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

         Table 9:        External Catchment A Peak Flow Rates
             Average Recurrence Interval            ARI     2          5        10          20           100
             Coefficient of Runoff                  C       0.60       0.67     0.70        0.74         0.84
             Area of Catchment (ha)                 A       0.042      0.042    0.042       0.042        0.042
             Average Rainfall Intensity (mm/h)      I       121.1      149.7    166.0       188.6        240.5
             Peak Flow Rate (m3/s)                  Q       0.008      0.012    0.014       0.016        0.024


3.4.5 External Catchment Channel Conveyance
         External Catchment A
         External Catchment A currently discharges stormwater through the site. Flows from External
         Catchment A will be captured in a field inlet in the subject site’s north eastern corner and conveyed to
         Council stormwater infrastructure on the north eastern boundary. For details and locations of the
         proposed field inlet refer to Odyssey Consulting Group, Stormwater Management Plan (Ref:
         K4277/P007/D) included as Appendix F.



4.0 STORMWATER DETENTION
         The development of land will potentially increase peak flow rates from the subject site due to
         increased impervious areas and a reduced critical time of concentration. The following section
         provides preliminary details of an onsite detention system ensuring there will be no adverse impacts
         associated with the increased runoff rate on downstream properties and infrastructure.

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 Q100 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.




K4277-0002-C                                        05 November 2018                                      Page 7 of 16