Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/598

Rh 574 RIVER ENGINEERING aw- >r irs. Iding- iweir. 56 lers draw- >rs. >vable irs. line ir. and its fall i3 correspondingly reduced. Accordingly, the dis- charging capacity of a river is materially diminished by the erection of overfall weirs ; and this is only partially remedied by the deepening of the channel for navigation, especially as deposit more readily accumulates in the lower part of a reach, owing to the reduction in velocity of the current by the conversion of the river into level reaches. Endeavours have been made to alleviate this defect by placing the weir in a wide place on the river, and at an angle to the cross section of the channel, thereby increasing the length of its sill, and consequently the discharge over it for any definite height of the river (fig. 2). The gain in length of an oblique weir is somewhat neutralized by the weir not being at right angles to the direction of the current ; and, even if the cross section of the channel above the sill of the weir is as large as the average section of the river bed, the change in shape, and the small hydraulic radius of the section over the weir, check the discharge of the stream. Draw-door Weirs. In order to afford a freer flow than is attain- able with the best-designed overfall weir, draw-door weirs are sometimes adopted, which serve equally well to retain the water above during dry weather, and provide a large opening for the discharge of the stream in floods. Draw-door weirs consist of a row of doors, or sluice gates, sliding vertically in grooves formed at the sides of frames, piles, or piers, which are shut down when the flow is small, but are raised to admit the passage of flood waters. These weirs generally serve to supplement an ordinary overfall weir ; and, whilst the overfall weir regulates the flow in dry weather, the draw-door weir provides for its more rapid discharge in flood time. The relief, however, afforded by draw-door weirs depends entirely on the opening they furnish ; they are rarely, if ever, made equal in section to the river channel, and their sills are usually raised some feet above the bed of the river ; but, nevertheless, they are much superior to overfall weirs in respect of drainage, especially when the river has a small fall and low banks. A large oblique overfall and draw-door weir has been erected across the Thames, by Mr Leach, at the limit of its tidal flow at Teddington, having a total length of 480 feet. This weir is divided into four bays, the two side bays being overfalls, whilst the two central bays, 172J feet and 69| feet wide respectively, are closed by large iron draw-doors sliding in grooves at the sides of strong iron frames supporting a foot bridge from which the doors are raised. The frames rest upon piles, and on the top of a rubble mound raised about a foot above low-tide level. 1 The friction of large draw-doors against the grooves, in being lifted, is considerable when there is a head of water on one side ; but this has been much reduced by Mr F. Stoney, in a large weir in Brazil, by making the doors, 20 feet in width, bear on each side against a row of free rollers suspended in the grooves. 2 Movable Weirs. Although draw-door weirs afford a much freer discharge for a river than overfall weirs, the vertical frames or piers in which the doors slide offer more or less impediment to the flow. This defect is avoided by movable weirs, which, whilst equally efficient in retaining the water when raised, can be entirely lowered or removed so as to leave the channel quite open in flood- time. There are three types of movable weirs which have been regularly adopted abroad, whilst other forms have been occasionally tried. The two types most extensively used are the frame or needle weir and the shutter weir, whereas the drum weir has been only erected on the Marne. The frame weir consists of a series of movable iron frames, placed at intervals across the channel of a river end on to the current, carrying a foot bridge at the top, and supporting a wooden water- tight barrier which forms the actual weir. Till recently the barrier was always composed of a series of long square wooden spars, or needles, placed close together and nearly vertical, resting against a sill at the bottom and against a horizontal bar con- necting the frames near the top (fig. 3). This type of weir has, accordingly, been very commonly called a needle weir (barrage a aiguilles) ; but this term would not now include every form of the frame weir. The first needle weir was erected across the Yonne in 1834 ; and till 1881 all the weirs on the Seine below Paris were of this form. The needle weir is opened by lifting each needle successively from the foot bridge ; one of the end frames is then disconnected from the rest by unfastening and withdrawing the connecting bars, the corresponding portion of the foot bridge is taken up, and the frame, which is hinged at the bottom, is lowered by chains on to the apron of the weir, and the whole of the frames are similarly lowered in succession, leaving the passage quite clear. The weir is closed again by a precisely reverse series of operations. As the weight of the needle, which should be readily lifted by one man, imposes a limit to the height of the weir, the large frame weir recently erected at Port-Villez near Vernon, having a height 1 L. F. Vernon- Harcourt, op. cit., plate iv. figs. 9 and 10. - Minutes of Proceedings Inst. C. E. t