were estimated using both the Rational Method and DRAINS modelling, providing
a benchmark for post-development assessments. A comparative analysis demonstrated that DRAINS provided a
more detailed hydrologic and hydraulic representation, capturing storage effects and flow routing dynamics not
accounted for in the Rational Method.



    2.   Stormwater Quantity Management

The post-development scenario introduced increased impervious areas, resulting in elevated runoff volumes and
peak flow rates across multiple storm events. To address these impacts, a bioretention basin was incorporated
into the stormwater management strategy. The hydrological modelling confirmed that the proposed bioretention
system effectively mitigates peak discharges, ensuring that post-development flows remain equal to or lower than
pre-development levels across all assessed storm events.

The basin was designed with the following key specifications:

    •    Storage Volume: 289 m³ for the 1% AEP event

    •    Excavated Level: 1.65 m AHD

    •    Top of Retaining Walls: 3.65 m AHD

    •    Outlet Pipe Size: 225 mm orifice into a 375 mm pipe, discharging Council’s dedicated reserve to the
         north of the Butler Street Cul de sac

The tailwater levels were appropriately set in accordance with QUDM Table 8.3.1, considering mean high water
springs (MHWS) and highest astronomical tide (HAT) conditions, ensuring resilience against potential tidal
influences.



    3.   Stormwater Quality Management

To ensure compliance with the Water by Design MUSIC Modelling Guidelines, a bio-retention basin was adopted
as the primary treatment measure for stormwater quality management. The MUSICX model was configured to
assess treatment effectiveness in removing pollutants generated from the development’s impervious surfaces.

Table 5-1 presents the pre-treatment pollutant loads, post-treatment residual loads, and the overall percentage
reductions achieved through the proposed stormwater treatment system.




 12 GYMPIE STREET, TEWANTIN
 Stormwater Management Plan                                                                                    20
 Client Reference: P0198-25-001        4 November 2025
 Prepared for Novato Group