RESERVOIR, a receptacle for storing water for any purpose, but chiefly for the supply of towns, for driving machinery, feeding canals. irrigation, or for some process of man ufactures. Generally, every water-works' establishment., for the supply of a town. requires to construct one or more reservoirs for providing compensation to the mills sit uated on the stream, for the water that is abstracted from any of its feeders.
The most advantageous position for a store rescryoh• is that where there is a narrow gorge in a valley widening out upward into a flat expanse, thereby enabling a compara tively small dam or embankment formed in the gorge, to impound a large body of water; but in many cases where there is no such choice, the embankment may require to be placed across a wide part of a valley which narrows as it ascends, thereby requir lag a great embankmeat, in proportion to the quantity of water impounded. • Some times reservoirs have to be formed on flattish ground affording no great natural facilities for storing water; and in such cases they may require to be embanked wholly or nearly round and round. Where a reservoir requires to be constructed on perfectly level ground, the excavation must be calculated to be exactly equal to the embanking. The worst possible situation for a store reservoir is on the slope of a hill.
In many cases, natural lakes are used as reservoirs, means being adopted for raising or lowering the surface of the water, the difference between the lowest and the highest level of the lake's surface, multiplied by its area, being the measure of the available storage. Instances of this occur in Loch Leven, Kinross-shire, for the supply of the mills on the river Leven: in Loch Katrine and Loch Vennachar, for the supply of the city of Glasgow, and for the compensation required by the millers on the river Teith, in consequence of the abstraction of the Loch Katriue water, and in many other similar cases both for the use of towns and for water-power.
The capacity of a reservoir necessary for making nearly the whole water of a dis trict available for use depends much on the climate. Where droughts are of long continuance, its capacity requires to be proportionately large, but generally in Great Britain a capacity of six or seven months' supply is reckoned sufficient.
As illustrative of the very different facilities afforded by different sites for storing water, an instance occurs of two reservoirs of the Edinburgh water trust, whereof one with an embankment containing 175,000 cubic yards of earthwork impounds only 17 millions of cubic ft. of water; while another, with an embankment of 53,000 cubic yards, impounds 85 millions of cubic ft., there being a single embankment across a valley in both cases. Generally, the structure for impounding water is an earthwork embankment, with a slope toward the water of 3 or 4 horizontal to 1 perpendicular, a breadth across•the top of from 6 to 12 ft. the height being from 4 to 7 ft. above the water, and an outside slope of from 2 to 24 horizontal to 1 perpendicular. The earth work ought to be formed in thin layers well rammed, and to have a puddle-wall of good well-worked clay in the center, the foundation of the puddle being a trench dug down to impervious rock or clay. The face toward the water requires to be protected by stones: and when a reservoir ii large those stones must be "pitched"—i.e., regularly set by hand—so as to be able to resist the lash of the wave. In all cases there is imperatively required a waste-weir, to allow flood-waters to escape without -isk of overflowing the dam. It ought, if possible, to be placed on the solid ground; and if it can be cut through solid rock, that is best, and saves a great expense for masonry. The width of the waste-weir must he regulated by the catchment or extent of gathering ground of the reservoir, and by the rain-fall of the district; but for a given catchment and rain-fall, a reservoir having a small area ought to have a larger waste-weir than one having a larger area, as the latter would allow flood-water to accumulate without rising to so high a level as it would in the former. Generally, however, from 12 to 20 ft. of length of waste-weir may suffice for a square mile of catchment. In some cases, dams across gorges, for the purpose of forming reservoirs, are constructed of walls of heavy masonry, instead of earthwork embankments. Those across rivers for dividing the water into mill-leads, and for retaining the water which would otherwise be wasted at meal-hours, are generally constructed of stone, hut sometimes of timber or iron.