Design Criteria for Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 62

- canals with an erodible boundary and carrying clean water : the canal
design is based on the determination of the maximum velocity for which
the bed material in the cross section does not move. The minimum cross
section with a maximum velocity that does not result in scouring of the
bed should be the end result of the design;
- canals with an erodible boundary and carrying water with sediment :
the design principle assumes that the canal should transport the water
as well as the sediment. The cross section must ensure flow velocities
as large as possible to convey the sediment and at the same time not
too large to prevent scouring of the bed. It is evident that it is difficult
to meet both restrictions simultaneously and for the whole irrigation
season. Therefore, the design must look for a stable canal over a longer
period and this means that the total sediment inflow during a certain
period must equal the total sediment outflow.
A major conclusion to be drawn from the previous section is the need
for the design of stable canals during the full operating life of the irri-
gation network. Sediments may be deposited during one phase of the
irrigation season and be eroded during another phase, but for the total
operation period, there should be a balance between erosion and deposition
in the canal.
Numerous researchers have developed a wide range of canal design
methods that are in use around the world. In the field of alluvial canal
design the earliest work was of Kennedy (1895). Several others followed
with width predictors for alluvial canals. The concepts of minimum stream
power (Chang, 1980), minimum energy dissipation (Brebner and Wilson,
1967; Yang et al . , 1981) and maximum sediment transport (White et al.,
1981) have been proposed to conquer limitations in the regime approach.
Lane (1955) presented the tractive force theory developed by many oth-
ers at the U.S. Bureau of Reclamation. This approach is more suitable
for canals that carry very little sediment and the problem is limited to
controlling the bed or bank erosion. Depending upon their fundamental
solution these methods can be broadly classified under regime, tractive
force and rational theories.
Chow (1983), Raudkivi (1990), HR Wallingford (1992), Simons and
Sentürk (1992), and others mention four methods for the design of stable
canals:
- the regime method;
- the tractive force method;
- the permissible velocity method;
- the rational method.
4.3.1 Regime method
The regime design methods present sets of empirical equations based on
observations of canals and rivers that have achieved dynamic stability.