Amended Stormwater Plan · MCU18/0019

                                                           Rational Method Analysis

                                                           Site Location:
                                                           Noosa
Q = CIA/360                                                IFD data from BOM
Where:                                                     DURATION 1 Year       2 years      5 years    10 years 20 years 50 years 100 years
Q = Flow rate (m3/s)                                       5Mins          130.00       165.00     202.00     224.00   253.00   291.00    320.00
C = Coefficient of discharge                               6Mins          122.00       155.00     190.00     210.00   238.00   274.00    302.00
I = average rainfall intensity (mm/hr)                     10Mins         100.00       127.00     156.00     173.00   197.00   227.00    250.00
A = area of catchment (ha)                                 20Mins          73.70        93.50     116.00     128.00   145.00   168.00    185.00
                                                           30Mins          60.20        76.50      94.70     105.00   119.00   138.00    152.00
I = Requires time of concentration (Tc)                    1Hr             40.60        51.90      64.90      72.40    82.60    96.10    106.00
Tc calculated by Friends Equation                          2Hrs            26.00        33.40      42.60      48.10    55.40    65.10     72.60
                                                           3Hrs            19.60        25.40      33.00      37.50    43.50    51.50     57.70
Friends Equation: Tc = (107nL0.333)/S0.2                   6Hrs            12.10        15.80      21.10      24.40    28.70    34.40     39.00
Where:                                                     12Hrs            7.65        10.10      13.80      16.20    19.20    23.30     26.60
n = Hortons surface roughness factor                       24Hrs            5.04         6.69       9.30      11.00    13.10    16.10     18.50
L = overland sheet flow path length                        48Hrs            3.35         4.47       6.28       7.45     8.96    11.00     12.70
S = slope of surface (%)                                   72Hrs            2.53         3.39       4.81       5.75     6.94     8.60      9.94


            Existing                                                                Existing
Tc (mins)              9                                              n                    0.035
                                                                      L                       35
Therefore intensities (I) using Tc are as follows:                    S                        5
                                                                      Tc = (mins)            8.9
   ARI      Existing
    1           101.28
    2           129.00
    5           160.14
    10          178.18
    20          203.03
    50          235.62
   100          260.59

1 hour 10 year intensity (1I10)=
       72.40
1
  I10 used with Fraction impervious (Fi) to determine C10 then multiplied by Frequency factor (Fy)

            Existing
Area (ha)        0.674                         **Note: C10 value derived for existing conditions as Intenisty, Area, and Flow were known values
Fi                   24
C10                0.76                        ARI         Frequency factor (Fy)
C1               0.608                                 1         0.8
C2               0.646                                 2        0.85
C5               0.722                                 5        0.95
C10              0.760                                10           1
C20              0.798                                20        1.05
C50              0.874                                50        1.15
C100             0.912                               100         1.2


Flow rates can then be calculated by Q = CIA/360:
                  Q (m3/s)
ARI        Existing
    1           0.115
    2           0.156
    5           0.216
    10          0.254
    20          0.303
    50          0.386
   100          0.445