DAMS AND SLUICES.
Uses of dam is a solid bank or barrier built across a stream to raise the level of the water. Structures of this character are designed for various purposes; as, for example, where it is desired to utilize the water for irrigation, or where it is desired to conduct the water through a side channel in order to apply the greater head thus obtained to the driv ing of water-wheels, turbines, etc., or, finally, where it is necessary to raise the surface of the water in rivers in order to obtain a sufficient depth for navigation. 'Where dams are_crected to obtain a greater depth of channel for purposes of navigation, suitable means must be provided for permitting the passage of vessels from one reach to another. For this purpose there are provided, in suitable side channels, locks, whose construction and ope ration are described under Canals. It is manifest that the height of struc tures of this kind should be no greater than is just necessary to accomplish the object of their establishment—not solely on account of the needless expenditure involved, but principally because the adjacent lands may be exposed to serious danger in periods of extreme high water.
Classes of Danis.—To this order belong the high masonry structures that are occasionally built across the narrow gorges of deep valleys to act as storage-reservoirs for retaining the flood-waters of rivers, which may be utilized for purposes of irrigation, for the water-supply of towns and cities, or incidentally to prevent destructive inundations. Again, the structure mav be so low that it will be covered even at low water; it is then called a " drowned dam." Dams of this kind have a very restricted application in special cases of river-improvement. The height of the dam commonly in use lies between extreme high-water level and extreme low-water level. At low water its top is dry, but as the water-level rises it is overflowed. The structures of-this class—called " overfall dams "—rank among the most important of those which fall in this category, and of late years their construction has been substantially improved. Although in providing for the rapid discharge of the flood-water these modifications are much superior to the simple overfall dam, they are inferior to the more modern structures called " movable dams," which may be lowered or removed either in part or wholly, and so allow the flood-water free exit. (See p. 309.)
Oblique ono' Curz,ea' Dains.—Dains are usually built at rig,ht angles to the shore—a disposition which not only reduces the cost of construction to a minimum, but also does away with the formation of currents and eddies by the overflow, which may endanger the bank, as may be observed in the case of dams built obliquely across the stream on the side toward which they are inclined. Dams curved up stream are still better in this respect, as they direct the overflow away from the banks, but they cost more to build. Where a passage-way for vessels is provided, the dam not infre quently is given an irregular form in order to direct the stream away from the channel and thus render the passage of the vessel easier. At the same time, it should be observed that the oblique dam, having an increased length of sill, presents a correspondingly greater surface for discharge, and consequently that the volume of the overflow for any given height of the water will be greater. The oblique dam will, therefore, to a certain extent prevent the damming of the flood-waters.
Masonry Dams.—The fixed or solid dams are constructed of various materials. Those built of stone are the most permanent, and are therefore to be preferred. In practice, two profiles for these structures are in vogue: either they may be given a trapezoidal cross-section when the overflow falls over them perpendicularly, or they may be given on the discharging side a gentle curvature sloping- down to the river-bed (p/. 57, fig. 14). The first-named and simpler construction is adopted where the river-bottom is rocky and cannot be disturbed by the impact of the falling water. Where the bottom, however, does not possess the solidity necessary to resist dis integration by the overfall water, it must be protected in some manner along- the foot of the dam as far as the action of the failing water is felt (for example, by a bed of concrete held between sheet-piling, etc.). Where the dam has a curved profile, the overflow should be conducted as uniformly as practicable, and without eddies, into the river-bed. To this end the protected surface below should be made broad enough to insure that the breaking up of the eddies formed by the mingling of the rapidly-flowing water from the dam with the slowly-flowing water of the lower section shall take place over the protected surface, and thus avoid the washing out of the river-bottom.