OPW16/0021- Operational Works Application 2 of 4

Time of Concentration Calcs                                                                                                                                      3



Time of Concentration (tc)
Step 1 Calculate Equal Area Slope

    Catchment flow path co-ordinates
    Must start at ch 00 at downstream end
        Point         Chainage Elevation             Slope      delta L                            Catchment A          Created A         Linear (Created A)

    A                         0         0            36.36%          165
    B                       165        60                                        70
    C
                                                                                 60
    D
    E
                                                                                 50
    F
    G                                                                            40

                                                                                 30
    Created
    Point           x             y                                              20
                        165                60.00
                                                                                 10
                           0                   0
         Equal Area Slope =           36.36%                                      0
                                                                                      0              50                  100                 150               200
Step 2 Estimate Time of Concentration

    Stream Length           0.165 km               mainstream length measured to catchment divide                                   165
                                     2
    Catchm't Area       0.011784 km

    Bransby Williams

          L                 0.165 km                                                                                     58L
          A             0.011784 km
                                     2
                                                                                                                 tc 
          Se                363.6 m/km
                                                                                                                        A0.1Se0.2
          tc                  4.6 min                  or            0.08 hr

    Pilgrim and McDermott


          A             0.011784 km
                                       2                                                                         t c  0.76 A 0.38

          tc                  8.4 min                  or            0.14 hr



    Queensland Main Roads Method for catchment areas up to 5 km2                                  Average Stream Velocities CA < 5 km2
                                                                                                   Type of Average Slope of Approx Stream
         L                 0.165 km2               This calc adds on 10 mins for                   Country    Catchment %      Velocity (m/s)
         Se                36.36 %                 overland flow to reach the stream              Flat               0 to 1.5               0.3
    V ave calced            3.00 m/s               if stream length is less than 1km              Rolling            1.5 to 4               0.7
                                                   (sect 3.5.9)                                   Hilly                4 to 8               0.9
          tc               10.92 min                     or           0.18 hr                     Steep               8 to 15               1.5
                                                                                                  Very Steep             >15                  3

    Queensland Main Roads for Catchments >5km2
    Modified Friend for Rural Catchments - use only if you know the stream properties

          L                 0.165 km               Length of flow path from catchment divide to outlet
          n                  0.04                  Manning's n             Rs is the hyd radius of the natural channel at the d/s end
          R                  0.56 m                Hydraulic Radius = 0.75Rs for uniform stream 0.65Rs for appreciable variation
                                                                                                  1/6
         Ch                  22.7                  Chezy coefficient for entire stream length = R /n

                                                                                                                                       8.5 L
                                     2
          A             0.011784 km                Catchment Area
          S                 36.36 %                Equal area slope
                                                                                                                     t 
                                                                                                                     c
          tc
        V ave
                               1.4 min
                             2.00 m/s
                                                       or            0.02 hr
                                                                                                                                    Ch. A 0 .1 S 0 . 4

Z:\AquaIntel Projects\2014022_Seaview_Tce_SWMP\hydrology\Rational Method_Seaview.xls                                                            9/24/2014 2:53 PM