Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/599

RIVER ENGINEERING 575 shutter of the navigable pass at Port-a-1'Anglais is 3J feet wide, and rises 12J- feet above the sill of the weir. Smaller shutter weirs, of a similar form, are placed on the top of overfalls to regulate the discharge, being so adjusted that the shutters dip when the water attains a certain height above them. Sometimes the regulation of the flow is effected through the large shutters by means of small shutters fixed in their upper panels, called butterfly valves, which open spontaneously when the water rises above the required height. The smaller shutters on the overfalls are entirely lowered in flood-time, like the large shutters, but, being un- provided with a foot-bridge, they are raised by aid of boats on the approach of the dry season. The earlier shutters erected across the navigable passes were similarly raised ; but, though a foot bridge adds considerably to the cost of a shutter weir, it greatly facilitates its working. The navigable passes in French rivers, which are always closed either by frame or shutter weirs, serve for the passage of vessels during floods when the locks are submerged. With the exception of the primitive movable stanches, there is only one example of a movable weir in England. This self-acting shutter weir has been erected across the Irwell at Thostlencst near Manchester ; and a section of it is given in fig. 6. The weir consists of a series of shutters turning on a central horizontal axis. "When the weir is closed, each shutter is inclined at an angle of 35 to the vertical, as shown in fig. 6, and revolves to a horizontal position for opening the weir. It resembles in fact the French shutter weirs, except that the shutters and their supports are not removed from the channel, so that the waterway at Thostlenest is not so unimpeded as in the French system. The shutters are so adjusted that they open when the river rises 2f feet above its ordi- nary level ; but, lest the rush of water, which would result from their sudden opening, should injure the river bed, an arrangement has been made for opening any of the shutters by means of a set of chains worked by crabs from each bank, so as to release the pent-lip waters more gradually. 1 This weir, designed by Mr Wiswall, consists of fourteen shutters, each 10 feet wide above the axis and 9 feet below, and 12 feet long. The actual height of the weir above the floor is only 10 feet, owing to the inclination of the shutters, so that it presents a surface of 1400 square feet to the stream when closed, which is reduced to 293 feet when open. )rum The drum weir, which has been adopted in several instances on reir. the river Marne, consists of an upper and an under iron paddle capable of making a quarter of a revolution round a horizontal central axis. The upper paddle forms the weir, and the under one revolves in a closed recess, shaped like the quadrant of a cylinder, laid below the sill of the weir, from which the term drum is derived (fig. 7). The under paddle and the drum are so formed that a space is left between the upper face of the paddle and the top of the drum when the paddles are horizontal ; and a similar space exists between the down-stream face of the paddle and the vertical wall of the drum when the paddles are vertical, as represented in fig. 7. These spaces serve as sluiceways by which water can be admitted into the series of drums on the upper or under side of the under paddles. The weir is closed by placing the upper sluiceway in communication with the upper pool, when the pressure of water on the upper faces of the under paddles overcomes the pressure of the stream upon the upper paddles, causing them to rise, and closing the weir against the stream. The weir is readily opened by shutting off communication between the upper pool and the upper sluiceway, and opening communication with the lower pool ; the stream then depresses the upper paddles, or the action can be quickened by opening communication between the upper pool and lower sluiceway. The pressure on either side of the under paddles can be easily adjusted with the utmost precision, enabling the paddles to be placed at any angle, so that the most absolute control is obtained over the discharge at the weir. The largest example of this type of weir is at Joinville, on the Marne, only a few miles above Paris. The weir is formed by forty-two paddles, 3| feet high and 4| feet wide, which are worked with great ease by means of sluice gates on the left bank, being opened or closed in three or four minutes by one man. This type of weir has the defect of not being suited for navigable passes, owing to the depth of the foundations for the drum below the floor of the weir having to exceed the height of the actual weir. Warn- Movable weirs possess the great merit over other forms of weirs ings of of offering little or no impediment to the passage of floods ; and this floods, advantage is still further enhanced by the system of warnings, organized for the Seine, the Loire, and other French rivers, whereby timely information of the approach of a flood is telegraphed to the various weir keepers, so that they may fully open the weirs before its arrival, and thus aid in facilitating its descent. By telegraphic intimation of the rise in the upper tributaries, and of the rainfall in the basin, it is possible to predict with remarkable accuracy the probable rise of the river at places lower down, and to afford valu- able warning of a coming flood to the riparian proprietors. 1 The Engineer, Sept. 1, 1882. Tidal and Tideless Rivers. Elvers may be broadly divided into two classes in respect of the lower portion of their course, for the tide is propagated up some rivers to a considerable distance from their mouth, commingling with the fresh water and pro- ducing an ebb and flow far into the interior ; whilst rivers flowing into tideless seas descend with an unimpeded current to their outlet. Tidal and tideless rivers, possess- ing very distinct' physical characteristics, necessarily pre- sent different features and require different methods of improvement, and will be therefore separately considered. Tidal rivers are the more numerous, owing to the greater extent of tidal seas. The great differences also in the tidal rise introduce numerous variations in the tidal influ- ence on rivers, and the rise of the river bed determines the distance to which the tidal flow extends. Accord- ingly, tidal rivers exhibit a greater diversity in their natural condition than tideless rivers, which are only affected by the volume of their fresh-water discharge, the amount of sediment carried in suspension, and the inclina- tion of their bed. These latter conditions affect also the state of tidal rivers, but their influence is greatly modified by the ebb and flow of a large volume of tidal waters. The effect of the tidal ebb and flow is most readily per- ceived in contrasting the mouths of tidal and tideless rivers. The mouths of the Mississippi, the Nile, the Danube, and the Ehone present very marked differences to the outlets of the St Lawrence, the Seine, the Thames, and the Severn. Tideless rivers divide into a number of mouths, whereas tidal rivers are confined to a single out- let ; and the effect of tidal influence on this difference is still further confirmed by the instance of the Maas, which, with a very slight tidal range, exhibits a tendency to deteriorate into the dispersion of mouths of a tideless river. The value of tidal flow in maintaining a river is fully manifested by comparing the navigable condition of the Thames or of the muddy Humber with the delta of the Nile or the Rhone, though the latter rivers possess a much larger fresh-water discharge. The tidal rise also frequently allows of the access of vessels to a river whose entrance is barred at low water. The general improvements of the upper portions of Reg both tidal and tideless rivers may be carried out on ** similar principles, though on approaching their mouths they need a totally different treatment. To give a river a uniform depth, its channel and flow require regulation. Hard shoals may be permanently removed by dredging; but silty shoals, even when dredged away, will re-form unless the channel is contracted. Formerly rivers were regulated by building out jetties at intervals at right angles to the banks, especially in wide shoal places, in order to contract the channel and concentrate the stream, so as to scour a deeper central channel. These cross jetties, however, whilst effecting a deepening in front of their extremities, caused irregularities in the flow of the current in the intervals between them, thus producing differences in depth. Continuous longitudinal jetties, or training banks, though more costly, are much more efficient in regulating a river, and are now generally adopted for procuring a uniform width, and, consequently, a regular depth. Where the fall alters, a corresponding variation must be made in the section of the channel ; and, in the case of tidal rivers, the section should gradually increase as it approaches the sea, so as to admit the increasing volume of sea water which enters but does not pass far up the estuary whose upper portion has been filled by the earlier flow. Most rivers, whether tidal or tideless, are more or less Cau impeded for navigation by a bar at their mouth. A bar tari is a ridge or shoal extending across the navigable channel,