Page:EB1911 - Volume 28.djvu/516

 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

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.

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.

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

the weir, and the down-stream one forms its support, and both fall flat upon the apron for opening the weir. 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. 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).

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.

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

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