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Dam

2007 Schools Wikipedia Selection. Related subjects: Engineering

   Scrivener Dam, in Canberra, Australia, was engineered to withstand a
   once-in-5000-years flood
   Enlarge
   Scrivener Dam, in Canberra, Australia, was engineered to withstand a
   once-in-5000-years flood

   A dam is a barrier across flowing water that obstructs, directs or
   retards the flow, often creating a reservoir, lake or impoundment. In
   Australian and South African English, the word "dam" can also refer to
   the reservoir rather than the structure. Most dams have a section
   called a spillway or weir over which or through which it is intended
   that water will flow either intermittently or continuously.

History

   Some of the first dams were built in Mesopotamia up to 7,000 years ago.
   These were used to control the water level, for Mesopotamia's weather
   effected the Tigris and Euphrates rivers and could be quite
   unpredictable.The earliest recorded dam is believed to have been on the
   nile river at Kosheish, where a 15m high masonary structure was built
   about 2900 B.C. to supply water to capital of Memphis.

Types of dams

   The Hoover Dam, a concrete gravity-arch dam in the Black Canyon of the
   Colorado River
   Enlarge
   The Hoover Dam, a concrete gravity-arch dam in the Black Canyon of the
   Colorado River

   Dams can be formed by human agency, natural causes, or by the
   intervention of wildlife such as beavers. Man-made dams are typically
   classified according to thier structure, intended purpose or height.

   Based on structure and material used, dams are classified as timber
   dams, embankment dams or masonry dams, with several subtypes.

   Intended purposes include providing water for irrigation or town or
   city water supply, improving navigation, creating a reservoir of water
   to supply industrial uses, 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.

   A saddle dam is an auxiliary dam constructed to confine the reservoir
   created by a primary dam either to permit a higher water elevation and
   storage or to limit the extent of a reservoir for increased efficiency.
   An auxiliary dam is constructed in a low spot or saddle through which
   the reservoir would otherwise escape. On occasion, a reservoir is
   contained by a similar structure called a dike to prevent inundation of
   nearby land. Dikes are commonly used for reclamation of arable land
   from a shallow lake. This is similar to a levee, which is a wall or
   embankment 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 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. Conversely, a wing dam is a structure that only partly
   restricts a waterway, creating a faster channel that resists the
   accumulation of sediment.

   A dry dam is a 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.

Diversionary dams

   A diversionary dam is a structure designed to divert all or a portion
   of the flow of a river from its natural course.

Timber dams

   A timber crib dam in Michigan, photographed in 1978.
   Enlarge
   A timber crib dam in Michigan, photographed in 1978.

   Timber dams were widely used in the early part of the industrial
   revolution and in frontier areas due to ease and speed of construction.
   Rarely built in modern times by humans due to relatively short lifespan
   and limited height to which they can be built, timber dams must be kept
   constantly wet in order to maintain their water retention properties
   and limit deterioration by rot, similar to a barrel. The locations
   where timber dams are most economical to build are those where timber
   is plentiful, cement is costly or difficult to transport, and either a
   low head diversion dam is required or longevity is not an issue. Timber
   dams were once numerous, especially in the North American west, but
   most have failed, been hidden under earth embankments or been replaced
   with entirely new structures. Two common variations of timber dams were
   the crib and the plank.

   Timber crib dams were erected of heavy timbers or dressed logs in the
   manner of a log house and the interior filled with earth or rubble. The
   heavy crib structure supported the dam's face and the weight of the
   water.

   Timber plank dams were more elegant structures that employed a variety
   of construction methods utilizing heavy timbers to support a water
   retaining arrangement of planks.

   Very few timber dams are still in use. Timber, in the form of sticks,
   branches and withes, is the basic material used by beavers, often with
   the addition of mud or stones.

Embankment dams

   Embankment dams are made from compacted earth, and have two main types,
   rock-fill and earth-fill dams. Embankment dams rely on their weight to
   hold back the force of water, like the gravity dams made from concrete.

