Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 11

Wetted perimeter (P) : It is the wetted length of channel across the cross-section
of the channel. It is the sum of the channel bed width plus two sloping sides.
Hydraulic radius (R) : It is the ratio of wetted area ( A ) to the wetted perimeter
( P ) of the channel cross-section, that is, R

A
P .

=

1.2.3 Considerations in Channel Design

1.2.3.1 Channel Geometry

The channel geometry design depends on site conditions and conveyance needs.
The channel cross-section may be trapezoidal, rectangular, parabolic, V-shaped, or
a combination of the geometric shapes.

1.2.3.2 Capacity Requirements

The capacity of canals or laterals should be as follows:

• Sufficient to meet demands of all the irrigation systems served and the amount of

water needed to cover the estimated conveyance losses in the canal or lateral

• sized to convey the available water supply in water-short areas, where irrigation

water is in demand.

• Capable of conveying surface runoff that is allowed to enter the channel, and

• Such that flow or runoff velocity must be non-erosive.

1.2.3.3 Permissible Velocity/Velocity Limitations

The design of an open channel should be consistent with the velocity limitations
for the selected channel lining to satisfy the condition of non-erosive velocity in
the channel. The velocity should not be too low to cause siltation in case of surface
drainage.
Permissible non-erosive velocity of a channel is dependent upon the stability of
lining materials and channel vegetation, as follows:

Maximum
velocity (m/s) Material

Sandy soil 0.6

Loam to silt 1.0

Silty clay 1.2

Stiff clay 1.5

Graded loam to silt 1.5

Hard pan/coarse gravel 1.5

Vegetative channel (grass cover of
alfalfa, weeping lovegrass)

1.2