Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 94

• degree of variation in the canal alignment, the size, shape and cross

section in the longitudinal direction.

The hydraulic resistance of water flowing in canals is also influenced
by the development of bed forms such as ripples, mega-ripples and dunes.
This resistance is measured in terms of a friction factor, for instance,
the de Chézy coefficient. Other examples of friction factors are the
Darcy-Weisbach and the Manning/Strickler coefficients.
The determination of the de Chézy coefficient of a movable bed is
complex and requires knowledge of the implicit process of flow condi-
tions and bed form development. The hydraulic resistance or roughness
depends on the flow conditions, such as velocity, water depth and sedi-
ment transport rate, but these conditions also greatly affect the bed form
development and hence the roughness. In fact, the dynamics of the bed
form development and the variety of bed configurations that may simul-
taneously occur frustrate the development of an equation that accurately
describes the roughness and the related friction factor.
The influence of the various factors will depend upon whether the
canal has a rigid or erodible boundary, and carries clear water or water
with sediment. Irrigation canals are manmade, so the degree of variation
in canal alignment, size, shape and cross section is normally small and
the effect of variation in alignment, size and shape can be ignored. For the
analysis of canals carrying sediment within an erodible boundary the bed
can be assumed to be free from vegetation and the effect of vegetation will
be on the sides only. The irregularities in the surface may be on both the
bed and sides, but the source of irregularities may be different. Irregularity
due to bed forms will occur on the bed only, while irregularity due to poor
construction or maintenance will be mostly on the sides. Once the canal is
operated the bed will be smoothed and the only irregularities will be the
bed forms. When the roughness is not uniform over the whole perimeter
then the shape of the cross section and the B / y ratio will influence the
overall roughness. Figure 5.3 shows the process for the determination of
the effective roughness in an erodible boundary canal.
The effective roughness is not constant for different flow conditions
and changes with time. Bed forms are a function of the flow and the
sediment characteristics, which will change during the operation. Also the
vegetation will change and there might be periodic maintenance activities
during the operation. Hence a methodology should be employed to predict
the roughness with time to study the performance of the design during
operation of the canal system.
An important feature that determines the flow in canals is the correct
estimation of the equivalent roughness. In many canals the flow very
often encounters a roughness along the bottom that is different from the
roughness of the sides. Some of the other characteristic conditions in
irrigation canals that might influence the flow of water and sediment are