   Rock-fill dams
   A rockfill dam
   Enlarge
   A rockfill dam

   Rock-fill dams are embankments of compacted free-draining granular
   earth with an impervious zone. The earth utilized often contains a
   large percentage of large particles hence the term rock-fill. The
   impervious zone may be on the upstream face and made of masonry,
   concrete, plastic membrane, steel sheet piles, timber or other
   material. The impervious zone may also be within the embankment in
   which case it is referred to as a core. In the instances where clay is
   utilized as the impervious material the dam is referred to as a
   composite dam. When suitable material is at hand, transportation is
   minimized leading to cost savings during construction. Rock-fill dams
   are highly resistant to damage from earthquakes. However, inadequate
   quality control during construction can lead to excessive fines in the
   embankment which can lead to liquefaction of the rock-fill during an
   earthquake. This problem can be eliminated by keeping susceptible
   material dry. New Melones Dam is a rock-fill dam.

   Earth dams
   A Farmer's Dam
   Enlarge
   A Farmer's Dam

   Earth dams, also called earthen, rolled-earth and earth-fill dams, are
   constructed of well compacted earth. A homogeneous rolled-earth dam is
   entirely constructed of one type of material but may contain a drain
   layer to collect seep water. A zoned-earth dam has distinct parts or
   zones of dissimilar material, typically a locally plentiful shell with
   a watertight clay core. Modern zoned-earth embankments employ filter
   and drain zones to collect and remove seep water and preserve the
   integrity of the downstream shell zone. An outdated method of zoned
   earth dam construction utilized a hydraulic fill to produce a
   watertight core. Rolled-earth dams may also employ a watertight facing
   or core in the manner of a rock-fill dam. An interesting 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.
   Oroville Dam is an example of an earth dam, and is the tallest dam in
   the United States.

Masonry dams

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

Gravity dams

   The Eder dam in Germany, built around 1910.
   Enlarge
   The Eder dam in Germany, built around 1910.

   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 that the moment around the turning
   point, caused by the water pressure is smaller than the moment caused
   by the weight of the dam. This is the case if the resultant force of
   water pressure and weight falls within the base of the dam. However, in
   order to prevent tensile stress at the upstream face and excessive
   compressive stress at the downstream face, the dam cross section is
   usually designed so that the resultant falls within the middle at all
   elevations of the cross section (the core). For this type of dam,
   impervious foundations with high bearing strength 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." Grand Coulee Dam is a solid
   gravity dam and Itaipu Dam is a hollow gravity dam.

   With a height of 285m the tallest gravity dam in the world is the
   Grande Dixence Dam in Switzerland.

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. 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.

   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. Jones Falls Dam, in
   Canada, is a constant radius dam. In a constant-angle dam, also known
   as a variable radius dam, this subtended angle is kept a constant and
   the variation in distance between the abutments at various levels are
   taken care of by varying the radii. Constant-radius dams are much less
   common than constant-angle dams. Parker Dam is a constant-angle arch
   dam.

   A similar type is the double-curvature or thin-shell dam. Wildhorse Dam
   near Mountain City, Nevada in the United States is an example of the
   type. This method of construction minimizes the amount of concrete
   necessary for construction but transmits large loads to the foundation
   and abutments. The appearance is similar to a single-arch dam but with
   a distinct vertical curvature to it as well lending it the vague
   appearance of a concave lens as viewed from downstream.

   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.

Steel dams

   Red Ridge steel dam, b. 1905, Michigan
   Enlarge
   Red Ridge steel dam, b. 1905, Michigan

   A steel dam is a type of dam briefly experimented with in around the
   turn of the 19th-20th century which uses steel plating (at an angle)
   and load bearing beams as the structure. Intended as permanent
   structures, steel dams were an (arguably failed) experiment to
   determine if a construction technique could be devised that was cheaper
   than masonry, concrete or earthworks, but sturdier than timber crib
   dams. Only two examples remain in the US.

Cofferdams

   A cofferdam during the construction of locks at the Montgomery Point
   Lock and Dam.
   Enlarge
   A cofferdam during the construction of locks at the Montgomery Point
   Lock and Dam.

   A cofferdam is a (usually temporary) barrier constructed to exclude
   water from an area that is normally submerged. Made commonly of wood,
   concrete or steel sheet piling, cofferdams are used to allow
   construction on the foundation of permanent dams, bridges, and similar
   structures. When the project is completed, the cofferdam may be
   demolished or removed. See also causeway and retaining wall.

Beaver dams

Spillways

   Spillway on Llyn Brianne dam, Wales soon after first fill
   Enlarge
   Spillway on Llyn Brianne dam, Wales soon after first fill

   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
   floodgates 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.

