Hydraulic Design of Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 249

be evacuated to drains or natural watercourses. A correct canal alignment
also depends on the bottom slope and one of the main design principles
is that the average slope of a main canal is more gentle than the average
slope of a secondary or branch canal (Leliavsky, 1983).
The final design of the horizontal and vertical alignment should be the
most economic solution in view of construction and maintenance costs
and this design is not always the shortest distance between two points.
The layout of an irrigation network depends on the location of the existing
natural, main drains; they form the physical boundary conditions for the
layout (in view of costs of land shaping). The horizontal alignment follows
as much as possible the topography of the terrain and is preferably located
on ridges, the main canal is situated along the watershed formed by the
main ridge and the branch canals follow the less significant ridges. Curves
in unlined canals should be as large as possible as they disturb the water
flow and have a tendency to induce siltation on the inside and scour on
the outside of the curve. The minimum radius for earthen irrigation canals
depends on the size and capacity of the canal, the type of soil and the flow
velocity. The recommended value for these curves is at least 8 times the
design width at the water surface.
The vertical canal alignment is a compromise between the following
design requirements:
•

Water levels should be high enough to irrigate the highest areas for
which irrigation is envisaged.

• Maintenance costs should be as low as possible; they are lower when

the water level is below the ground surface. These lower water levels
might hamper the illegal diversion of water by gravity.

• Balance between cut and fill is an economic criterion in view of the

construction costs; but when this criterion results in canals in high fill
it has to be mentioned that these canals are more difficult to construct
and in general will lose more water by seepage.

C.3 WATER LEVELS

The water level or command is the location of the water surface in relation
to a datum (reference level). The water level in a canal reach should be high
enough to supply water to the highest farm plots for which irrigation is
envisaged. In a gravity irrigation system water flows from the secondary
canal through a turnout to a tertiary canal, and next through the farm
canal system to the farm plot. The water level at the turnout, taking into
account the distance to and the ground level of the critical point, should
be sufficient high to convey the water to the farm plot. The design water
levels of a canal follow from the water level diagram . This diagram
presents the longitudinal profile of the ground surface and indicates all