Page:History of the Periyár project. (IA historyofperiyar00mack).pdf/33

I.] disadvantage of enormous cost as long as the dam was of earth with a very wide base and the necessity of allowing no water to flow over it was paramount. The inherent disabilities of an earthen dam more than 150 feet high were considered to have been overcome by the use of the "Silting process," of which much was at that time expected. Meanwhile two new sites, marked 2 and 3 on the plans, were examined and found to possess material advantages over No. 1, but these were ultimately abandoned in favour of the site marked 4, 7 miles lower down the river, which was, in many respects, superior to all the upper sites. The river bed here is 34 feet lower than No. 1 site, but, by its adoption, the water can cross the watershed at the point marked B, which is 47 feet lower than A, showing a saving of 13 feet in the height to be overcome. The ground between the sites 1 and 4 is flat and open as compared with that above No. 1, so that a dam 168 feet high at No. 4 stores more than double the quantity impounded by a dam of 220 feet at No. 2, while the drainage area available is 305 square miles against 250. No. 4 site was therefore definitely adopted, and it was further decided that the relative levels of dam and offtake should be such as to allow of the storage of sufficient water to overcome all fluctuations in the discharge of the river, and to allow a regular equable supply to be passed, under complete control, into the Suruli valley. A project complete in all details was finally submitted by Mr. R. Smith in April 1872. His proposals included—

(1) A dam, 175 feet in height, to be constructed of earth by the silting process, with an escape 400 feet in length blasted out of the saddle on the right flank of the dam. The construction of the dam involved, as a subsidiary work, the excavation of a tunnel 423 feet in length with an area of 1,064 square feet, with cuttings at each end aggregating in contents nearly $4 1⁄2$ millions of cubic feet for the passage of the water temporarily.

(2) A tunnel 7,000 feet in length under the watershed ridge, with cuttings at its two ends. The sill of the cutting on the lake side was to be 113 feet above the river bed, the space between this level and that of the escape crest (+144) allowing 6,815 millions of cubic feet of water to be stored. For controlling the entrance of water into the tunnel an elaborate system of regulating sluices estimated to cost Rs. 71,000 was provided.

(3) A series of regulating sluices for passing the Periyár water round the flanks of the various anicuts on the Suruliyár.