Att 5 - Preliminary Stormwater Detention

The Chief Executive Officer
Noosa Council
PO Box 141
Tewantin QLD 4565


10 September 2018


Attention:    Development Services, Noosa Council


RE: 34 Elm St – Preliminary Stormwater Management Layout

A preliminary Stormwater Management Layout has been included as part of the application
for the proposed development at 34 Elm St, Cooroy.

The development will need to demonstrate non-worsening of peak flow discharges. It is
proposed to demonstrate non-worsening of peak flows as part of a detailed Stormwater
Management Plan to be submitted as part of an Operational Works application.

The preliminary stormwater management layout attached to this application is designed to
demonstrate that there is sufficient room available to accommodate the required
stormwater detention system.

The Brisbane City Council – Simplified Detention Storage Sizing Method, contained in
Planning Scheme Policy for Infrastructure Design, Chapter 7 has been used to ensure that
there is adequate allowance on site to install an effective non-worsening system. From
experience, the Brisbane City Council deemed to comply solution tends to provide results
that are oversized in the order of 50% to 100%. As such, using the Brisbane City Council –
deemed to comply solution is estimated to provide an oversized solution that will be refined
during the Operational Works Stage of the Development.

The existing fraction impervious area for 34 Elm St is approximately 70%. The deemed to
comply solution states “Sites with greater than 60% existing sealed impervious surfaces will
generally not require stormwater detention because there is little change in peak flow, and
redevelopment will often improve and augment older roof-water and stormwater drainage
systems.”

However, to provide a conservative solution, the site has been deemed to be a D3 site
(which has an existing impervious area greater than 40% but less than 60%). A D3 site
requires 120m3/ha of stormwater detention.

Based on a site area of 2,014m2, the required stormwater detention required by the
deemed to comply method is 24m3.