A committee was appointed by Lord Canning to consider the views as to irrigation held by Colonel Sir Arthur Cotton and Colonel Crofton, land they decided in favour of Colonel Crofton's views against those of Sir A. Cotton. Of the reasons given for its decision one was their ob , jection to the construction of a weir across the Ganges, below the confluence of the Solari, at the estimated cost of £1,128,631, to pass a volume of water over the Ganges of 30,000,000 cubic yards. But the Godavery weir, to pass above 200,000,000, only cost £90,000; the Cauvery weir, to pass the same volume as the Ganges, 30,000,000 cubic yards, only cost £15,000; the Kistna weir, to pass 180,000 cubic yards, only £100,000 ; the Tumbudra weir, for about double the volume of the Ganges, only £30,000. The average estimate for weirs on similar rivers in Madras is about £500 per million cubic yards of volume per hour ; while the estimate of the Ganges Canal Committee was nearly £40,000 per million.
Only rivers of the larger class, which have a continuous flow for several months, are available for extensive irrigation projects. The smaller rivers are merely torrents, which quickly carry off heavy falls of rain, and then become dry again. The water, however, is in many cases intercepted by chains of tanks, of- the second or third class, built across these torrents.
The deltas of large rivers, being the most easily irrigated lands, have been so treated for ages, and the works have been much extended and improved under the British Government, by the construction of permanent weirs of great lengths at the heads of the deltas, such weirs being built on the sandy beds of wide rivers subject to heavy floods. This seemed to have been beyond the skill of the ancient native rulers. They, however, built many weirs on the large rivers in the middle part of their courses, the situations being skilfully chosen, but the construction was rude and im perfect. They were generally built on a reef of rocks, with loose rubble, faced with large blocks of granite laid dry, and sometimes fastened with iron clamps. The modern weirs in similar situa tions are of masonry, with a vertical or slightly battering face on the down-stream side, and with heavy copings. In rivers having sandy beds, it is usual to build the body of the weir on a founda tion of brick wells, sunk to the low-water level, and filled with concrete. On the lower side there is an apron, having a slope of 1 in 12 from the crest, with a toe wall ; and if the slope be long, intermediate walls are also built on wells, and below all there is a broad layer of rough rubble of large dimensions.
The ancient irrigation channels were generally defective in design, being too small, and having much too great a fall. In consequence of these
channels being so near the river, they irrigated only a narrow strip of land ; and the current being too great, excessive annual repairs were required. This system necessitated numerous off takes from the river, involving the expense of many weirs, and a great aggregate length of un productive channel, from the offtake to the point where the channel reached such a level as to com mand the surface of the country. On the other hand, a canal of large dimensions, taken off from one head, having a slower current and less fall, would soon so gain on the level of the river that it would reach districts remote from it, and con sequently more in need of artificial supplies of water ; and it would also command a much larger extent of country than, it could supply entirely with water. This was an advantage, because it would bo many years before a district could be completely changed from dry to wet cultivation, as it would require to have its population trebled. It also afforded means of assisting dry crops in years of drought, and thus preventing famine. In many districts complete failure of the crops now grown occurred every few years, and a good crop was a rare occurrence. There should therefore be facilities for completely irrigating detached areas at considerable intervals, and of giving occa sional irrigation to dry crops.
Distribution was effected from the second class of tanks directly, by means of sluices in the bund. From the third, and more especially from the first class, it was commonly effected indirectly ; thus the natural channel of the river or rivers, which had been dammed to form the tank, were used to carry part of the water for irrigation, weirs being built across them at suitable places, and artificial channels taken off from above them. By these means the surplus of the water, which was gene rally wastefully used by the ryots, was saved, being collected by drainage into the stream, and redis tributed at the next weir. Distribution was most economically effected from a canal, when the latter ran along a ridge ; but as this could rarely be accomplished in the case of a canal taken off from a main drainage, it was next best effected by leading the main distribution channels down the ridges crossed by the canal. Distribution could be carried out in the Ceded Districts for 5s. per acre, including sluices in the main canal, and all necessary road and water crossings, but excluding the coast of terracing the land to prepare it for wet cultivation, this being done by the occupier. The nature of the ground was occasionally such that the drainage was effected naturally, no works being required for that purpose beyond small open trenches in the rice fields.