1911 Encyclopædia Britannica/Weir
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 up the river channel to the extent of their height, and consequently raise the flood-level above them. This serious defect of solid weirs, where the riparian lands are liable to be injured by inundations, can be slightly mitigated by keeping down the crest of the weir somewhat below the required level, and then raising the water-level at the low stage of the river by placing a row of planks along the top of the weir.
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 masonry reservoir 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 regulated as required and considerably increased in flood-time by introducing a series of openings in the centre of a solid weir, with sluice-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 or lowered as desired from a foot-bridge.
Fig. 1.—Lifting-gate Weir and Foot-bridge at Richmond, Surrey.
This arrangement has been provided at several weirs on the Thames,
to afford control of the flood discharge,
Fig. 2.—Mechanism of Lifting-gate,
Richmond. and reduce the extent of the
inundations; 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 24214 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.[1]
Ordinary draw-doors, sliding in grooves of
moderate 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 considerable
head, the friction
of the surfaces in contact
offers a serious impediment
in raising them. This friction
has been greatly reduced
by making the draw-doors,
or sluice-gates, slide
on each side against a vertical
row of free-rollers suspended
by an encircling
chain; and the working
is much facilitated by
counterpoising the doors.
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,
the doors are rotated automatically at the top by grooves at
the sides of the piers, so as to assume a horizontal position and
pass out of sight in the central space between the two foot-ways
(fig. 2). The barrage at the head of the Nile delta, and the
regulating sluices across the Nile at Assiut and Esna in Upper
Egypt below Assuan, are examples of draw-door weirs, with their
numerous openings closed by sluice-gates sliding on free rollers,
which control the discharge of water from the river for irrigation.
Fig. 3.—Plan of Works at Richmond.
Movable Weirs.—There are three main types of movable weirs, namely frame weirs, shutter weirs and drum weirs, which, however, present several variations in their arrangements.
The ordinary form of frame weir consists of a 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 river and carrying a foot-bridge along the top, from which the actual barrier, resting against the frames and crossbars at the top and a sill at the bottom, is put into place or removed Frame weir. for closing or opening the weir.
Fig. 4.—Needle Weir, River Moldau.
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 or sheet-iron closing the space between adjacent frames and sliding in grooves at the sides, and rolling-up curtains composed of a series of horizontal wooden laths connected by leathern hinges, have also been employed. The needle weir, so called from the long, 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 1812 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.
Fig. 5.—Spar Weir, Louisa, Big Sandy
River, U.S.A.
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 current is strong, they are drawn to the bank by a rope attached to them previously to their release. The trestles of this weir are, as usual, hinged to the apron, so that in flood-time they can be completely lowered into a recess across the 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 obviate this, the frames have, in a few instances, been suspended from an overhead foot-bridge. The system was first proposed in view of the canalization of the Rhone, which brings Suspended frame weir. down large quantities of shingle and gravel; but it was first adopted for two weirs on the lower Seine under quite different conditions (fig. 7). The frames hang vertically from the bottom of the overhead bridge, and rest against a sill at the bottom when the weir is in operation, the openings between the frames being closed below the water-level by rolling-up curtains or sliding panels, which are lowered or raised by a travelling winch carried by a small foot-bridge formed by hinged brackets at the pack of the frames, and situated a little above the highest flood-level.
Fig. 6.—Frame Weir with Rolling-up Curtain, Port Villez, Lower Seine.
The weir is opened by removing the sliding panels or rolling up the curtains, and then lifting the hinged frames to a horizontal position under the overhead bridge by means of chains worked by a winch on the bridge. This system, which has been employed for the lowest weir on the Moldau, and for a weir at the upper end of the Danube canal near Vienna to shut out floods and floating ice, as well as on the Seine, possesses the merits of raising all the movable parts of the weir out of water in flood-time, and rendering the working of the weir very safe and easy. On the other hand, it involves the expense of a wide foot-bridge for raising the frames, and wide and high river piers. Especially for the navigable passes where the bridge has to be raised high enough to afford the regulation headway for vessels at the highest navigable flood-level (fig. 7), so that its adoption should be restricted to positions where the conditions are quite exceptional.
Fig. 7.—Suspended Frame Weir, Poses, River Seine.
