2. MOVABLE WEIRS Movable weirs are barriers capable of being lowered so as to present no obstruction to the flow of water in flood time. They are constructed either upon a foundation or sill having its surface approximately level with that of the river bed or on the crest of a solid weir structure raised above the river bed.
A simple form of needle weir was employed in France about the end of the i8th century. Small wooden spars called aiguilles (needles), which bore on the bottom against a masonry sill and at the top against wooden beams supported on masonry piers, formed the barrier. This type, however, only pro vided a series of small openings between the piers. Poiree, a French engineer, devised in 1834 the movable frame weir which bears his name and can be lowered so as to leave the whole width of the river free from obstruction. In its ordinary form the Poiree needle weir consists of a series of iron frames placed across a river, end on to the current, 3 to 4 ft. apart, hinged to a masonry apron on the bed of the river and carrying a foot-way across the top, from which 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. A winch is used to handle the frames.
A needle weir built in 1891-97 across the Big Sandy river at Louisa, Ky., was the first constructed in the United States and is higher than any in Europe.
A modification of the Poiree needle weir is the Boule gate introduced in 1874 in which panels of boards or sheet-iron set in tiers one above another are used instead of needles to form the barrier between the frames. The panels are set and removed by a small derrick crane travelling on top of the footbridge. The system has the advantage of forming a tighter dam which can be more easily although less rapidly manoeuvred than needles. Boule gates have been used to a considerable extent in France and on the Moskowa and other rivers in Russia.
The curtain weir, invented by Camere, was first introduced in 1876-80 at Port Villez on the lower Seine. In it wooden curtains that can be rolled up from the bottom were substituted for the needles in the Poiree weir. The curtains are raised and removed, and the frames lowered by winches travelling over the service bridge. The manoeuvring of the frames and curtains is a troublesome operation and all the curtain weirs subsequently constructed have been designed for suspension from a fixed over-head bridge (vide infra.).
The earliest practical application of falling shutters to overfall weirs was made on the river Orb in France in the latter part of the i8th century. A gate or shutter turning on a horizontal axis at the bottom was supported by a prop when raised against the stream and fell flat on the apron when the prop was drawn aside. The difficulty experienced in raising such a shutter against a head of water was overcome by Thenard about 1837 who placed a second row of shutters (counter-shutters) on the up-stream side of the weir which, rising with the stream, were retained in an upright position by chains and stopped the flow of water while the lower shutters were being raised and propped. Shutters of modified and improved forms, some of them automatic in action, are in use at many irrigation weirs in India and other countries. A modern form of shutter falling automati cally is illustrated in Plate, fig. 4.
The inconveniences attending the use of counter-shutters of the Thenarcl type were overcome by Chanoine, another French engineer, who, in 1857, at the Conflans weir on the Seine, employed shutters turning on a horizontal axis a little above their centre of pressure. The axis rests on an iron trestle at the back of each shutter, which is hinged to the apron of the weir and is supported when raised by an iron prop resting against a shoe fastened on the apron. The weir is opened by releasing the 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. 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 usually effected by partially tipping some of the shutters by chains from a foot-bridge. Occasionally the balance is so arranged that the shutters tip automatically when the water level in the upper pool reaches a certain height.