Catchments & Creeks guidelines

The minimum sill lengths presented in Table 2 have been determined assuming a Manning’s
roughness for 50-150mm (Class D) grassed surfaces based on Equation 1. The sill length is
sensitive to the selection of Manning’s roughness. Variations between Table 2 and other
published design tables for is due to variations in the assumed Manning’s roughness, which is
highly variable depending on the type and length of grass, and local growing conditions.
                                           R1/ 6
Class D roughness:          n=                                                       (Eqn 1)
                                   . + 20.77 log10 (R1.4 . S0.4 )
                                 5124

                      Table 3 – Minimum dimension of level spreader
   Discharge (m3/s)      Entrance width (m)           Depth (m)           End width (m)
       0 to 0.28                 3.0                     0.15                  0.9
     0.29 to 0.57                4.9                     0.18                  0.9
     0.58 to 0.85                7.3                     0.21                  0.9

Construction of a level spreader may require formation of flow control banks as shown in
Figures 1 to 3.




Figure 1 – Example of a level spreader used for flow diversion around a soil disturbance




                        Figure 2 – Typical layout of level spreader


© Catchments & Creeks Pty Ltd            V2 February 2010                             Page 3