Stormwater Management Plan

GOOD SHEPHERD LUTHERAN COLLEGE                                                              ODYSSEY CONSULTING GROUP
Lot 30 on SP170751                                                               Conceptual Stormwater Management Plan
115 Eumundi Noosa Road, Noosaville                                                                     Project No: K4277

         This objective shall be achieved by detaining site runoff within a detention tank located within the
         development.

4.2      Hydraulic Model
         A calculation of the required detention volume to mitigate any increase in total site discharge rates
         has been made using the DRAINS software programme.

         DRAINS modelling has been adopted at the preliminary planning stage to ensure that the detention
         tank volume is estimated with a higher degree of confidence. As finished site levels and internal pipe
         levels are still approximate, this initial calculation is an estimate but is accurate enough to progress
         the design with confidence.

         The DRAINS model comprised of two post development Catchments. Catchment A flows shall be
         directed towards existing stormwater infrastructure located in Eumundi Noosa Road via a detention
         basin. Catchment B flows shall be directed towards existing stormwater infrastructure located in
         Walter Hay Drive.

         The 2, 10 and 100 year ARI storm events were analysed for all standard durations ranging from 10
         minutes to 180 minutes. It was determined that the 25 minute storm is the critical duration for the
         peak site discharge for both the pre development and post development scenarios. The combined
         peak discharge rates for the site calculated by the DRAINS model is shown below for both scenarios.

         Table 13:      Peak Discharge Rate for Site (m3/s)
                                                      Q2                   Q10                         Q100


                 Pre Development                    0.104                 0.169                       0.277


         Post Development (unmitigated)             0.165                 0.266                       0.429


         As the development will increase peak discharge rates from the site, onsite detention will be required
         to attenuate runoff to pre development levels.

4.3      Detention Volume
         The following detention storage parameters were found to achieve the target mitigated pre
         development flow rates:

         Table 14:      Detention Tank A Parameters
                                                                                               2
           Detention Basin Base Surface Area                                           100 m
                                                                                               2
           Detention Basin Top Surface Area                                            100 m
           Detention Tank Invert Level                                                 9.14 m
           Detention Tank Height (including 300mm freeboard)                           1.53 m
           Low Flow Outlet Pipe Diameter (at invert of detention
                                                                                      300 mm
           area)
           Low Flow Orifice Diameter                                                  245 mm
           Mid Flow Orifice Diameter (at 0.40 m above invert of
                                                                                      115 mm
           detention area)
           High Flow Spillway (at 0.92 m above invert of
                                                                                         1m
           detention area)
                                                                                               3
           Detention Volume                                                            133 m

         The 25 minute design storm was the critical storm event for determining the required volume within
         the tank. A comparison of the pre development, post development and mitigated flow rates based on
         the above arrangement is shown in Table 15 followed by the hydrograph in Figure 3 for the critical
         duration of the Q100 storm event.



K4277-0002-D                                          14 February 2019                                        Page 8 of 16