The earliest form of shutter weir, known as a bear-trap, introduced in the United States in 1818, and subsequently erected across the Marne in France, consists of two wooden gates, each turning on a horizontal axis laid across the apron, inclined towards one another and abutting together at an angle in the centre when the weir is closed; the up-stream one serves as Shutter weir. the weir, and the down-stream one forms its support, and both fall flat upon the apron for opening the weir.[2] This weir is raised by admitting water under pressure beneath the gates through culverts in connexion with the upper pool; and is lowered by unfastening the raised gates and letting the water under them escape into the lower pool. This old form of bear-trap has been used for closing an opening 52 ft. wide to provide for the escape of drift at the Davis Island weir across the Ohio. Improvements, however, in the beartrap have been introduced in the United States, one of the best novel forms being shown in fig. 8. whereby the pass of a weir 80 ft. in width can be readily closed, opened or partially opened under a maximum head of 16 ft. by means of chains worked by a winch.[3] The shutter weir, introduced on the upper Seine about the middle of the 19th century and subsequently adopted for weirs across several rivers in France, Belgium and the United States, consists of a row of wooden or iron shutters turning on a horizontal axis a little above their centre of pressure, borne by an iron trestle at the back of each shutter, which is hinged to the apron of the weir, and supported when raised by an iron prop resting against an iron shoe fastened on the apron (fig. 9).
Fig. 8.—Bear-trap Weir, U.S.A.
The weir is opened by releasing the iron props from their shoes, either by a sideways pull of a tripping bar with projecting teeth laid on the apron and worked from the bank, or by pulling the props clear of their shoes by chains fastened to the bottom of the shutters; the unsupported trestles and shutters fall flat on the apron on the top of the props, as shown by dotted lines in fig. 9. The weir is raised again by pulling up the shutters to a horizontal position by their bottom chains from a special boat, or from a foot-bridge on movable frames, together with their trestles and the props which are replaced in their shoes. The discharge at the weir whilst it is raised is effected either by partially tipping some of the shutters by chains from a foot-bridge, or by opening butterfly valves resembling small shutters in the upper panels of the shutters. The addition of a foot-bridge greatly facilitates the raising and lowering of these shutter weirs, and also aids the regulation of the discharge; but it renders this form of weir much more costly than the ordinary frame weir, and where large quantities of drift come down with sudden floods, the frames of the bridge are liable to be carried away, and therefore boats must be relied on for working the weir.
Fig. 9.—Shutter Weir with Foot-bridge, Port a l'Anglais, Upper Seine.
The drum weirs erected across shallow, regulating passes on the river Marne in 1857–1867 comprise a series of upper and under wrought-iron paddles, which can make a quarter of a revolution round a central axis laid along the sill of the weir. The straight, upper paddles form the weir, and can be raised against the stream by making the water from the upper Drum weir. pool press upon the upper faces of the slightly larger lower paddles, crooked for the purpose, causing them to revolve in a quadrant of a cylinder under the sill, known as the drum; and they can be readily lowered by cutting off the flow from the upper pool and putting the drum in communication with the lower pool, which connexions can be adjusted by see-saw sluice-gates, so as to put the upper paddles in any intermediate position between vertical and horizontal (fig. 10). The merits of this weir in being easily raised against a strong current and in allowing of the perfect regulation of the discharge, are unfortunately, under ordinary conditions, more than counterbalanced by the necessity of carrying the drum and its foundations to a greater depth below the sill of the weir than the height of the weir above it. Accordingly, for several years its use was restricted to the Marne; but in 1883–1886 drum weirs were adopted for closing the timber passes alongside the needle weirs placed across the Main, with a single upper paddle 3913 ft. long and 5 ft. 7 in. high in each case; and a still larger drum weir was erected about the same time for closing the navigable pass of a weir across the Spree at Charlottenburg, with an upper paddle 3234 ft. long and 916 ft. high (fig. 10).
Fig. 10.—Drum weir, Charlottenburg, River Spree.
A peculiar and cheaper form of drum weir has been constructed across ten bays each 75 ft. wide on the Osage river near its confluence with the Missouri, where a hollow, wooden, cylindrical sector, stiffened inside by iron framing and revolving on an axis laid along the crest of the solid part of the weir, fits into a drum at the back lined with planking having a radius of 9 ft The weir is raised by admitting water from the upper pool into a wedge-shaped space left below the sector when it is lowered in the drum which by its pressure lifts the sector out of the drum, forming a barrier 7 ft high, closing each bay of the weir. Provision has also been made for rendering the sector buoyant by forcing air into it, so that it can be raised when the head of water is insufficient to lift it by the pressure of the water from the upper pool. In spite of its high cost, the drum weir furnishes a valuable hydraulic contrivance for situations where it is very important to be able to close a weir of moderate height against a strong current and to regulate with ease and precision the discharge past a weir. (L. F. V. H.)