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Att 5 - Preliminary Stormwater Detention
Attached to the e-mail Retail Bus Type 2 Shop & Salon, Multiple Housing Type 4 Conventional (2 units) & Boundary Realignment (3 lots into 3) - MCU18/0158 & RAL18/0017.
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.
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Two locations have been identified to provide the storage tanks required. Tank 1 is proposed to be located under the 8 scooter bays adjacent to Shop 2 and will discharge to the stormwater pit near the Elm St and Opal St intersection. Tank 2 is proposed to be located under car parks 1 and 2 and discharge to the stormwater pit located on Opal St near the eastern property boundary. Allowing for 1% fall from the invert of the tanks to the invert of the pit and 400mm for the top of the detention tank and wearing course, the combined storage capacity of tanks 1 and 2 is in excess of 30kL. These could be constructed from poured in-situ tanks or by pre-constructed structures such as Humes Storm Trap (or equivalent product). It is expected that once a stormwater quantity model is completed as part of the Operational Works application, the likely required stormwater detention will be between 5m3 and 10m3 (considerably less than the 24m3 as required by the Brisbane City Council deemed to comply solution). This correspondence and the accompanying Preliminary Stormwater Management Layout demonstrates that there is adequate area within the proposed development to accommodate the required on-site detention. If you have any queries in relation to the proposed stormwater detention for 34 Elm St, Cooroy, please do not hesitate to contact Chris Gibberd on 0490 530 793. Yours sincerely, Chris Gibberd (RPEQ) Principal Engineer 360 Civil Consulting Pty Ltd