RFI - P0478-26-001_45_Maple_Street_Cooroy_SWMP

4.4         Required Mitigation – Installation of a Detention Tank
Due to the projected increase in post-development peak flows, mitigation measures are required to ensure
compliance with Noosa Shire Council stormwater management requirements and to prevent potential downstream
impacts. The most suitable solution is the use of a detention tank; modelling will ensure the designed tank will
achieve the following outcomes:

     •    Attenuate peak discharge by temporarily storing excess runoff and releasing it at controlled, pre-
          development rates

     •    Control flow velocities, thereby reducing the risk of downstream erosion and infrastructure overloading
          and

     •    Improve water quality by facilitating the settlement of suspended solids and associated pollutants prior to
          discharge.

The detention tank will be designed to ensure that post-development peak flows do not exceed pre-development
flow conditions for all relevant design storm events. Further hydrological modelling will be undertaken to
appropriately size the tank and confirm its effectiveness in mitigating increased flow volumes and velocities.


4.5         Stormwater Detention and Mitigation Strategy

The use of a detention tank is proposed to be utilised for stormwater detention and will be sized to retain all site-
generated flows. The tank is located in the southeastern portion of the development, as shown on the Civil Set of
Plans (Drawing No. P0478-26-001_Cooroy_Development_OPW), for Council’s consideration.

The detention tank has been designed to detain and control stormwater flows for all assessed events, up to and
including the 1% AEP, ensuring post-development peak discharges do not exceed pre-development levels in
accordance with QUDM. Attenuation is provided across all design storm events, with tank sizing and outlet controls
configured to achieve compliant discharge rates. Refer to Table 4-2 for tank design parameters.

The site naturally drains towards the east in accordance with the existing topography. The proposed development
is designed so that stormwater from the site, including roof areas and hardstand, will be captured by a detention
tank located in the southeastern portion of the site, redirecting overland flow away from adjacent properties.

This approach ensures controlled attenuation of the increased peak flows generated by the proposed
development’s additional impervious surfaces prior to discharge via a new 375mm RCP pipe into council’s existing
stormwater network within Myall Street. Existing site overland flow paths will continue to be conveyed towards
the east in accordance with predevelopment conditions.

Hydrological modelling adopts a consistent catchment framework for both pre- and post-development scenarios,
enabling direct comparison and demonstrating compliance with Council’s non-worsening drainage requirements.
Table 4-2: Detention Tank Parameters

 Parameter                                                 Value

 Tank Footprint                                            50.00 m2
 Internal Tank Height                                      1.50 m
 1% AEP Storage Volume                                     74.53 m³

 1% AEP Water Depth                                        1.49 m

 Lower Discharge Control                                   170 mm orifice at 116.84 m AHD

 Secondary Discharge Control                               245 mm orifice at 117.84 m AHD

 Outlet Pipe Size                                          375mm dia RCP

 Outlet Pipe Invert Level                                  116.84 m AHD




 45 MAPLE STREET, COOROY
                                         24 June 2026                                                P a g e | 14
 Stormwater Management Plan
 Client Reference: P0478-26-001
 Prepared for Myall Rise PTY LTD