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

   It was the inadequate design of the spillway that caused the
   overtopping of a dam that caused the infamous Johnstown 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. The primary
   function of the dam's structure is to fill the gap in the natural
   reservoir line left by the stream channel. The 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 and engineering geology considerations
   when building a dam include:
     * permeability of the surrounding rock or soil
     * earthquake faults
     * landslides and slope stability
     * 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

   The reservoir emptying through the failed Teton Dam
   Enlarge
   The reservoir emptying through the failed Teton Dam

   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 reservoir to be lowered
   or even drained in the event of such problems. Another solution can be
   rock grouting - pressure pumping portland cement slurry into weak
   fractured rock.

Environmental impacts

   ( Source: Canadian Geographic)

   More than half of the world’s large rivers have been dammed, regulating
   and flooding approximately 400,000 square kilometres of land worldwide.
   These diversions have an effect on diverse ecosystems and habitats
   around the globe, replacing them with uniform structures and reservoirs
   and ultimately changing the way otherwise balanced, stable ecosystems
   function.

Stream flow

   The life of a river is closely tied to its stream flow, which
   constantly fluctuates. Damming a river and altering its flow pattern
   generates a number of physical and biological impacts. The disruption
   of a river’s flow obstructs its natural current and affects the water’s
   habitat.

   One of the largest impacts a lack of current has on a river is the
   sediment flow, which is normally carried down the river by the current.
   When trapped by a dam, the sediment is held in the reservoir and
   settles to the bottom while clear water containing very little sediment
   is released down the river.

   Over time, the easily erodible material from the riverbed is carried
   away with no sediment being deposited to replace it. This leaves a
   rocky stream bed, resulting in a poorer habitat for aquatic fauna.

Barrier to migration

   The most visible and obvious effect of dams is that they fragment
   rivers and make migration difficult for fish and other aquatic life.
   Species such as salmon and eels, which migrate to spawn, may not make
   it to their destination or may suffer injury or death while traveling
   through turbines or over spillways. Fish that do make it through are
   often disoriented and become more susceptible to predators.

   Some dams are equipped with fish passage structures, or fish ladders,
   to attempt to accommodate the migration of a river’s aquatic life.
   Questions have been raised as to whether fish ladders are actually too
   stressful for adult fish and reduce their chances for successful
   spawning.

Water quality impacts

   When water is held in the reservoir of a dam, the quality of water is
   affected in several ways, the extent of which depending on how long it
   is held there.

   The initial creation of a reservoir on a floodplain submerges the
   existing vegetation and soil, causing much of the organic material to
   decompose over time which can deplete oxygen from the water supply.

   The establishment of a deep reservoir will almost always lead to
   thermal stratification during summer months. Water warmed by the sun
   forms an upper warm layer called the Epilimnion which is well
   oxygenated. The bulk of the water is held in the lower, cold unmixed
   layer, the Hypolimnion. This cold water receives relatively little
   light, has no contact with the air and is often depleted in oxygen. The
   boundary between these two layers is the thermocline.

   Where draw-off towers or sluices in dams release water from the
   Hypolimnion into the downstream river, the water discharged may be
   unusually cold and may be low in oxygen and high in metals such as
   Manganese. All these properties can have seriously adverse effects on
   the normal biota of a river.

   Mercury, which can exist at very low levels in the soil, may be
   transformed by bacteria into methyl mercury once the soil is flooded if
   the benthic conditions become substantially anoxic. Methyl mercury is a
   cumulative toxin to veterbrate species and may enter the food chain
   from consumption of reservoir fish. Such circumstances are
   theoretically possible but very rare in practice.

Examples of failed dams

     * Baldwin Hills Reservoir - 1963
     * Banqiao and Shimantan Dams - 1975
     * Big Bay Dam, Mississippi - 2004
     * Buffalo Creek Flood - 1972
     * Camará Dam - 2004
     * South Fork Dam - 1889
     * Kelly Barnes Dam - 1977
     * Lawn Lake Dam - 1982
     * Malpasset, Côte d'Azur, France - 1959
     * Opuha Dam - 1997
     * Shakidor Dam - 2005
     * St. Francis Dam, Los Angeles, California - 1928
     * Taum Sauk reservoir - 2005
     * Teton Dam - 1976
     * Tous Dam, Valencia, Spain - 1982
     * Vajont Dam - 1961
     * Val di Stava Dam Collapse - 1985

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   with only minor checks and changes (see www.wikipedia.org for details
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