Amended Stormwater Plan

                                                           Catchment (G) Rational Method Analysis

                                                           Site Location:
                                                           Tewantin
Q = CIA/360                                                IFD data from BOM
Where:                                                     DURATION1 Year        2 years      5 years    10 years 20 years 50 years 100 years
Q = Flow rate (m3/s)                                       5Mins          129.00       164.00    202.00     223.00   253.00    291.00    321.00
C = Coefficient of discharge                               6Mins          121.00       154.00    189.00     210.00   238.00    275.00    302.00
I = average rainfall intensity (mm/hr)                     10Mins          99.30       126.00    156.00     173.00   196.00    227.00    250.00
A = area of catchment (ha)                                 20Mins          73.10        92.90    115.00     128.00   145.00    168.00    185.00
                                                           30Mins          59.70        76.00     94.20     105.00   119.00    138.00    152.00
I = Requires time of concentration (Tc)                    1Hr             40.40        51.60     64.70      72.30    82.70     96.30    107.00
Tc calculated by Friends Equation                          2Hrs            25.90        33.40     42.80      48.40    55.90     65.90     73.50
                                                           3Hrs            19.60        25.50     33.20      37.90    44.10     52.50     58.90
Friends Equation: Tc = (107nL0.333)/S0.2                   6Hrs            12.20        16.00     21.50      25.00    29.50     35.60     40.40
Where:                                                     12Hrs            7.72        10.20     14.10      16.60    19.90     24.30     27.90
n = Hortons surface roughness factor                       24Hrs            5.09         6.78       9.51     11.30    13.60     16.70     19.30
L = overland sheet flow path length                        48Hrs            3.37         4.51       6.38      7.61     9.18     11.40     13.10
S = slope of surface (%)                                   72Hrs            2.54         3.41       4.88      5.86     7.10      8.83     10.20


            Existing Developed                                                     Existing     Developed
Tc (mins)          8.3       5                                       n                    0.045
                                                                     L                       25
Therefore intensities (I) using Tc are as follows:                   S                       14
                                                                     Tc = (mins)            8.3       5.0 **Standard inlet time adopted
   ARI      Existing Developed
    1           106.99  106.99
    2           135.98  135.98
    5           167.93  167.93
   10           186.05  186.05
   20           211.29  211.29
   50           244.32  244.32
   100          269.30  269.30

1 hour 10 year intensity (1I10)=
       72.30
1
  I10 used with Fraction impervious (Fi) to determine C10 then multiplied by Frequency factor (Fy)

            Existing Developed
Area (ha)        0.398   0.357                **Note: C10 value derived for existing conditions as Intenisty, Area, and Flow were known values
Fi                    0       0
C10                 0.7     0.7               ARI          Frequency factor (Fy)
C1               0.560   0.560                         1         0.8
C2               0.595   0.595                         2        0.85
C5               0.665   0.665                         5        0.95
C10              0.700   0.700                        10           1
C20              0.735   0.735                        20        1.05
C50              0.805   0.805                        50        1.15
C100             0.840   0.840                       100         1.2


Flow rates can then be calculated by Q = CIA/360:
                  Q (m3/s)
ARI        Existing Developed               % Change to Boundary
     1          0.066      0.059               -10.9%
     2          0.089      0.080               -10.9%
     5          0.123      0.111               -10.9%
    10          0.144      0.129               -10.9%
    20          0.172      0.154               -10.9%
    50          0.217      0.195               -10.9%
   100          0.250      0.224               -10.9%