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

3.3.2 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.

         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

K4277-0002-C                                           05 November 2018                                             Page 5 of 16