Design Criteria for Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 70

Table 4.5. Reduction of the limiting boundary shear
stresses in non-straight canals.

Slightly sinuous 10%

Moderately sinuous 25%

Very sinuous 40%

As discussed in this chapter, the design of irrigation canals needs at
least four equations to establish the main dimensions of the canal. For
earthen canals the side slope will be based on the (estimated) canal depth
and soil properties and therefore, three equations are required to deter-
mine the remaining variables. Hence, the application of the tractive force
method requires two other equations: for example one equation to com-
pute the discharge (Manning, Strickler or de Chézy) and another for the
relationship between the bottom width and the water depth ( B/y ).
The hydraulically optimal cross-section has a minimized perimeter P ,
which results in a maximum flow velocity at minimum cost. However, the
optimum hydraulic section is hardly ever applied; this is because it will
not be stable due to relatively deep excavations and also any change in
discharge severely affects the water depth and the velocity. A deep section
is nevertheless applied wherever possible, because the expropriation costs
will be less, the velocity is higher in a deep rather than in a shallow canal
and the sediment transport capacity is larger in deeper canals (the transport
capacity is linear with the bottom width, but exponential with the water
depth). To limit the excavation and expropriation cost, canal side slopes
are designed to be as steep as possible. Soil material, canal depth and the
danger of seepage determine the maximum slope of a stable side slope. The
side slope has to be stable under normal conditions , also against erosion.
For deep excavations, an extra berm can be included to improve the slope
stability. Table 4.6 presents some values for side slopes in irrigation canals.

Table 4.6. Side slopes in canals.

Material Side slope: 1 V:mH

Rock 0.0

Stiff clay 0.5

Cohesive medium soils 1.0-1.5

Sand 2.0

Fine, porous clay, soft peat 3.0

The top width of canal embankments depends on the soil type, the
side slope and special requirements in view of maintenance and operation.
Water levels may rise above the design water level due to deterioration of
the canal embankment, a sudden closure of a gate or unwanted drainage
inflow. A freeboard, being the distance between the design water level