India and Indian Engineering. / Three lectures delivered at the Royal Engineer Institute, Chatham, in July 1872

by J. G. Medley

However much tanks may differ in locality and size, they may all be said to consist of the following parts:—1. A Dam or bund, natural or artificial, or both, by which the water is restrained. 2. A Stream, by which the tank is fed, unless it is fed by direct rainfall. 3. A Sluice or sluices, by which the ponded water is drawn off to be used for irrigation. 4. A Waste Weir, overfall, or Calingula, by which, when the tank is full, surplus water can escape safely. The Dam is generally the most important work in a tank, and the determination of its site, and the construction of it when chosen, often demand engineering skill of a high order. It is, of course, first of all necessary to take a very careful series of levels in order to determine whether the amount of water that can be ponded up by a given length and height of dam will be enough to pay for the cost of the work. Next, you must determine whether there is a sufficient area of land in the vicinity demanding water, and whether you are pretty sure of a good supply of water for the tank.

The Dam may be built of earth or stone, or of a combination of the two; and in the case where a great depth of water is ponded up, the foundations of the dam must, of course, be constructed in the most substantial and careful manner, for a failure may involve not merely the destruction of the tank, but the loss of hundreds of lives. The question of the best form of retaining wall to withstand the pressure of great depths of water, has excited a good deal of discussion, and you will find some of the best sections in the ‘Papers,’ as well as a good description and diagram of calculation of the Moota Dam in Bombay.

The other chief point in a tank besides the dam is the Waste Weir, which is generally built at the exit from the tank of the original channel by which it is fed. It must be made of sufficient length to discharge the maximum quantity of water which your calculations have shown you is likely to pass over it, and it must be strong enough, both in body and foundation, to resist both the pressure and the shock of this water. For this latter purpose, the principle of the vertical canal fall, already described, is often used, and the falling water is received into a cistern, by which its shock is broken.

You will find a chapter devoted to tanks in the ‘Roorkee Treatise,’ and several examples of the different kinds employed.

As to the irrigating duty of tank water, it is roughly calculated that a cubic yard of water is required for every square yard of land; this, however, refers to rice, which takes far more water than wheat. We have not time to pursue the subject further.

Before closing the subject of irrigation works, I may perhaps be allowed to name one or two of those engineers who have added most to our knowledge of that science. Foremost stands the name of the late Sir Proby Cautley, F.R.S., of the late Bengal Artillery, the designer and constructor of the Ganges Canal; then of Sir A. Cotton, of the Madras Engineers, his great rival, head and founder of the Madras school of irrigation; Colonel Baird Smith, F.R.S., chief engineer at the siege of Delhi; and then the late Colonel Dyas, of the Bengal Engineers, one of the most scientific officers in India.

Though not exactly under the head of irrigation works, the question of [River Works], or river improvements, is not very far removed from it, and though we have but little time to spare for it, it would not be right to exclude such a subject altogether from your notice, for many of you will probably have a good deal to say to it sooner or later.

An engineer is generally called upon to devise works for the improvement of a river, either to increase its navigable facilities or to prevent its waters from inundating the country. With regard to the first object, the work in India generally consists in removing obstacles, such as shoals, kunkur banks, sunken trees or boats, and I cannot do better than draw your attention to the admirable paper on the Gogra River Works, sent to me by the late Lieutenant Carroll, R.E., whom I have already mentioned. The systematic improvement of rivers by the lock and dam method, as in the Thames and other small streams, is hardly applicable to such rivers as the Ganges and Indus, except at a cost which puts it altogether out of the question. It is greatly to be regretted that it is so, for the navigable capabilities of Indian rivers are far inferior to what we should suppose, judging from their length and size; much inferior to those of America, for example, though the Mississippi has much the same general characteristics as the Ganges.

During the rainy season, Inundations in the lower part of the course of many of these Indian rivers often extend far inland; the delta of the Ganges is, in fact, a vast sheet of water at that time of year, and in the dry season it is an enormous jungle intersected with watercourses, and inhabited only by tigers, snakes, and other wild animals, with the exception of a few wretched fishermen or woodcutters.

Higher up the country, again, rivers, such as the Damooda, are restrained within artificial embankments, which, unfortunately, have not been systematically planned, but have been erected from time to time by the different villages threatened with inundation along its course. There are often many miles of such embankment (or levees, as they are termed in America,) under a young engineer’s charge, and very anxious work it is, as he has to guard a long line from the attack of an insidious foe, often with a very imperfect garrison. Such embankments as these are always constructed of earth, and when consolidated by time and covered with grass, a very thin earthen bank will successfully resist until it is absolutely overtopped, while new earthwork of twice and three times the sectional area will be soaked through and breached readily. Another source of danger arises from rats and other vermin, which perforate your embankments in hundreds of places.