Dam

A dam (a common Teutonic word, compare to Dutch dam, Swedish and German damm, and the Gothic verbfaurdammjan, to block up) is a barrier across flowing water that obstructs, directs or retards the flow, often creatinga reservoir , lake or impoundment. Most damshave a section called a spillway, over which or through which it is intended that water will flow.

Types of dams

Dams may be classified according to structure, intended purpose or height.

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 damsserve 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 majordam is over 150 metres in height. Alternatively, a low dam is less than 30 m high; a medium-height dam isbetween 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 flooding .

An overflow dam is designed to be overtopped. A weir is a type of smalloverflow 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 dam is dam designed to control flooding. It normally holds back nowater and allows the channel to flow freely, except during periods of intense flow that would otherwise cause floodingdownstream.

Diversionary dams

A diversionary dam is a dam that does not completely block a river . Some of the flow is siphoned off into a separate lake , in front of which is the dam.

Timber dams

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

Embankments

Embankments are made from fill material not joined by mortar.

Rock-fill dams

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

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 highlattitudes is the frozen-core dam, in which a coolant is circulated through pipes inside the dam to maintain a watertight regionof permafrost within it.

Masonry dams

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

Gravity dams

In a gravity dam, stability is secured by making it of such a size and shape that it will resist overturning, sliding andcrushing at the toe. The dam will not overturn provided the resultantforce falls within the base. However, in order to prevent tension at the upstream face and excessive compression at thedownstream face, the dam cross section is usually designed so that the resultant falls within the middle third at all elevationsof 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 thatlends an atmosphere of permanence, stability, and safety. When built on a carefully studied foundation with stresses calculatedfrom completely evaluated loads, the gravity dam probably represents the best developed example of the art of dam building. Thisis significant because the fear of flood is a strong motivator in many regions, and hasresulted 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 damis 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 isordinarily made an ogee curve .

Arch dams

In the arch dam, stability is obtained by a combination of arch and gravity action. If the upstream face is vertical theentire weight of the dam must be carried to the foundation by gravity, while the distribution of the normal hydrostatic pressurebetween vertical cantilever and arch action will depend upon the stiffness of the dam in a vertical and horizontal direction. When the upstream face issloped the distribution is more complicated. The normal component of the weight of thearch ring may be taken by the arch action, while the normal hydrostatic pressure will be distributed as described above. For thistype 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 narrowcanyon 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 typeemploys the same face radius at all elevations of the dam, which means that as the channel grows narrower towards the bottom ofthe dam the central angle subtended by the face of the dam becomes smaller. In a constant-angle dam, this subtended angle is kepta constant and the variation in distance between the abutments at various levels is are taken care of by varying the radii. Thesafety of an arch dam is dependent on the strength of the side wall abutments, hence not only should the arch be well seated onthe side walls but also the character of the rock should be carefully inspected. The multiple-arch dam consists of a number ofsingle-arch dams with concrete buttresses as the supporting abutments. The multiple-arch dam does not require as many buttressesas the hollow gravity type, but requires good rock foundation because the buttress loads are heavy. See Geotechnical engineering .

Spillways

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

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

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. Theprimary function of the dam's structure is to fill the gap in the natural reservoir line left by the stream channel. The mostdesirable sites are usually those where the gap becomes a minimum for the required storage capacity. The most economicalarrangement is often a composite structure such as a masonry dam flanked by earthembankments. The current use of the land to be flooded should be dispensable.

Significant other engineering considerations when building a daminclude

• permeability of the surrounding rock or soil
• earthquake faults
• peak flood flows
• reservoir silting
• environmental impacts on river fisheries, forests and wildlife (see fishladder )
• 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 ofdam-building, beginning: "The cost of building dams is always underestimated" and concluding: "...cost-benefit analysis is nearlyalways 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 seepagefrom drains in, and around, larger dams is necessary to anticipate any problems and permit remedial action to be taken beforestructural failure occurs. Most dams incorporate mechanisms to permit the resevoir to be lowered or even drained in the event ofsuch problems. Another solution can be rock grouting - pumping cement slurry into weak fractured rock under pressure.

Examples of dams

• Three Gorges Dam , China
• Itaipu Dam , Brazil / Paraguay
• Aswan Dam , Egypt
• Benmore Dam , NewZealand
• Glen Canyon Dam , United States
• Grand Coulee Dam , United States
• Hoover Dam , UnitedStates
• Hume Dam , Australia
• Lake Pedder - LakeGordon , Australia
• Kariba Dam , Zambia / Zimbabwe
• Vishvesvaraya Dam , India

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

External links

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