Irrigation works

by E. S. Bellasis

On some Indian irrigation canals made about 30 years ago, great sums of money were wasted in making the canals navigable. There is nothing like enough navigation to pay for the extra cost. The idea has now been quite given up except as regards timber rafting from upstream. This requires no curtailment of the velocity in the channels. The requirements of the irrigation and navigation were always in conflict. The mere fact that branches have to be worked in turns is enough to prevent navigation succeeding.

In India the water used for irrigation is paid for, not according to the volume used but according to the area irrigated. The volume used in any particular watercourse is not known. The areas sown are measured. Certain kinds of crops use up more water than others and the charges are fixed accordingly.

In the canals which have their headworks among the mountains of Western America there are frequent tunnels and syphons and the canals often run in steep sidelong ground. There are great lengths of tunnel and syphon in the Marseilles and Verdun Canals and there are long tunnels in the Periyar Canal in Madras and in the Upper Swat Canal in the North West Frontier Province of India.

The Tieton Canal, Washington, U.S.A., traverses steep sidelong ground which would be liable to slip if a large cutting were made. The cross-section of the channel is a circle, 8-ft. 3¹⁄₂-ins. in diameter, with the upper part removed, so that the depth is 6 feet. It is made of reinforced concrete 4 inches thick and the sides are tied together by iron bars which run across the channel above the water. In the Santa Ana Canal the channel consists for 2¹⁄₂ miles of a flume made of wooden “staves.”

A canal constructed in Wyoming, U.S.A., after taking off from a river, passes through a tunnel into another valley and is turned into another stream which thus becomes the canal. This is said to save loss of water by percolation. The stream is winding while a canal could have been made straighter. There may, owing to the ground near the stream being saturated, have been less loss of water at first than there would have been in the artificial channel but, owing to the smaller wetted area, there would probably have been an eventual saving in adopting the latter. The real advantage of adopting the natural stream was probably a saving in the cost of construction. (Min. Proc. Inst. C.E. Vol. CLXII.)

Irrigation from canals which are supplied from reservoirs differs in no respect from that from other canals. The principles on which reservoir capacities should be calculated and earthen and masonry dams constructed are given in River and Canal Engineering. Sometimes, as for instance when a reservoir becomes seriously reduced in size owing to silt deposit, the water is run off after the bed of the reservoir has been soaked, and crops are grown on the soaked soil.

The distribution of the water of a canal as between the main channel and the branches, is effected by means of the regulators at the bifurcations. When the supply is ample and the demand great, the channels may all be running nearly full. When the demand exceeds the supply, the water may be reduced proportionately in each branch but this may result in the water of a branch being too low to give proper supplies to the distributaries or some of them, and in the water of a distributary not commanding the higher ground. Moreover it violates the principle of keeping the water in bulk as far as possible. It is more usual to give each branch full supply, or a certain large fraction of the full supply, in turn, and similarly with the distributaries.

The method of distribution from a distributary to the watercourses varies. In many modern canals there is, at each watercourse head, a sluice which is adjusted at frequent intervals according to the supply and the demand. One method, which is excellent because it fulfils in the highest degree the principle of keeping the water in bulk, is to have very large watercourses and, by means of regulators which are built at frequent intervals, to turn the whole of the water of the distributary into a few watercourses at a time, beginning with those nearest the head of the distributary and working downstream. But a system which seems eminently suitable may be impracticable because of local circumstances. In India, any such arrangement would need an army of officials and would lead to unbounded corruption.

In India the water from a distributary enters the watercourses through “outlets” which are small masonry tubes passing through the banks of the distributary. There is no easy way of closing these outlets or at least of keeping them closed if the cultivators choose to open them, but it is easy to close a whole distributary and so regulate the supply. This is the chief reason why watercourses in India do not usually take off direct from the canals.

The presence of silt in the water of a river from which a canal is drawn is often spoken of as being a great evil. If it is an evil at all it is a very mixed evil. The deposits of silt in the channels have been enormously reduced by the application of scientific principles of design. The clayey silt which remains in the water and reaches the fields, brings to them greatly increased fertility.