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A dam (a common Teutonic word, cf. Swedish and German damm, and the Gothic verb faurdammjan, to block up) is a barrier built across flowing water in order to hold it back, often creating a water reservoir or lake behind the dam. Dams may be built to provide water for irrigation or town water supply, control the amount of water in rivers or to provide hydroelectric power. Dams may also be built to control effluent from industrial work sites such as miness or factories.

The best place for building a dam is a narrow part of a deep river valley; the valley sides can then act as natural walls. The primary function of the dam's structure is to fill the gap in the natural reservoir line left by the stream channel. The most desirable sites are usually those where the gap becomes a minimum for the required storage capacity. The best economical arrangement is often a composite structure such as a masonry dam flanked by earth embankments. The current use of the land to be flooded should be dispensable.

The dam also serves to divert water to the intake works.

Dams may be classified according to their height. A large dam is used to describe a dam higher than 15 metres, a major dam for constructions over 150 metres.

Dams may be classified as follows:

  1. Timber dams
  2. Rock-fill dams
  3. Earth dams
    1. plain
    2. core wall
    3. hydraulic fill
  4. Masonry dams
    1. gravity, solid and hollow
    2. arch, single and multiple

The timber dam is rarely used by humans because of its short lifespan and the limitation in height to which it can be built. The locations where timber dams are most economical to build are those where timber is plentiful, cement is costly and difficult to transport, and only a submerged diversion dam is required. Timber is the basic material used by beavers, often with the addition of mud or stones.

Rock-fill dams are embankments of loose rock with either a watertight upstream face of concrete slabs or timber or a watertight core. Where suitable rock is at hand, a minimum of transportation of materials can be realized with this type of dam. Like the earth embankment, rock-fill dams resist damage from earthquakes quite well.

Earth dams are constructed as a simple homogeneous embankment of well-compacted earth, sometimes with a watertight concrete core or upstream face, or sometimes with a hydraulic fill to produce a watertight core. A type of temporary earth dam occasionally used in high lattitudes is the frozen-core dam, in which a coolant is circulated through pipes inside the dam to maintain a watertight region of permafrost within it.

Masonry dams are of either the gravity or the arch type. In a gravity dam, stability is secured by making it of such a size and shape that it will resist overturning, sliding and crushing at the toe. The dam will not overturn provided the resultant force falls within the base. However, in order to prevent tension at the upstream face and excessive compression at the downstream face, the dam cross section is usually designed so that the resultant falls within the middle third at all elevations of the cross section. In the arch dam stability is obtained by a combination of arch and gravity action. If the upstream face is vertical the entire weight of the dam must be carried to the foundation by gravity, while the distribution of the normal hydrostatic pressure between vertical cantilever and arch action will depend upon the stiffness of the dam in a vertical and horizontal direction. When the upstream face is sloped the distribution is more complicated. The normal component of the weight of the arch ring may be taken by the arch action, while the normal hydrostatic pressure will be distributed as described above. Hence, for the gravity type, good impervious foundations are essential, but for the arch type, firm reliable supports at the abutments (either buttress or canyon side wall) are more important. The most desirable place for an arch dam is a narrow canyon with steep side walls composed of sound rock. When situated on a suitable site, a gravity dam inspires more confidence in the layman than any other type; it has mass that lends an atmosphere of permanence, stability, and safety. When built on a carefully studied foundation with stresses calculated from completely evaluated loads, the gravity dam probably represents the best developed example of the art of dam building. This is significant because the fear of flood is a strong motivator in many regions, and has resulted in gravity dams being built in some instances where an arch dam would have been more economical.

Gravity dams are classified as "solid" or "hollow." The solid form is the more widely used of the two, though the hollow dam is frequently more economical to construct. Gravity dams can also be classified as "overflow" and "non-overflow." If the dam is meant to serve as a spillway section, its downstream face is ordinarily made an ogee curve with the curvature such that there will be no tendency of the water to leave the surface of the concrete, even with the maximum water elevation at the crest.

Two types of single-arch dams are in use, namely the constant-angle and the constant-radius dam. The constant-radius type employs the same face radius at all elevations of the dam, which means that as the channel grows narrower towards the bottom of the dam the central angle subtended by the face of the dam becomes smaller. In a constant-angle dam, this subtended angle is kept a constant and the variation in distance between the abutments at various levels is are taken care of by varying the radii. The safety of an arch dam is dependent on the strength of the side wall abutments, hence not only should the arch be well seated on the side walls but also the character of the rock should be carefully inspected. The multiple-arch dam consists of a number of single-arch dams with concrete buttresses as the supporting abutments. The multiple-arch dam does not require as many buttresses as the hollow gravity type, but requires good rock foundation because the buttress loads are heavy.

Significant engineering considerations when building a dam include permeability of the surrounding rock or soil, earthquake faults, peak flood flows, reservoir silting, environmental impacts on river fisheries, forests and wildlife, impacts on human habitations and compensation for land being flooded as well as population resettlement, and removal of toxic materials and buildings from the proposed reservoir area.

Dam failures are generally catastrophic if the structure is breached or significantly damaged. Routine monitoring of seepage from drains in, and around, larger dams is necessary to anticipate any problems and permit remedial action to be taken before structural failure occurs. Most dams incorporate mechanisms to permit the resevoir to be lowered or even drained in the event of such problems. Another solution can be rock grouting - pumping cement slurry into weak fractured rock under pressure.

Some dams:

Some Dutch cities are named after dams; Amsterdam (dam on the Amstel) and Rotterdam (dam on the Rotte).

Compare the usage of the word barrage.

See also: List of reservoirs and dams