24-7-19 Attachment F Rev Stormwater Mgmt Plan

18 July 2019


The detention tank will discharge via a new piped connection to the stormwater infrastructure
constructed as part of the masterplan ROL development.

The basement will not be subject to direct rainfall runoff as the basement entries are covered by
roof areas and ramps.

It is proposed to construct a sump and pump for any stormwater that may track in off cars parked
within the basement. This system will not be required to cater for direct rainfall or stormwater
runoff. Current geotechnical testing notes that no groundwater was present to a depth of 4m
below surface level. It is unlikely that any permanent dewatering will be required due to the depths
of the basement excavation.

Due to the levels of the basement it is envisaged that any basement subsoil drainage may require
treatment to neutralise the acidic prior to discharge from the site to the sensitive downstream
receivers. Levels of treatment will be determined in accordance with the site specific acid sulfate
soils management plan to ensure levels of acidity are consistent with the downstream
environment. It is 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 combined
bioretention/detention basin located along the Northern boundary and a detention tank located
under the main entry to the new building. The detention system will be oversized to cater for the
areas that will bypass detention.

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




Excellence with Economy in Engineering                                                                 9
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