RFI - Attachment H - Stormwater Management Plan_Issue C_

15 May 2019


noted that the downstream environment is slightly acidic and as such treatment of any groundwater is
likely to be minimal.

Stormwater discharged from site will also be consistent with that which is currently collected on site and
discharged into the downstream system. A detention system will be installed to ensure that peak runoff
from the site doesn’t exceed the current volumes discharged from the currently undeveloped site.
Stormwater treatment will be implemented to remove sediments, nutrients, gross pollutants and
hydrocarbons in accordance with the requirements of Noosa council and the state planning policy.

The proposed conceptual stormwater drainage layout is depicted on MPN Plan 7700-DA.01 attached in
Appendix 3.

5.3.2 Stormwater Quantity Management Strategy

For the management of stormwater quantity for the proposed development, the DRAINs computer
model was used to calculate the stormwater runoff quantity for the existing site and developed site
conditions. Runoff flows for the Average Recurrence Intervals of 2, 5, 10, 20, 50 and 100 years and
durations of 5 minutes to 120 minutes were calculated to ensure that peak runoff flows from the
proposed site with detention would not exceed peak runoff flows from the existing site.

Stormwater runoff from the proposed development will be detained in a detention tank located at the
main entry to the new building. The detention system is fully contained within the developable areas of
the site.

Stormwater events varying from 5 minutes to 120 minutes were analysed with results for the peak
storm flows shown in Table 1 below.


                                             Existing Discharge                        Proposed Discharge
        ARI Year
                                             From Site (m3/sec)                         From Site (m3/sec)

             2                                        0.196                                  0.195

             5                                        0.271                                  0.254

            10                                        0.310                                  0.296

            20                                        0.359                                  0.349

            50                                        0.390                                  0.386

           100                                        0.435                                  0.435

                                       Table 1 – Existing and Developed Peak Flows

The proposed detention tank has a volume of 155m3.

5.3.3 Stormwater Quality Management Strategy

In order to satisfy the requirements of the State Planning Policy July 2017 for the adequate treatment
of stormwater runoff, mechanical treatment solutions have been provided to remove hydrocarbons,
suspended solids and nutrients prior to being discharged from site.

5.3.3.1 Potential Pollutants Generated


Excellence with Economy in Engineering                                                                       10
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