vol. Ix. p. 88. of 18 feet, has been closed by a sort of wooden hinged shutter which spans the interval of 3 feet between each frame, and can be rolled up from the bottom and removed when the weir is to be opened. A series of similar hinged shutters are designed to close double intervals between the frames, about 7 feet wide, of the frame weir in progress at Poses, the next weir above Martot weir which is at the boundary of the tidal Seine above Rouen. Poses weir has a form quite distinct from all frame weirs hitherto constructed, for its vertical frames are suspended from an overhead girder, and rest against a sill at the bottom when down, but can be raised entirely out of water into a horizontal position when the weir is open. The girders carry a foot bridge, from which the frames and hinged shutters are raised and lowered, and rest on masonry piers dividing the river in to seven bays, the two navigable passes being 106J feet and the five shallower passes 99 feet wide. The girders spanning the two navigable passes leave a clear headway of 17 feet above the navigable high water, whilst the rest of the girders are merely placed above flood level. The fall at the weir is 13 feet. The system is costly, with its girders and piers, but it secures all the movable parts of the weir from injury in flood, and enables the weir to be worked with perfect ease and safety. Sliding panels have been adopted for closing the frame weir across the Rhone at Mulatiere, near Lyons, erected in 1882; and a comparison is being made of the relative durability of sliding panels and hinged shutters by placing the two systems side by side at Suresnes weir just below Paris. The earliest form of shutter weir consisted of a gate, or shutter, Shutter turning on a horizontal axis at the bottom, supported by a prop wen-, when raised against the stream, and falling flat on the apron of the weir when the prop was withdrawn. As considerable force would be required to raise such a shutter against a strong stream, a second up-stream gate is usually provided, which, rising with the stream and being retained by chains, relieves the pressure on the down -stream gate, and enables it to be readily raised and propped up. The waterlevel is then equalized on both sides of the upper gate, which is then lowered ; and the lower gate forms the actual weir, which can be opened by merely releasing the prop. In India, where this form of shutter weir has been adopted on a large scale, the strain on the retaining chains was so great, when the upper shutter was raised in a strong current to shut off the river from the irrigation canals, that hydraulic brakes have been substituted, by Mr Fouracres, for controlling the motion of the up-stream shutters. A closed cylinder full of water is fixed on the apron of the weir above the upper shutter, in which a piston, attached to the upper side of the shutter, is fitted. Directly the current tends to lift the shutter, the piston is drawn against the cushion of water in the cylinder, which controls its motion. The pressure of the piston forces the water gradually out of some small orifices along the side of the cylinder, so that the piston is enabled to travel slowly along the cylinder. In its progress, however, it passes by some of the orifices, whereby the rate of efflux of the remaining water is reduced, and a greater resistance offered to the motion of the piston. Whilst therefore the shutter in rising presents a greater surface to the stream, and consequently exerts a greater pull upon the piston, the retarding force is similarly increased, and the shutter is thus gradually raised without any jar. At Brulee Island weir, on the Yonne, the up-stream shutter has been dispensed with; and the shutter forming the weir is raised against the stream by a piston working in a hydraulic press (fig. 4), the water pressure being supplied from an accumulator which is charged by means of a turbine worked by the fall of water at the weir. The weir is closed by seven shutters, 11J feet long and 6 feet high, which can be raised in five minutes ; and the power for damming up the stream is actually obtained from the stream itself. The form of shutter weir most commonly adopted in France is shown in fig. 5, representing a section of the largest and most recent weir of this type, erected at Port-a-1'Anglais weir on the Seine, two or three miles above Paris ; and weirs on the same system have been erected across the Great Kanawha river in the United States. The shutter revolves upon a horizontal axis placed just above the centre of pressure on the down-stream side of the shutter. The axis is fastened on an iron tressel hinged to the apron of the weir ; and the shutter and tressel are supported in position by a wrought-iron prop resting against a cast-iron shoe fixed on the apron. When the weir is closed, the shutter butts against a sill at the bottom, as is shown on fig. 5. The weir can be more or less opened, from the foot bridge, by means of chains fastened to the top and bottom of the shutter; and it can be completely lowered in flood time by releasing the prop from its shoe, when the prop, tressel, and shutter fall flat upon the apron, their fall being regulated by aid of the chains ; the frames also supporting the foot bridge, being hinged to the apron, can be lowered as in the ordinary frame weir. The weir is raised by first reinstating the foot bridge, and then raising the shutters, with the connected tressels and props, by means of the chains. Each