Catchments & Creeks guidelines

                      Table 1 – Allowable flow velocity (m/s) for grassed surfaces [1]

  Percentage                                          Gradient of grass surface (%)
  grass cover               1         2         3       4       5         6        8    10      15      20
             [2]
      70%                  2.0        1.8       1.7    1.6     1.6        1.5     1.5   1.4     1.3     1.3
              [3]
      100%                 2.0        2.0       2.0    2.0     2.0        2.0     2.0   1.9     1.8     1.7
                    [3]
 Poor soils                1.5        1.4       1.3    1.2     1.2        1.1     1.1   1.1     1.0     0.9
[1]   Maximum allowable flow velocity limited to 2.0m/s due to shallow water flow and resulting high shear
      stress. High flow velocities are allowable on reinforced grass.
[2]   70% cover would be typical for most grasses recently established by seed, but only when there is
      sufficient plant establishment time.
[3]   “Poor soils” refers to the soil’s high erosion potential, such as dispersive clays (Emerson Class 1 and
      2) such as sodic, yellow and red soils. Unstable, dispersible clayey sands and sandy clays, such as
      yellow and grey massive earths formed on sandstones and some granites. Highly erodible soils may
      include: lithosols, alluvials, podzols, siliceous sands, soloths, solodized solonetz, grey podzolics,
      some black earths, fine surface texture-contrast soils, and Soil Groups ML and CL.


       Table 2 – Level spreader sill length B metres per unit discharge (m per m;/s) [1]

   Land                          Allowable down-slope velocity over well grassed surface (m/s)
 slope (%)                1.0             1.2         1.5           1.8         2.0       2.2         2.5
      1.0                 3.5*          2.5*          1.6*       1.1*           0.9*     0.8*         0.6*
      2.0                 5.2           3.8*          2.5*       1.8*           1.4*     1.2*         0.9*
      3.0                 6.6             4.8         3.2*       2.3*           1.8*     1.5*         1.2*
      4.0                 7.7             5.6         3.8*       2.7*           2.2*     1.8*         1.4*
      5.0                 8.7             6.3         4.3*       3.1*           2.5*     2.1*         1.6*
      6.0                 9.5             7.0         4.7        3.4*           2.8*     2.3*         1.8*
      7.0                 10.3            7.6         5.2        3.7*           3.1*     2.6*         2.0*
      8.0                 11.0            8.2         5.6        4.0*           3.3*     2.8*         2.2*
      9.0                 11.8            8.7         6.0        4.3*           3.5*     3.0*         2.4*
      10.0                12.4            9.2         6.3        4.6*           3.8*     3.2*         2.5*
 Caution the release of water onto grass slopes steeper than 10%.
      15.0                15.2         11.3           7.8           5.7         4.8      4.0*         3.2*
      20.0                17.4         13.1           9.1           6.7         5.6       4.7         3.7*
      25.0                19.4         14.6           10.3          7.6         6.3       5.3         4.3*
      33.3                22.1         16.8           11.9          8.8         7.4       6.2         5.0
      50.0                26.6         20.3           14.5      10.8            9.1       7.8         6.3
                                                                          3
* Sill length limited to minimum 4m for discharges less than 0.85m /s.


Design example:
Design a level spreader to release a flow rate of 0.5m3/s down a 10% slope containing a good
(70%) grass cover on moderately erodible soil.

Solution:
From Table 1, choose a maximum flow velocity of 1.4m/s as best representative of a good grass
cover on a moderately erodible soil.
From Table 2, select a sill width per unit flow rate of 7.3m/m3/s.
Therefore, the sill length would need to be 0.5 x 7.3 = 3.65m < 4m (minimum).
Conclusion, specify a sill length of 4m.



© Catchments & Creeks Pty Ltd                          V2 February 2010                               Page 2