Stormwater Management Plan

Good Shepherd Lutheran College                                                                   Odyssey Consulting Group
Proposed Unit Development                                                         Conceptual Stormwater Management Plan
Lot 30 on SP170751                                                                                      Project No: K4277
115 EUMUNDI NOOSA ROAD, NOOSAVILLE

4.1     Objective
        In accordance with Urban Stormwater Quality Planning Guidelines 2010 (DERM USQG) the following
        objective has been set for stormwater discharge from the site and proposed development:

                   No net increase in peak flows from the subject site for all events up to the Q 100 storm event
                    during the post developed condition.

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


                 Pre Development                    0.104                  0.169                       0.277


         Post Development (unmitigated)             0.174                  0.280                       0.440


        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:




K4277-0002                                            14 March 2018                                            Page 8 of 16