Page:Journal of the Asiatic Society of Bengal Vol 1.djvu/507

1832.] time, and with the least possible labor; at the same time avoiding the application of superstructure in the shape of a fixed road-way and lifting gates; in short, to throw the whole dam as open as possible, and to relieve it from any obstructions that may interfere with the free passage of the floating timber, &c. It will appear evident, that, the best sort of dam in a position of this nature would be a simple flooring of masonry, with flanks of the same material; the bund or bank for retaining the water being constructed of earth, gabions, &c. which although it would be annually washed away during the rains, would be repaired at a trifling expense, and the space of masonry between the flanks would provide an escape sufficiently ample, and at the same time perfectly free and unimpeded by any interruptions from piers, &c. in its width. This species of dam would best suit a situation, where the heavy freshes only rise once during the year, and springs of sufficient abundance occur below the works to keep the southern line of canal supplied with water; but in the present case, the destruction of these bunds or dams, and the loss of the head supply from the mountain streams, would leave the canal nearly dry, as there are but a few scanty springs in the southern limits. To meet this difficulty the following form of sluice has been introduced in the dams over the Rogaon and the Muskára rivers, on the Doab canal works; and as the experience of two rainy seasons has proved its efficacy, and shewn a facility of working, which was in a measure unexpected, I cannot do better than enter into a short description, accompanying it by a sketch, which will also exhibit the high-water mark of the floods that have occurred within the last two seasons.

Fig. 1—Represents a tranverse section through the centre opening of the Muskára dam. The gate being closed, the flank revetment is shewn at the back. The horizontal lines explain the different levels at which the canal supply may be regulated. The two upper lines shew the high-water marks of floods in the rains of 1830 and 1831. The former of 8 feet 9 inches, and the latter of 19 feet, from the flooring of the dam.

Fig. 2—Shews an elevation of three openings of the dam, with the sluice gates at different angles, the windlasses and chains fixed, &c.

Fig. 3—Shews three similar openings, with the gates dropped, and the windlasses, chains, &c. removed, as happens in the height of the rains, when the rivers are subjected to continual floods. The gates moreover act as self- regulators on certain occasions, especially on the approach of the flood water from the hills, which is generally sudden, although its approach is perfectly well known to the inhabitants of this part of the country.

The gates are made to fit the grooves in the masonry as closely as possible, so as not to fall by their own weight, but to depend on the increased pressure of the rise of water for their removal in these sudden floods: the arrangement on these occasions is simple, the catches fixed to the bit-heads are removed from the ratchet wheels