Appendix G - Stormwater Management Report

RPS Project Management c/- Lutheran Services                                                      Page 4 of 10
PF80 - Cooroy Care Farm
28 May 2018                                                                                 Ref. No. 2017.0616




3      STORMWATER MANAGEMENT STRATEGY
       3.1           Existing Drainage and Legal Point of Discharge
The site forms part of the Lake McDonald catchment.

Dial Before You Dig, Council records (Noosa City Council IntraMaps), survey data and site records
indicate that there is no piped stormwater drainage system through the site. Given there is no potable
water service, the drainage for the existing houses is presumed to be via rainwater tanks before
discharging to the rear of the site.

It is considered that the legal point of discharge of the site is the existing creek at the rear of the site.
Two smaller gullies currently run through the site as per Appendix D and are considered the appropriate
stormwater discharge points. Catchments (at the development boundary) are calculated to be 2.9ha and
0.9ha for the western and eastern gullies respectively.

       3.2           Stormwater Quantity Strategy
It is estimated that the fraction impervious of the site would increase from 5-10% up to 50% after the
proposed development (7.5 ha total development area). This increase would generate an increase of
stormwater runoff discharge from the site. To mitigate the possible adverse impacts created by this
runoff increase, some form of flow control / detention would need to be implemented.

Based on an analysis using the probabilistic rational method (QUDM 2013), the current peak site
discharge under a 1% AEP storm event is estimated to be 3.4 m³/s, and the 20% AEP site discharge is
2.3 m³/s.
The development should ensure that there is no change to these peak flows resulting from increased
impervious surfaces and more efficient drainage systems, however given the site’s location in relation to
the waterway at the rear of the site, the requirement for stormwater detention needs to be explored
further in liaison with the local authorities.
As the architectural and landscape design is still in development at the time of preparing this report, the
detailed design of the stormwater system will occur in due course. It is recommended that water
management be incorporated into the architectural and landscape design, potentially in the form of:
       ▪ Rainwater tanks
       ▪ Permeable pavement/surfaces
       ▪ Landscaping which slows and detains runoff.
       ▪ Integrated water management
The grade and vegetated nature of the site lends itself to use of swales/surface stormwater
management to maximise the amenity and environmental sustainability.

       3.3           Stormwater Quality Strategy
The site is greater than 2500 m² in area. As such, it is above the threshold in the State Planning Policy
(SPP) for requiring compliance with stormwater quality provisions.

There are likely some simple and effective ways that the development might be able to minimise
impacts on stormwater quality while enhancing the amenity of the development, including the use of
permeable paving, using self-watering street trees along the road frontages, and directing roof and
pavement runoff into gardens and landscape areas.

During the detailed design stage, sufficient detail will need to be provided to demonstrate that the
proposed development works comply with the SPP requirements. The pollutant target objectives are
summarised in Table 2 below.