Alignment of Canals
Irrigation canals can be aligned in any of the three ways:
1. As watershed canal
2. As contour canal; and
3. As side â€'slope canal
Watershed Canal. The dividing line between the catchment area of two drains (streams) is called the watershed. Thus, between two major stream, there is the main watershed which divides the drainage areas of the two. Similarly, between any tributary and the main stream, and also between any two tributaries there, are subsidiary watersheds, dividing the drainage between the two streams on either side.
Figure: Alignment of a Water â€'shed canal along the water â€'shed
For canal system in plain areas, it is often necessary as well as advantageous to align all channels on the watersheds of the areas, they are designed to irrigate. The canal, which is aligned along any natural watershed, is called a watershed canal. From such a canal irrigation, water is taken out by gravity on either side of the canal, directly or through small irrigation channels.
Moreover, cross â€'drainage works are avoided, as the natural drainage will never cross a watershed, because all the drainage flows away from the watershed. Sometimes, watershed may have to be abandoned in order to bypass localities settled on the watershed.
Figure: Alignment of a Contour canal
(ii) Contour Canal: The above arrangement of providing the canals along the watershed is not possible in hill areas. In the hills, the river flows in the valley, while the watershed or the ridge line may be hundred of metres above it. It becomes uneconomical to take the canal on top of such a ridge. The channel, in such cases, is generate sufficient flow velocities, are given to it.
The maximum designed slope that can be provided in the canal without generating excessive velocities, is generally less than the available country slope. The difference is accommodated by providing canal falls at suitable places. A contour channel irrigates only on one side, because the areas on the other side are higher.
As the drainage flow is always at right angles to the ground contours, such a channel would definitely have to cross drainage lines. Suitable cross drainage works are then provided.
Figure: Alignment of a Side Slope canal
(iii) Side Slope Canal: A side slope channel is that which is aligned at right angles to the contours, i.e. along the side slopes, as shown in figure.
Such a channel is parallel to the natural drainage flow and hence, does not intercept cross drainage, and hence no cross drainage works are required.