Thursday, January 19, 2012

The Periyar dam

The book, “Irrigation in India” by Herbert Wilson and published in 1903 carries an excellent description (page 189) of the Periyar dam and the planning that went into it. It also has some sketches and a couple of photographs of the construction in progress.

The Periyar project for the irrigation of the Vaigai Valley, in Madras Presidency, is probably the most interesting illustration of the combined storage work and irrigation canal system to be found in India, especially as it was sanctioned as a protective work. The project includes the construction of a dam to close the valley of the Periyar River to store 300,000 acre-feet of water, of which 150,000 acre-feet are above the sill of the outlet tunnel and are thus available for irrigation; the construction of a tunnel through the watershed dividing the valley of the Periyar from that of the Vaigai River for the purpose of drawing off the water from the reservoir, with the necessary sluices and subsidiary works for controlling the passage of the supply of the Periyar down the valley of the tributary called the Sooroolly, by which it reaches the Vaigai; and finally, the construction of the works necessary for the regulation and distribution of this supply for the command of 107,050 acres of land in the Vaigai Valley, of which 76,445 acres were irrigated in 1898-99.

As the waters of the Periyar flow westward into the Arabian Sea, they are thus diverted across the peninsular divide, the Ghauts, to the eastern coast of India, where they enter the Bay of Bengal. The project has come of further examinations made by Mr. Smith and Major Pennycuick, though to the latter are due most of the later details, and under him is being conducted the construction of the works. It was in this report that Major Pennycuick submitted the first proposals for the substitution of a masonry dam for one of earth. These final proposals were submitted in 1882, and included the construction of a dam located at the same point as that chosen by Mr. Smith, 7 miles below Major Kyves's site, the height to be 155 feet above the bed of the river, and the summit surmounted by a parapet 5 feet high and 4 thick. The dam proper is 12 feet thick at the top and 114 feet at the lowest part. It is constructed throughout of concrete composed of 25 parts of hydraulic lime, 30 of sand, and 100 of broken stone. The front face is covered with a plaster composed of equal parts of lime

The area of country which is irrigated by this project was described by Major Ryves, in his early report on the project in 1867, as being about 1,200 square miles in extent, with a population of nearly half a million. Up to the present time irrigation has been practiced from native tanks, most of which, however, have become very shallow, and from which the waste of water by evaporation is at least 30 per cent. In very good years the water supply from the Vaigai itself is sufficient to irrigate 20,000 acres. Agricultural operations in this region are rarely rewarded by a good crop, although the land where water can be provided is of the most fertile character. During the famine of 1876 as much as $600,000 was expended in relief in this district.

The idea of utilizing the water of the Periyar for the irrigation of the Vaigai is an old one. It was first reported in 1808 by Sir James Caldwell, who condemned the project as decidedly chimerical and unworthy of further regard. The subject was occasionally discussed from time to time, but it was not until 1867 that it was practically brought forward by Major Ryves. Major Ryves's proposals included an earth dam 162 feet high, with an escape crest 142 feet above the river bed, and the water was to be diverted into the Vaigai Valley by a cutting having a maximum depth through the watershed of 52 feet. Other examinations were made, and finally a project was submitted by Mr. Smith in 1872, which included a dam 171 feet in height, to be constructed by the silting process and having an escape of 400 feet in length blasted out of the saddle at the right bank.


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