Dam Types Of Dams Diversionary Dams Timber Dams Embankments

Based on structure and material used dams are timber dams, embankment dams or masonry dams (arch or gravity type).

Intended purposes include providing water for irrigation or town or city water supply, improving navigation, generating hydroelectric power, creating recreation areas or habitat for fish and wildlife, flood control and containing effluent from industrial sites such as mines or factories. Few dams serve all of these purposes but some multi-purpose dams serve more than one.

According to height, a large dam is higher than 15 metres and a major dam is over 150 metres in height. Alternatively, a low dam is less than 30 m high; a medium-height dam is between 30 and 100 m high, and a high dam is over 100 m high.

What is sometimes called a saddle dam is actually a dike, a wall built at the edge of a lake to protect nearby land from flooding. This is similar to a levee, which is a wall built along a river or stream to protect adjacent land from floodA flood (in Old English flod a word common to Teutonic languages; compare German Flut Dutch vloed from the same root as is seen in flow, float is an overflow of water, an expanse of water submerging land, a deluge. In the sense of "flowing water", the woring.

An overflow dam is designed to be overtopped. A weirCoburg lake in Victoria (Australia) after heavy rainfall. Sturminster Newton on the River Stour, Dorset. A weir is a small overflow type dam commonly used to raise the level of a small river or stream. Weirs have traditionally been used to create mill pon is a type of small overflow dam that can be used for flow measurement.

A check dam is a small dam designed to reduce flow velocity and control soil erosion.

A dry damA dry dam is a dam constructed for the purpose of flood control. Dry dams typically contain no gates or turbines, and are intended to allow the channel to flow freely during normal conditions. During periods of intense rainfall that would otherwise cause is dam designed to control flooding. It normally holds back no water and allows the channel to flow freely, except during periods of intense flow that would otherwise cause flooding downstream.

1.1 Diversionary dams

A diversionary dam is a dam that does not completely block a riverMurray River in Australia Australia A river is a large natural waterway. It is a specific term in the vernacular for large streams, stream being the umbrella term used in the scientific community for all flowing natural waterways. In the vernacular, strea. Some of the flow is siphoned off into a separate lake, in front of which is the dam.

1.2 Timber dams

The timberTimber is a term used to describe clusters of trees. It is also used to describe wood throughout its processing from the time it is cut down to the time it is used as a structural material. The word timber is also used as an exclamation when a tree falls. 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, cementIn the general sense, a cement is any material with adhesive properties. The term cement is also commonly used to refer more specifically to powdered materials which develop strong adhesive qualities when combined with water. These materials are more prop 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.

1.3 Embankments

Embankments are made from fill material not joined by mortar.

1.3.1 Rock-fill dams

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.

1.3.2 Earth dams

Earth dams are constructed as a simple homogeneous embankment of well-compacted earth, sometimes with a watertight concrete or clay 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 latitudes 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.

1.4 Masonry dams

Masonry dams are of either the gravity or the arch type.

1.4.1 Gravity dams

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. For this type of dam, good impervious foundations are essential.

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" (spillway) and "non-overflow."

Any cavitation or turbulence of the water flowing over the spillway slowly eats the dam. To minimize that erosion (especially with maximum water elevation at the crest), the downstream face of the spillway is ordinarily made an ogee curve.

1.4.2 Arch dams

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. For this type of dam, 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.

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. See Geotechnical engineering.

1.4.3 Cofferdams

A cofferdam is a temporary barrier constructed to exclude water from an area that is normally submerged. Made commonly of wood, steel, or concrete sheet piling, cofferdams are used to allow construction on the foundations of permanent dams, bridges, and similar structures. When the project is completed, the cofferdam is demolished.

2 Spillways

A spillway is a section of a dam designed to pass water from the upstream side of a dam to the downstream side. Many spillways have gates designed to control the flow through the spillway.

A service spillway or primary spillway passes normal flow. An auxiliary spillway releases flow in excess of the capacity of the service spillway. An emergency spillway is designed for extreme conditions, such as a serious malfunction of the service spillway. A fuse-plug spillway is a low embankment designed to be overtopped and washed away in the event of a large flood.

It was the inadequate design of the spillway that caused the overtopping of a dam that caused the infamous Johnstown Flood.

3 Other considerations

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 most 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.

Significant other 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 (see fish ladder)
impacts on human habitations
compensation for land being flooded as well as population resettlement
removal of toxic materials and buildings from the proposed reservoir area

Kenneth E. Boulding's poem The Ballad of Ecological Awareness (see External links) discusses the social and ecological impact of dam-building, beginning: "The cost of building dams is always underestimated" and concluding: "...cost-benefit analysis is nearly always sure/To justify the building of a solid concrete fact/While the Ecologic Truth is left behind in the Abstract."

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.

4 Examples of dams

4.1 Failed dams

South Fork Dam - 1889
St. Francis Dam - 1928
Baldwin Hills Dam - 1963
Banqiao and Shimantan Dam s - 1975 (Estimates of as many as 85,000 killed directly and 100,000 plus additional due to water borne disease and farmland distruction.)
Teton Dam - 1976
Kelly Barnes Dam - 1977
Lawn Lake Dam - 1982
Opuha Dam - New Zealand

5 See also

List of reservoirs and dams

6 External links

International Commission On Large Dams (ICOLD)

Structurae: Dams and Retaining Structures
The Ballad of Ecological Awareness (N. B.: pdf version)

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1 Types of dams