Irrigation works

by E. S. Bellasis

It was stated in [Chapter II.] that the construction of masonry outlets on a distributary is not usually a final settlement of the matter. In many cases a proper proportion of water does not reach the tail. Even in such a case matters have occasionally been left alone, or the old and pernicious system of closing the upper outlets has been resorted to. In such circumstances the irrigation of a group of tail villages will be found to be less than that of a group higher up, the people to some extent acquiescing in the old idea that a tail village must be a sufferer. Government, or at least the Irrigation Department, has no particular direct interest in the matter. The total area irrigated, will probably be very much the same in any case. But an engineer who takes an interest in this part of his work will not allow matters to remain long in the state described. He will, of his own accord, adjust the outlets and equalise, as far as possible, the irrigated percentages. The people will disturb matters to some extent by enlarging watercourses, but there is a limit to this and it can be met by an occasional reduction of an outlet. A distributary, when once its outlets have been carefully adjusted, attains to something approaching perfection in its working. Any excess in the supply is taken partly by the upper outlets but part of it gets to the tail. Similarly any deficiency in the supply is distributed over the channel. The outlets which have a poor command and small head are most affected in either case. On the whole they do not lose or gain more than the others. The working of such a distributary causes great satisfaction to the engineer and not the least ingredient in this is the knowledge that he has wholly destroyed the power of his native subordinate.

In an inundation canal division in the Punjab, some dozen distributaries, varying in length from 5 to 28 miles, and with discharges ranging up to 300 c. feet per second, were dealt with as above in one season. The engineer in charge being specially desirous that sufficient water should reach the tails, reduced the sizes of some outlets too much. When an outlet of 1 or 2 sq. feet has to be reduced to a small fraction of its size it is not easy to say what the fraction shall be. Water reached the tails of all the channels in sufficient quantity, in some cases in rather more quantity than was necessary. When the irrigation register was examined, it was found that the general results were entirely satisfactory. In a small proportion of cases outlets had irrigated too little and had to be re-enlarged somewhat. After a second season hardly any changes were needed. When any silt clearance or berm-cutting seemed necessary the irrigation register again came into play. If, for instance the tail outlets, as a whole, were receiving too little water, enlargement of the upstream reaches was effected with consequent lowering of the water level there.

In the case above described the channels flowed for only five months in the year. Some of them silted a good deal but as this silting was roughly the same every year, it did not greatly affect the question of outlet sizes. On a perennial distributary of which the head reach silts during part of the year and scours during the other part, a proper distribution of supply by adjustment of outlet sizes alone may be more difficult. If the silt was frequently cleared, this would cause needless expense and interference with irrigation. In cases where the distributary is not run constantly, something can be done by attending to the regulation. When there is silt in the head reach, the discharge can be reduced and the period of flow proportionately increased. The lowered water level reduces the supplies of the upper outlets, and increases the discharges of those lower down. Moreover the periodical silting and scour are not always serious. Also it is not essential that the supply to each watercourse should be exactly the same every year. There are always good and bad seasons. It is sufficient if a watercourse is not allowed to suffer on the whole, and is never allowed to suffer much. There is no doubt that it is possible to deal satisfactorily in the above manner with very many distributaries. It is frequently reported that “difficulty is experienced in getting water to the tail.” This is owing to timidity in reducing the sizes of outlets. The suitable plan is to reduce them to such an extent as to cause a proper supply to reach the tail and then, if necessary to enlarge some. It has been already remarked that only a short length of the barrel need be altered. The cost of this is very small. The real difficulty in the case is not the impossibility of securing good results, but the impracticability, in many cases, of securing the constant attention which the procedure demands.[38]

[38] See also [Chapter V. Art 3].

8. Miscellaneous Items.

At the headworks of a canal there is a permanent staff of men who work the gates and look after the works. They assist in discharge observations and in reading the gauges, and they may have to take soundings in the river to see what changes are taking place. Some one is on watch day and night and reads the gauges at frequent intervals. The officer in charge occasionally inspects the works at night without notice. Detailed rules regarding the above matters, and any others that are necessary owing to special local conditions, are drawn up. Sometimes there is difficulty in getting the staff to attend properly to the regulation of the supply in the canal at night. Probably some “tell-tale” watches would be useful. They would at least show the times at which the men concerned went to the gauges or other points.

At the headworks, and at all important regulators, a stock of concrete blocks should be kept ready for the execution of any urgent repairs.

Regarding the ordinary maintenance work on the channels, details are given in [Appendices B] and [C]. [Appendix D], reprinted from Punjab Rivers and Works, contains rules for watching and protecting any banks or embankments which require it.

Silt clearances and berm cutting of channels have been mentioned in [Art. 1]. Special attention should be given to the accurate ranging of the centre line. Otherwise the channel may become crooked. The great defect in the earthwork ordinarily met with in the banks of canals and distributaries is that the clods are not broken. In consequence of this new banks are extremely liable to breach, and much trouble and expense result. Sometimes a dam is thrown across a new distributary, and the channel upstream of it is gradually filled with water, the bank being watched and leakages made good. The dam is then shifted to a place further down. In this way the banks are consolidated.

When a distributary is closed for silt clearance or other work, if the head regulator has planks and a double set of grooves, it is possible to stop all leakage by filling in earth between the two sets of planks and ramming it, but otherwise it is necessary to construct an earthen dam just below the regulator. Upstream of the dam the water, owing to the leakage through the planks, gates or needles, rises to the same level as the water in the canal. Native subordinates have a remarkable aptitude for allowing such dams to break while the work in the distributary is in progress or before it is measured. Now and then the dam is wilfully cut. The remedy is to make the dam of proper strength—the top should be 8 feet wide and a foot above the water,—and to have it watched day and night.