ilfozyrbk Dams.—As the solid dams fulfil their intended office only when the stream is at normal or low-water level, and are rather objection able than otherwise during periods of high water by reason of the impedi ment which they form to the free discharge of the stream—a defect which even the addition of draw-doors does not fully remedy—movable dams have been devised. These are structures which, at,high water, may be in part or wholly lowered. In the former case the dam consists of a submerged fixed portion and an upper removable part, resting- upon it and extend ing up to the crest of the dam. In the latter case the entire structure admits of being lowered to the river-bed or removed, leaving the channel unobstructed. The constructions of this order are distinguished from the above-described sluice-dams (which, strictly speaking*, are also movable dams) in requiring less power and less time for their operation.
Frame- or Ncalc-dams.—The so-called "frame." or "needle-darn" (p. 5-„ figs. r6, r7) is a form of the movable dam that has come into general use. It consists, substantially, of a series of iron frames arranged parallel with the channel, bv which frames the solid barrier constituting* the dam is supported. This barrier is formed of a series of wooden stakes of proper length standing approximately vertical. At their lower ends they bear against a sill, and at their upper ends they rest against a horizontal bar, which, in turn, connects with the iron frames. The frames also support a foot-bridge, from which the i.vork of raising or lowering the clam is effected. To lower the dam, the wooden stakes (or " needles," as they are termed) between two contiguous frames (beginning at the end-frames) are raised from the foot-bridge; one of the end-frames is then freed by removing the horizontal bar connecting them; the section of the bridge is taken up and the frame, which is hinged at the bottom, is lowered on to the apron of the dam, by means of chains. In the same manner each succeeding frame is disconnected and lowered to the bottom, until, when the last is reached, the channel is left free from obstruction. In some cases the needles are not raised up singlv, as above described, but are freed in groups by successively withdrawing the connecting-bars against which their upper ends rest. I3y this means all the needles between two adjoining frames are simultaneously freed. The re-erectiou of the structure is accomplished by reversing these operations.
Modifications of the a modification of the needIe-dam, the barrier closing the space between two adjacent frames is formed by -a wooden•hiuged shutter, which is rolled up from the bottom when it becomes necessary to open the dam. In another, more elaborate, and expensive con struction the vertical iron frames are suspended from an overhead girder which rests on piers of masonry. In this construction the frames are raised entirely out of the water into a horizontal position when the dam is to be opened. The apertures between the frames, when lowered, are closed by means of hinged shutters. In other modifications of the frame-dam, sliding
panels have been employed for the same purpose.
In the system devised by Thenard the upper or movable part of the dam is formed of gates or shutters which are hinged at the bottom. They are supported in the vertical position against the force of the current by means of iron props; and when these aye removed, they fall to the bottom. In this modification it is necessary to provide a second, up-stream gate, to relieve the main dam from the pressure of the current in order to raise the latter into position after lowering. A number of modifications of this form also exist in which the use of up-stream shutters is dispensed with. Fig ure 9 054 56) exhibits a form of movable frame adopted by the United States government engineers, and used in the construction of the Ohio River improvement at Davis Island. In another form the dam is self acting, the shutters being attached in such a manner, in a position inclined to the current, to a horizontal axis upon which they may turn, that they will be lowered by the weight of the water above them when this reaches a certain predetermined height for which they have been adjusted.
The which has been introduced in France, is formed of an upper and a lower iron paddle, so devised as to be capable of a quarter revolution about a horizontal axis. The upper paddle constitutes the dam, and by suitably admitting water through sluices into the drum in which the lower paddle is enclosed, so that it shall act upon the upper or lower side of the under paddle as may be desired, the upper paddle, forming the dam, will be raised or lowered accordingly.
or as the capacity of the stream is materially reduced by the erection of a solid barrier, there will be a disposition to deposit sediment in the lower part of the reach near the location of the dam, owing to the reduced velocity of the current. This action, in turn, by raising the bed of the stream, will gradually en large the area of the inundated tract behind the dam. To obviate these objectionable features, openings are sometimes provided through a portion of the dam, reaching to the bottom of the river-bed and furnished with draw-doors (sluice-gates) or other devices by which they may be closed at pleasure. These gatcs or doors are kept closed during periods of low water, but are raised in flood seasons. The arrangement thus provides for the more rapid discharge of the water at high-water stages of the stream and neutralizes the tendency to the deposition of sediment, with its attend ant objectionable features. Such structures are called " draw-door " or "overfall dams" (p. 57, jigs. 14, 13). To provide for the prompt dis charge of the water if the rise of the stream is very considerable, it may be necessary to increase the number of these openings until their united areas approximate to that of the cross-section of the stream (figs. 13, 19).