RESERVOIR, a large and commodious receptacle for storing water for any purpose, but more commonly for supplying cities and towns, feeding canals driving machinery and the like. A dam differs from a reservoir in that the name applies only to the artificial structure that holds back the water, while the reservoir includes also the entire artificial lake and its appurtenances. The construction of a reservoir is of much importance and requires great engineering skill. In the selection of a site the great object should be to choose a posi tion which will give the means for collecting a large supply of rainfall with as little recourse as possible to artificial construction. An ad vantageous site would be, for instance, in the narrow gorge of a valley through which a stream passes, and at a place where the gorge widens out on the side looking toward the source of the stream. Particular care, however, is necessary to discover whether the position chosen communicates with the courses of under d waters or springs, as disastrous results frequently been occasioned by ignoring this As a general rule the best and safest method is to establish reservoirs in valleys filled in with clays derived from the decomposition of the primary formations, which are very slightly Permeable. The embankments or dams may be constructed either of masonry or earth-work, the latter being generally the more economical method. Where the reservoir requires to be constructed on perfectly level ground the exca vation should be calculated ao as to equal the embankment. The earth-wqk of dams should be as much as possible of material — such as a mixture of clay and gravel—which would resist the infiltration of, the water; but as it is im possible always to obtain such material in suffi cient quantities, engineers have recourse to what is termed a puddle-wall, formed in the middle of the embankment. This wall is constructed of well-worked clay, the foundation of the puddle being a trench dug down to impervious rock or clay, and its breadth should be on an average about. one-third the height of the em bankment. The inside slope of the embankment is usually 1 perpendicular to 3 horizontal, some times it is steeper; the outside slope may be 1 perpendicular to 2 horizontal; and the height above the surface of the water is often 4 to 7 feet. The earth-work ought to be con structed of thin layers, carefully rammed so as to secure their equal settlement; the inner face requires to be protected by stones in order to resist the action of the water. The waste-weir, to admit of the surplus water flowing over, should be from 4 to 6 feet below the surface line of the embankment, built of strong mason-work and of a width such that with a flow of 2 feet over the crown it should be able to discharge all the water brought down by the severest floods ever known to fall upon the basin supplying the reservoir. Many of the
accidents that have occurred were due to defi ciencies in the waste-weirs. In the reservoirs of which the dams are built of masonry there is no necessity for a waste-weir, as then the water may be allowed to overflow the wall; there being no fear of its endangering the work. The outlet at the bottom, by which the water to be used is drawn off from the reservoir, may consists either of a tunnel, culvert or iron pipes provided with suitable sluices, and these should be so constructed that access may be had to them even when the reservoir is full.
The true principles for the proportioning of masonry dams were not thoroughly investigated until about the middle of the 19th century and subsequently. This was first done by French engineers, and many such dams have since been constructed in France, Algeria, the United States, etc. The highest of all such dams is that of the New Croton reservoir, New York, its height being 248 feet; thickness at bottom, 185 feet; at top, 16. The first great masonry dam built in France on the newer principle was that of Furens (1860-65) ; height, 184 feet; thickness at bottom, 162; at top 11. San Mateo reservoir dam near San Francisco is 170 feet high; thickness at bottom, 176; at top, 20. The Periyar dam, India 0890-95), has a height of 173 feet; bottom thickness, 139; top, 12. In the famous reservoir of Alicante, Spain, execu ted 1579-94, the wall is 67 feet thick at the top, 112 feet at the bottom, and the height 141 feet. In England the preference is generally given to earthen darns. Sometimes natural lakes are used as reservoirs, instances of which are Loch Katrine, for the water supply of Glasgow, and Lake Thirlmere, in Cumberland, for the supply of water to Manchester. Distributing reservoirs for towns are generally built of masonry, but sometimes largely of steel. Rein forced crete is now generally favored. They ought to be placed high enough to command the highest part of the town, and ought to be capacious enough to contain half a day's supply, their chief use being to store the surplus water during the night. Among notable modern storage res ervoirs for towns are those of the Vyrnwy, for supplying Liverpool, containing 11,900 million gallons; Vehar (10,800 million gallons), for Bombay; San Mateo (31,000 million gallons), for San Francisco; and the New Croton (32,000 million gallons), for New York. See DAMS;