Appendix C - Stormwater Management Plan

TfA Project Group Pty Ltd                                                                         Stormwater Management Plan
                                                                                                       Development Application




   Overland run-off from the undeveloped upstream catchment area will be conveyed via spoon drains and gully pits
   installed along the top of a proposed retaining wall located on the Eastern boundary of the proposed development
   discharging to the legal point of discharge on Eumundi Noosa Road. Details of the spoon drain and pit locations,
   as well as hydraulic calculations, will take place in the detail design stage of the development.


   6       WATER QUANTITY ASSESSMENT

   The purpose of this part of the assessment is to investigate whether there is a need to attenuate stormwater flows
   to negate any adverse impacts on upstream or downstream environments. Stormwater runoff from both pre and
   post-development scenarios will be calculated and the results analysed and possible solutions proposed.


   6.1      Catchment Analysis
   The total proposed development area is 2,448m² (0.248 Ha). Details of the pre-development catchment are
   shown in Table 2 below, which shows the surface types, areas, percentage imperviousness and fraction
   impervious.


   Table 2: Pre-development catchment characteristics

           Surface Type                Area (m²)        Percentage Imperviousness                    Fraction impervious
         Roof and Paved                    0                              100
              Area                                                                                           0.00
         Landscaped Area                 2,448                              0


   Details of the post-development catchment are shown in Table 3 below, which shows the surface types, areas,
   percentage imperviousness and fraction impervious.


   Table 3:Post-development catchment characteristics

           Surface Type                Area (m²)        Percentage Imperviousness                    Fraction impervious
         Roof and Paved                  1,673                            100
              Area                                                                                           0.68
         Landscaped Area                  775                               0


   Tables 2 and 3 above show that the fraction impervious for the post-development case is higher than the fraction
   impervious of the pre-development case, this will result in increased peak flows in the post-development case
   therefore onsite stormwater detention is required.


   6.2      Stormwater Detention Sizing


   6.2.1      Rational Method
   The rational method was used to calculate the stormwater runoff flows from the site for the pre and post-
   development scenarios. The formula is as follows:


              Qy = (2.78 * 10-3)) Cy*A* tIy                                     (QUDM 2007 Equation 4.02)



                            123 Eumundi Road (Cnr Walter Hay Drive), Noosaville QLD 4566                                    Page 4