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

3.3     Post Development

3.3.1   Catchment Definition and Lawful Point of Discharge
        The post development scenario consists of two internal catchments that convey the stormwater to
        the south and west of the site. Catchment A discharges into existing Council stormwater
        infrastructure on Eumundi Noosa Road on the western site boundary. Catchment B discharges into
        existing Council stormwater infrastructure on Walter Hay Drive off the southern corner of the site.
        These discharge points represents the Lawful Points of Discharge (LPOD) for the site.

        The post development catchment area and lawful point of discharge is shown on the Odyssey
        Consulting Group, Post Development Catchment Plan (Ref: K4277/P006/A) included as Appendix D.

3.3.2   Coefficient of Runoff
        The post development coefficient of runoff (C year) was determined based on the fraction impervious
                                                                                                                2
        (fi) method as specified in QUDM. The proposed development has a total impervious area of 4,513 m
                                                                                                            1
        which equates to a fraction impervious value of 0.9. Using a one hour, ten year rainfall intensity ( I10)
        of 72.3 mm/hr, a C10 value of 0.88 has been adopted for the post development scenario.

        With reference to QUDM Table 4.05.3, applying the frequency factors for the standard storms of 2,
        10, 20 and 100 years results in the following post development coefficients of runoff as shown in
        Table 5.
        Table 5:      Post Development Catchment Impervious Properties
                                          2                          2
          Catchment    Impervious Area (m )    Catchment Area (m )              Fraction Impervious             C10
              A                 3,220                   3,578                          0.90                     0.88
              B                 1,292                   1,436                          0.90                     0.88

        With reference to QUDM Table 4.05.2, by applying the frequency factors for the standard storms of 2,
        5, 10, 20, 50 and 100 years result in the following pre development coefficients of runoff as shown in
        Table 6.

        Table 6:      Post Development Coefficients of Runoff

          Catchment             2                5                       10                    20                  100

             A&B            0.75               0.84                      0.88                 0.92                 1.06

3.3.3   Time of Concentration
        Both post development catchments will have a standard time of concentration of five (5) minutes for
        paved areas.

3.3.4   Design Flow Rates
        Design storm flow rates have been calculated for standard storms with an ARI of 2, 5, 10, 20, 50 and
        100 years for the post development case 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 required
        design flow rates for the subject site. The calculated post development peak flows for this subject site
        are presented in Table 7.




K4277-0002                                           14 March 2018                                                 Page 5 of 16