Contents preface (VII) introduction 1—37

5
CANAL IRRIGATION
5.1. CANALS
A conveyance subsystem for irrigation includes open channels through earth or rock formation, flumes constructed in partially excavated sections or above ground, pipe lines installed either below or above the ground surface, and tunnels drilled through high topographic obstructions. Irrigation conduits of a typical gravity project are usually open channels through earth or rock formations. These are called canals.
A canal is defined as an artificial channel constructed on the ground to carry water from a river or another canal or a reservoir to the fields. Usually, canals have a trapezoidal cross-section. Canals can be classified in many ways.
Based on the nature of source of supply, a canal can be either a permanent or an inundation canal. A permanent canal has a continuous source of water supply. Such canals are also called perennial canals. An inundation canal draws its supplies from a river only during the high stages of the river. Such canals do not have any headworks for diversion of river water to the canal, but are provided with a canal head regulator.
Depending on their function, canals can also be classified as: (i) irrigation, (ii) navigation, (iii) power, and (iv) feeder canals. An irrigation canal carries water from its source to agricultural fields. Canals used for transport of goods are known as navigation canals. Power canals are used to carry water for generation of hydroelectricity. A feeder canal feeds two or more canals.
A canal can serve more than one function. The slope of an irrigation canal is generally less than the ground slope in the head reaches of the canal and, hence, vertical falls have often to be constructed. Power houses may be constructed at these falls to generate power and, thus, irrigation canals can be used for power generation also.
Similarly, irrigation canals can also be utilised for the transportation of goods and serve as navigation canals. Inland navigation forms a cheap means of transportation of goods and, hence, must be developed. However, in India, inland navigation has developed only to a limited extent. This is mainly due to the fact that irrigation canals generally take their supplies from alluvial rivers and, as such, must flow with sufficient velocity to prevent siltation of the canal. Such velocities make upstream navigation very difficult. Besides, the canals are generally aligned on the watershed¹ so that water may reach the fields on both sides by flow. This alignment may not be suitable for navigation which requires the canal to pass through the areas in the vicinity of industries.
¹ Watershed is the dividing line between the catchment areas of two drains (see