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

3.4.3 Time of Concentration
         To calculate the time of concentration for the external catchment the Friends Equation (tc =
                0.333 0.2
         (107nL )/S ) from QUDM has been applied for overland sheet flow with the remaining distance
         calculated as concentrated flow. Please refer to Table 11 below for the calculated time of
         concentration.

         Table 11:        External Catchment C

                                                                                        Time of concentration
                          Catchment          Catchment
             Catchment            2                                   Overland flow             Concentrated flow
                           Area (m )         Properties                                                                   Total tc
                                                                    Friend’s Equation           QUDM Figure 4.09
                                                                   Horton’s (n) = 0.035
                                             Light cover
                                                                         L = 31 m
                 A            449          bushland / poor                                             n/a                11 min
                                                                     Slope = 4.38%
                                             grass cover
                                                                      t = 11.04 min

3.4.4 Design Flow Rates
         Design storm flow rates have been calculated for standard storms with an ARI of 2, 10, 20 and 100
         years for the external catchments using design rainfall intensities from the Bureau of Meteorology.
         The Rational Method (Q = 2.78 x 10-3 CIA) has been used to calculate the design flow rates.

         The peak flows for the external catchments are presented in Table 12.

         Table 12:        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           0.045      0.045         0.045      0.045        0.045
             Average Rainfall Intensity (mm/h)            I           121.1      149.7         166.0      188.6        240.5
             Peak Flow Rate (m3/s)                        Q           0.009      0.012         0.015      0.017        0.025


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/E) 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 Q 100 storm event
                 during the post developed condition.



K4277-0002-D                                                  14 February 2019                                          Page 7 of 16