Attachment 4 - Engineering Report · MCU18/0015

       6.2     Legal Point of Discharge

       This existing natural drainage gully is considered to represent the legal point of
       discharge for the proposed development.

       6.3     Stormwater Quantity Management

       The site will be subject to limited development works and is expected to contain
       ≤ 25% total impervious area in the post developed case (up from 0%) which will
       result in a slight increase to peak discharge of stormwater runoff leaving the site.

       Non-worsening of peak discharge will be achieved by providing a combined
       rainwater attenuation/reuse tank for each proposed new lot with connection to
       be made to roof areas only. Stormwater runoff generated over the proposed
       access driveway will discharge unattenuated to the existing discharge point.
       Roof water runoff will effectively be ‘over-attenuated’ to compensate for those
       areas of the site not subject to detention measures.

       Balance areas of the site not subject to development works have not been
       considered in attenuation calculations.

               6.3.1 Time of Concentration – Pre/Post Developed

               Stormwater runoff from the pre-developed site is generally directed to the
               central gully as sheet and channel flow. This flow regime will be
               maintained in the developed case.

               A pre-developed TOC of 10mins has been adopted for each sub-catchment
               within the site. Given the steep topography of the site, this is considered a
               conservative estimate.

               A post developed TOC of 6 minutes (5 minute standard inlet time + 1
               minute piped flow) has been adopted for all sub-catchments for the
               developed case.
               6.3.2 Pre/Post Developed Discharge

               A Microsoft Excel spreadsheet has been prepared for each sub-catchment
               which quantifies pre-developed and post-developed (detained) runoff
               volumes. Refer to Appendix E for screenshots detailing inputs and results
               of the proposed attenuation measures.



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C5756/17/Engineering Report - 27 Attunga Heights, Noosa Heads          3
January 2017   JW/jl