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Encyclopedia Britannica - Main :: WAT-WIL |
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WEIR (from O. Eng. wer, a dam; cognate with werian, to defend, guard; cf. Ger. Wehr, defence) , a barrier placed across rivers to raise the water-level for catching fish, for mills, for navigation or for irrigation, the discharge of the river taking place over the crest or through openings made for the purpose. Rough weirs, formed of stakes and twigs, were erected across English rivers in Saxon times for holding up the water and catching fish, and fish-traps, with iron-wire meshes and eel baskets, are still used sometimes at weirs. Weirs are essential for raising the head of water for water-wheels at mills, and for diverting some of the flow of a river into irrigation canals; but they have received their greatest and most varied extension in the canalization of rivers for navigation. There are three distinct classes of weirs, namely, solid weirs, draw-door weirs, including regulating sluices for irrigation, and movable weirs, which retain the water above them for navigation during the low stage of the river, and can be lowered or removed so as to leave the channel quite open in flood-time. Solid Weirs.The simplest form of weir is a solid, watertight dam of firm earthwork or rubble stone, faced with stone pitching, with cribs filled with rubble, with fascine mattresses weighted with stone, or with masonry, and protected from undermining by sheet piling or one or more rows of well foundations. These weirs, if solidly constructed, possess the advantages of simplicity, strength and durability, and require no superintendence. They, however, block
Waste weirs resemble ordinary solid weirs in providing for the surplus discharge from a reservoir of an impounded river or mountain stream over their crest; but in reality they form part of a masonryreservoir dam for storing up water for water- supply or irrigation, kept purposely lower than the rest of the dam to allow the excess of water to escape down the valley (see WATER-SUPPLY ).Draw-door Weirs.The discharge of a river at a weir can be .egulated as required and considerably increased in flood-time by introducing a series of openings in the centre of a solid weir, with sl::ice-gates or panels which slide in grooves at the sides of upright frames or masonry piers erected at convenient intervals apart,and which can be raised o. lowered as desired from a foot-bridge. This arrangement has been provided at several weirs on the Thames, to afford control of the flood discharge, and reduce the extent of theinundations; the largest of these composite weirs on that river is at the tidal limit at Teddington, where the two central bays, with a total length of 242ii-, ft., are closed by thirty-five draw-doors sliding between iron frames supporting a foot-bridge, from which the doors are raised by a winch.' Ordinary draw-doors, sliding in grooves ofmoderate size and raised against a small head of water, can be readily worked in spite of the friction of the sides of the doors against their supports; but with large draw-doors and a con- siderable head, the friction of the surfaces in contact offers a serious impediment in raising them. This fric- tion has been greatly re- duced by making the draw- doors, or sluice-gates, slide on each side against a verti- cal row of free-rollers sus- pended by an encircling chain; and the working a is much facilitated by FIG. 2.Mechanism of Lifting-gate, counterpoising the doors. Richmond. By these arrangements the large draw-door weir across the Thames at Richmond, with three spans of 66 ft. closed by lifting doors, each 12 ft. high and weighing 32 tons, can be fully opened in seven minutes by two men raising each door from the arched double foot-bridge (figs. 1, 2 and 3). This weir retains the river above it at half-tide level, in order to cover the mud-banks which had been bared at low tide between Richmond and Teddington by the lowering of the low-water level, owing to the removal of various obstructions in the river below. The weir is raised out of the river as soon as the flood-tide on its lower side has risen to half-tide level, so as not to impede the flow and ebb of the tide up to Teddington above that level, and is not lowered till the tide has fallen again to the same level. In order that the doors when raised may not impede the view under the arches, ' L. F. Vernon-Harcourt, Rivers and Canals, 2nd edition, p. 114, and plate iii. figs. 15 and 16.the doors are rotated automatically at the top by grooves at the sides of the piers, so as to assume a horizontal
control the discharge of water from the river for irrigation.Movable Weirs.There are three main types of movable weirs, namely frame
The ordinary form of frame
series of iron frames placed across a river end on to the current, between 3 and 4 ft.apart, hinged to a masonry apron on the bed of the Frame river and carrying a foot-bridge along the top, from which weir. the actual barrier, resting against the frames and cross- bars at the top and a sill at the bottom, is put into place or removed for closing or opening the weir. The barrier was originally formed of a number of long square wooden spars which could be readily handled by one man, being inclined slightly from the vertical and placed close together for shutting the weir; but panels of wood
horizontal
employed. The needle weir, so called from the Iong, slender spars being termed aiguilles in France, had the merit of simplicity in its earliest form; and by means of some ingenious contrivances, comprising a hook, winch, lever and rotating bar, for assisting the weir-keepers in placing and releasing the needles, the system has been applied successfully to the weirs of greater height required on the Meuse, the Main and the Moldau (fig. 4). The needle weir has, however, attained its greatest development in the United States across the Big Sandy river at Louisa, where, instead of needles 3 to 4 in. square, beams 12 in. square and 182 ft. long have been resorted to, provided with a steel eye at the top and a ring near the centre of gravity to enable them to be worked (fig. 5). The needles are put in place one by one against the raised frames, or trestles, by a derrick on a barge lifting them by their ring, whilst a man on the foot-bridge, taking hold of the eye at the top, arranges them in position close together. The weir is opened by joining the needles of each bay by a chain passed through the eyes at the top and a line of wire through the central rings, so that when released at the top by the tilting of the escape bar by the derrick, they float down as a raft, and are caught by a man in a boat, or, when the cur- rent is strong, they arer,a0oosgo OHIO
drawn
their release. The trestles of this weir are, as usual, hinged to the apron, so that in flood-time they can be completely lowered FIG. 5.Spar Weir, Louisa, Big Sandy into a recess across the River, U.S.A. apron by means of chains actuated by a winch, leaving the channel perfectly open for the discharge of floods and for the passage of vessels when the lock is submerged. Whereas, however, ordinary frames placed nearer together than their height overlap one another when lowered on to the apron, the trestles of the Louisa weir lie clear of each other quite flat on the apron. The frame weir closed by sliding panels or rolling-up curtains (fig. 6) possesses the advantage that the panels or laths can be diminished in thickness towards the top in proportion to the reduced water-pressure; whereas the needles, being of uniform cross-section, have to be made stout enough to sustain the maximum bottom pressure. An objection has occasionally been urged against frames lowered on to the bed of a river that they are liable to be covered over by detritus or drift brought down by floods, and consequently are subject to injury or impediments in being raised. In order to /Ate ! lllCAWaft ._8 : gNmaI~II^ ^I~ ~ End of Article: WEIR (from O. Eng. wer, a dam; cognate with werian, to defend, guard; cf. Ger. Wehr, defence) If you wish, you can link directly to this article.
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