Design Criteria for Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 71

and the canal bank, is provided to safeguard the canal against overflow-
ing during unexpected water level fluctuations and wave actions. USBR
recommends a freeboard F

=

Cy , where the coefficient C varies between
0.5 to 0.6 with a minimum of 0.15 or 0.20 m.

4.3.3 Permissible velocity method

Depending on the fact whether a canal will be erodible or non-erodible,
the permissible velocity concept can be used as a design criterion for
stable canals.
• The minimum permissible velocity is defined as that velocity that will

not result in sedimentation or induce the growth of aquatic weeds. This
velocity depends on the sediment transport capacity of the canal.

• The maximum permissible velocity is that velocity that will not cause

erosion. This velocity is difficult to ascertain and is very variable; it can
only be estimated with experience and sound judgment (Chow, 1983).

Table 4.7 presents the maximum, permissible velocities depending on
the bed material (Simons and Sentürk, 1992). The USBR has derived from
these velocities the corresponding tractive force values (Chow, 1983).

Table 4.7. Maximum permissible velocities and the corresponding tractive force values.

Clear water Silt-loaded water

τ (N/m 2 ) τ (N/m 2 ) Material Manning nv (m/s) v (m/s)

Fine sand, colloidal 0.02 0.46 1.30 0.76 3.61

Sandy loam, non-colloidal 0.02 0.53 1.78 0.76 3.61

Silt loam, non-colloidal 0.02 0.61 2.31 0.91 5.29

Alluvial silts, non-colloidal 0.02 0.61 2.31 1.07 7.22

Ordinary firm load 0.02 0.76 3.61 1.07 7.22

Volcanic ash 0.02 0.76 3.61 1.07 7.22

Stiff clay, very colloidal 0.025 1.14 12.51 1.52 22.13

Alluvial silts, colloidal 0.025 1.14 12.51 1.52 22.13

4.3.4 Rational method

The design of irrigation canals typically tries to find the four unknown
dimensions, which include the bottom slope S o , bottom width B , water
depth y and side slope m . The side slope m should be based on the soil
properties and the (estimated) canal depth y . Hence, three equations are
required to determine the remaining variables. They might include an
alluvial friction predictor (de Chézy, Manning, and Strickler), a sediment
transport predictor and a minimum stream power or maximum sediment
transport efficiency. Sometimes a regime relationship is used to provide
a relationship for bottom width and water depth (HR Wallingford, 1992).