Design Criteria for Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 61

of sediments. The need to convey different quantities of water to meet the
irrigation requirements at the required water level is the main criterion for
canal design. Furthermore, the design must be compatible with a particular
sediment load in order to avoid silting and/or scouring (Lawrence, 1990
and 1993). The diverted discharge should meet the irrigation requirements
and at the same time it should result in the least deposition and/or erosion
in the canal network.
Vanoni (1975) stated that the canal design must be based on a compre-
hensive assessment of the canal operation to determine the future pattern
of the water demand. In that way the sediment transport characteristics
along a canal network can be established in terms of place and time.
Sediment may be deposited during one phase of operation and eroded
during another phase, resulting in a balanced or stabilized condition.
According to FAO (1981), the objective of a canal design is to select
the proper bottom slope and geometry of the cross section so that during
a certain period the sediment flowing in is equal to the sediment flowing
out of the canal. Changes that occur in the equilibrium conditions for
sediment transport will result in periods of deposition and/or erosion.
Chang (1985) suggested that in view of these sediment problems,
the bottom slope and the canal geometry must be interrelated in order
to maintain the best possible sediment transport equilibrium. He stated
that the sediment problem can be controlled by maintaining the con-
tinuity in sediment transport in the irrigation canals during the design
stage.
Dahmen (1994) pointed out that an irrigation network should be
designed and operated in such a way that:
- the required flow passes at the design water level;
- no erosion of the canal bottom and banks occurs;
- no deposition of sediment in the canal takes place.
The design of a canal that has to convey a certain sediment load requires a
set of equations related to the water-sediment flow to provide the unknown
variables of bottom slope and cross section (bottom width and water
depth). The geometry of an irrigation canal that carries water and sed-
iment will be the end result of a design process in which the flow of water
and the transport of sediments interact.
In view of the design, irrigation canals can be divided into three
categories (Ranga Raju, 1981) that can be described as follows:
- canals with a rigid boundary : the design is based on the determination
of the velocity at which any sediment entering into the canal will not
settle on the canal perimeter. High velocities are allowed, but they
should not damage the lining or create large disturbances in the water
surface. A numerical simulation of the most likely changes in the flow
conditions during the irrigation season becomes an important tool to
ensure that the sediment is not deposited, even when the velocity is low;