When a channel is remodelled, the remodelling of the outlets may consist in alterations of the number or sites or in alterations of their sizes.
Regarding the former, a map should be prepared showing all watercourses, chaks and contours.[35] On this map new lines for the watercourses can be shown, the principles enunciated in [Chapter II., Art. 9], being generally followed, but in such a way as to utilise existing watercourses and outlets as far as possible. The work often consists in the abolition of a certain number of watercourses, when these are too close together and run parallel to one another. There may, however, be little gain in amalgamating two such watercourses if they serve two different villages. There is nothing to prevent the people from dividing the watercourse into two as soon as it gets away from the canal, and they are likely to do this in many cases. When one branch has a flatter slope than the other it would lose command if it took off further down. The people on the steeper branch might not agree to using the flatter one because of silt trouble, or increased height of embankment. In a new project it is not difficult to get the people to do what is needed, but when once irrigation has become established it is often difficult to get suitable changes made. The advantages of amalgamating watercourses, though appreciable, have been a good deal exaggerated. The chief advantage is gained by reduction in the sizes of outlets. Then, however many branches the watercourse may have, they can only run in turns and not all together. It may happen that two watercourses, though taking off near one another, run in different directions and that the chaks are of suitable shapes and sizes. In such a case the only advantage of amalgamating is that it saves an outlet in the canal bank. No saving in the length of watercourse will be effected because there will be a bifurcation as soon as the watercourse leaves the canal boundary. If both outlets are of suitable design and proper size or require only slight alteration, both can remain but otherwise amalgamation can be effected. In some cases amalgamation might give a discharge greater than that usually allowed for an outlet but this need form no obstacle. The chief reason for limiting the discharge is the alleged inability of the farmers to manage a large channel. This matter is exaggerated as already stated ([page 74]). In the case under consideration it obviously makes no difference whether there are two watercourses each discharging 5 c. ft. per second, or one discharging 10 c. ft. per second, and immediately dividing into two. Very small watercourses should, when possible, be joined to others but if there is no other near enough they must generally remain, however small they may be.
[35] In small remodelling schemes the lines of existing watercourses show how the country slopes, and a contour plan is not a necessity.
Regarding the alterations in sizes of outlets, whether or not there are alterations in their number and position, information as to the actual duties on the watercourses should be obtained. The discharge of the watercourses should be observed several times and added up and checked with the discharge of the distributary. The areas irrigated are known from the irrigation register. If the duties are abnormal the causes can be gone into, and a judgement can be formed as to how far they will remain in existence, and whether any watercourse is often kept closed. If so the outlet is too large. The duties, modified so far as may seem desirable, can be used for calculating the sizes of the remodelled outlets. But alterations of the sizes after a year or two years’ working will probably be necessary. The above procedure is also applicable to a case where the old watercourses had no masonry heads but were merely open cuts as on some inundation canals.
A common case is that in which the channel is not remodelled—or at least its water level remains very much as before—but merely the outlets are altered in number, position or size, or in any or all of these. If the land irrigated by an outlet is high, the irrigation may be far short of what was expected, and the size of the outlet may have to be increased or its site shifted, generally upstream. This is often done at the request of the people, and at their expense.[36]
[36] On some of the more modern canals the people are not allowed to pay for outlets, so that no question of ownership can arise.
Old outlets should always be removed when superseded by others. Otherwise they are apt to be reopened or claims set up regarding them.
Near the tail of a channel the discharge of an outlet may be an appreciable fraction of that of the channel. In such a case the adjustment of the size of the outlet, and that of the channel or of any weir or fall in the channel, should be considered together, the irrigation on the outlet and that on the channel downstream of it being compared. And similarly as to the sizes of any two or more tail outlets. Such outlets are sometimes left without masonry heads on the ground that this injures no one. It may injure an outlet upstream of them by drawing down the water. Tail outlets often need constructing or reducing in size to raise the water level in the reach upstream of them.
Whenever the size of an old outlet is altered the design should be altered if unsuitable. The parapets should be brought into proper line, the roadway corrected, the floor level adjusted and any splayed wing walls abolished. If the outlet is skew it should be made square. All this should also be done to all old outlets or heads of minors even if the sizes are correct, whenever remodelling of outlets on any channel is undertaken.[37]
[37] Wherever an outlet is built or altered, a template, made to the exact size of barrel required, should be supplied to the subordinate in charge of the work.