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

        Table 7:       Catchment A - Post Development Peak Flow Rates


                                                        Catchment A
          Average Recurrence
                                          ARI           2             5           10            20            100
          Interval
          Coefficient of Runoff            C           0.75         0.84         0.88          0.92           1.06
          Average Rainfall Intensity
                                           I          164.1        201.6         223.0         252.6         320.6
          (mm/h)
          Area of Catchment (ha)           A          0.358        0.358         0.358         0.358         0.358
                             3
          Peak Flow Rate (m /s)            Q          0.122        0.168         0.195         0.232         0.337

                                                        Catchment B
          Average Recurrence
                                          ARI           2             5           10            20            100
          Interval
          Coefficient of Runoff            C           0.75         0.84         0.88          0.92           1.06
          Average Rainfall Intensity
                                           I          164.1        201.6         223.0         252.6         320.6
          (mm/h)
          Area of Catchment (ha)           A          0.144        0.144         0.144         0.144         0.144
                             3
          Peak Flow Rate (m /s)            Q          0.049        0.067         0.078         0.093         0.135


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/A) included as Appendix E.

3.4.2   Coefficient of Runoff
        The coefficient of runoff (Cy) was determined based on the fraction impervious (f i) 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.



K4277-0002                                         14 March 2018                                           Page 6 of 16