SBSMR 20004 Revision D - Response to RFI

5.1 PIPED DRAINAGE SYSTEM – PRE-DEVELOPMENT

Total Catchment is 3105m2


Coefficient of Runoff

The coefficient of runoff for the existing site has been determined using light cover bush with a
high soil permeability (sand) for zero fraction impervious, giving a C10 value of 0.53.

Time of Concentration

Slope of catchment: Length 55m, fall 4m.
Therefore Slope = 4/55 x 100 = 7.3%
For average grassed surface n = 0.045
Using Friend’s Equation:
       t       = 107nL0.333/S0. 2
               = 107 x 0.045 x 55.333/7.30.2      =          12.3 minutes
However, adopt a time of 10 minutes.

                        EY %     ARI               10 min Rainfall Intensity
                          1       1                          102
                         0.2      5                          155
                        0.11     10                          181
                        0.05     20                          205
                        0.02     50                          236
                        0.01     100                         259
                                       Table 1 – 10min Rainfall Intensity


Using these values the pre-development flows are as follows

                     EY %       ARI             Pre Development Flows m3/s
                       1         1                         0.037
                      0.2        5                         0.067
                     0.11       10                         0.083
                     0.05       20                         0.098
                     0.02       50                         0.124
                     0.01       100                        0.142
                                        Table 2 - Site Pre-Development




NMA-SBSWMR-20004 Noosa Youngcare                                                                    8