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      External Catchments
         The subject site and the surrounding area were examined to determine if any external catchments
         will contribute to the subjected site. The size of external catchments has been determined based on
         ground contours, existing roads and existing stormwater infrastructure. An external catchment has
         been identified to the North of the subject site. The external catchment has been analysed to ensure
         drainage has been sufficiently sized to convey external flows.

         The rational method has been utilised to calculate the peak flow rates. The parameters adopted in the
         rational method have been selected to reflect the current use of the external catchment.

3.4.1 Catchment Definition
         One external catchment has been considered for analysis. External Catchment A has an area of 419
         m² and currently discharges stormwater along the northern boundary of the subject site.

         The external catchment is shown on the Odyssey Consulting Group, External Catchment Plan (Ref:
         K4277/P011/C) included as Appendix E.

3.4.2 Coefficient of Runoff
         The coefficient of runoff (Cy) was determined based on the fraction impervious (fi) method as
         specified in QUDM. A fraction impervious value of 0.10 has been adopted in accordance with QUDM,
         Table 4.05.1 Fraction impervious vs development category. The adopted fraction impervious value
                                                                                                                1
         reflects the potential fully developed future land use. Using a one hour, ten year rainfall intensity ( I10)
         of 72.3 mm/hr, a C10 value of 0.70 has been adopted for External Catchment A.

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 3 below for the calculated time of
         concentration.

         Table 8:       External Catchment

                                                                                Time of concentration

                           Catchment         Catchment                              Concentrated
           Catchment               2                          Overland flow             flow
                            Area (m )        Properties                                                     Total tc
                                                            Friend’s Equation       QUDM Figure
                                                                                       4.09

                                              Open            Horton’s (n) =
                                          Space/parks –            0.035
               A               449         Light cover           L = 31 m                n/a               11.04 min
                                         bushland / poor      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 9.




K4277-0002-C                                        05 November 2018                                           Page 6 of 16