8 DESIGN OF STABLE CHANNELS 8.1. GENERAL Surface water for irrigation is conveyed from its source to the field by means of canals or channels. These channels generally have alluvial boundaries and carry sediment-laden water. A hydraulic engineer is concerned with the design, construction, operation, maintenance, and improvement of such channels.
Lane (1) gave the definition of stable channel as follows:
‘‘A stable channel is an unlined earth channel : (a ) which carries water, (b) the banks and bed of which are not scoured objectionably by moving water, and (c) in which objectionable deposits of sediment do not occur’’. This means that over a long period, the bed and banks of a stable channel remain unaltered even if minor deposition and scouring occur in the channel. Obviously, silting and scouring in a stable channel balance each other over a long period of time.
An irrigation channel can have either a rigid boundary or one consisting of alluvial material. These channels may have to carry either clear water or sediment-laden water. Accordingly, there can be four different types of problems related to the design of a stable channel.
(i) Rigid-boundary (i.e., non-erodible) channels carrying clear water, (ii) Rigid-boundary channels carrying sediment-laden water,
(iii) Alluvial channels carrying clear water, and
(iv) Aluvial channels carrying sediment-laden water.
The design of a stable channel aims at obtaining the values of mean velocity, depth (or hydraulic radius), width and slope of the channel for known values of discharge Q, sediment discharge QT, sediment size d, and the channel roughness characteristics without causing undue silting or scouring of the channel bed.
