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
                                                                                          1
         (fi) method as specified in QUDM. Using a one hour, ten year rainfall intensity ( I10) of 72.3 mm/hr, the
         following coefficient of impervious (C10) values have been adopted for the post development
         scenario.

         Table 5:       Post Development Catchment Impervious Properties
                                               2                                2
           Catchment     Impervious Area (m )       Catchment Area (m )                    Fraction Impervious             C10
                A                 2,709                         3,573                             0.76                     0.85
                B                 1,042                         1,181                             0.88                     0.88
                C                   0                            260                              0.00                     0.70

         With reference to QUDM Table 4.5.2, 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              0.72                  0.81                           0.85                 0.89                 1.00
                B              0.75                  0.84                           0.88                 0.92                 1.00
                C              0.60                  0.67                           0.70                 0.74                 0.84


3.3.3 Time of Concentration
         Post development catchments A & B consist of paved areas and have therefore been assumed to have
         a standard time of concentration of five (5) minutes as per T.4.6.2 – Recommended standard inlet
         time. The time of concentration for Catchment C is indicated below in Table 7.

         Table 7:       Catchment C Time of Concentration

                                                                                             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=5m
               C            260           bushland / poor                                                      n/a                5 min
                                                                      Slope = 6%
                                            grass cover
                                                                      t = 4.5min

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 Catchment A, B
         and C are presented in Table 8, 9 and 10 respectively.




K4277-0002-D                                                 14 February 2019                                                 Page 5 of 16