Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 10

mat, clay lining, and compaction are durable for 3, 2, 1, and 1 year, respectively.
Nowadays, irrigation conveyance is being done by low-cost rubber pipes, hose
pipes, and underground pipe systems in many developing countries.

1.2 Designing Open Irrigation Channel

1.2.1 Irrigation Channel and Open Channel Flow

An irrigation channel is constructed to convey irrigation water from the source of
supply to one or more irrigated areas. A channel or lateral is needed as an integral
part of an irrigation water conveyance system.
In an open channel, water flows at atmospheric pressure, under the force of grav-
ity. In most cases, a gentle slope is provided in the open channel to facilitate the
flow.
The words “ Canal ” and “ Channel ” are interchangeably used in the literature
and also in this topic. Basically, “canal” is artificially constructed (man-made), and
“channel” is a natural water passage.

1.2.2 Definition Sketch of an Open Channel Section

The cross-sectional view of a trapezoidal channel with definition sketch is shown in
Fig. 1.3

Channel top width

Freeboard

Flow
depth

1

z

Channel bed

Fig. 1.3 Definition sketch of a trapezoidal channel section

Top width (T) : It is the width of the channel at the surface.
Freeboard : It is the additional/extra height of the channel above the design flow
depth. It is provided as a safety factor.
Channel bed : It is the bottom width of the channel.
Side slope : Channel side slope is generally expressed as horizontal:vertical (i.e.,
H : V ). For convenience in computation, the vertical value is reduced to 1, and
the corresponding horizontal value is expressed as Z .