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A dam is a barrier that impounds
water or underground streams. Dams generally serve the primary purpose of
retaining water, while other structures such as floodgates or levees (also
known as dikes) are used to manage or prevent water flow into specific land
regions. Hydropower and pumped-storage hydroelectricity are often used in
conjunction with dams to provide generate electricity. A dam can also be used
to collect water or for storage of water which can be evenly distributed
between locations.
History
The word dam can be traced back to Middle English,[1] and before that, from
Middle Dutch, as seen in the names of many old cities.
Most early dam building took place in Mesopotamia and the Middle East. Dams
were used to control the water level, for Mesopotamia's weather affected the
Tigris and Euphrates rivers, and could be quite unpredictable.
The earliest known dam is the Jawa Dam in Jordan, 100 km northeast of the
capital Amman. This gravity dam featured a 4.5 m high and 1 m wide stone wall,
supported by a 50 m wide earth rampart. The structure is dated to 3000 BC. The
Ancient Egyptian Sadd-el-Kafara Dam at Wadi Al-Garawi, located about 25
kilometers south of Cairo, was 102 m long at its base and 87 m wide. The structure
was built around 2800 or 2600 B.C. as a diversion dam for flood control, but
was destroyed by heavy rain during construction or shortly afterwards.
Roman dam construction was characterized by "the Romans' ability to plan
and organize engineering construction on a grand scale". Roman planners
introduced the then novel concept of large reservoir dams which could secure a
permanent water supply for urban settlements also over the dry season. Their
pioneering use of water-proof hydraulic mortar and particularly Roman concrete
allowed for much larger dam structures than previously built, such as the Lake
Homs Dam, possibly the largest water barrier to date, and the Harbaqa Dam, both
in Roman Syria. The highest Roman dam was the Subiaco Dam near Rome; its record
height of 50 m remained unsurpassed until its accidental destruction in 1305.
Roman engineers made routine use of ancient standard designs like embankment
dams and masonry gravity dams. Apart from that, they displayed a high degree of
inventiveness, introducing most of the other basic dam designs which had been
unknown until then. These include arch-gravity dams, arch dams, buttress dams
and multiple arch buttress dams, all of which were known and employed by the
2nd century AD (see List of Roman dams). Roman workforces also were the first
to built dam bridges, such as the Bridge of Valerian in Iran.
Eflatun Pınar is a Hittite dam and spring temple near Konya, Turkey. It's
thought to the time of the Hittite empire between the 15th and 13 century BC.
The Kallanai is a massive dam of unhewn stone, over 300 meters long, 4.5 meters
high and 20 meters (60 ft) wide, across the main stream of the Kaveri river in
Tamil Nadu, South India. The basic structure dates to the 1st century AD. and
is considered one of the oldest water-diversion or water-regulator structures
in the world, which is still in use. The purpose of the dam was to divert the
waters of the Kaveri across the fertile Delta region for irrigation via canals.
Du Jiang Yan is the oldest surviving irrigation system in China that included a
dam that directed waterflow. It was finished in 251 B.C. A large earthen dam,
made by the Prime Minister of Chu (state), Sunshu Ao, flooded a valley in
modern-day northern Anhui province that created an enormous irrigation
reservoir (62 miles in circumference), a reservoir that is still present today.
In Iran, bridge dams such as the Band-e Kaisar were used to provide hydropower
through water wheels, which often powered water-raising mechanisms. One of the
first was the Roman-built dam bridge in Dezful, which could raise 50 cubits of
water for the water supply to all houses in the town. Also diversion dams were
known. Milling dams were introduced which the Muslim engineers called the
Pul-i-Bulaiti. The first was built at Shustar on the River Karun, Iran, and
many of these were later built in other parts of the Islamic world. Water was
conducted from the back of the dam through a large pipe to drive a water wheel
and watermill. In the 10th century, Al-Muqaddasi described several dams in
Persia. He reported that one in Ahwaz was more than 3,000 feet long, and that
and it had many water-wheels raising the water into aqueducts through which it
flowed into reservoirs of the city. Another one, the Band-i-Amir dam, provided
irrigation for 300 villages.
In the Netherlands, a low-lying country, dams were often applied to block
rivers in order to regulate the water level and to prevent the sea from
entering the marsh lands. Such dams often marked the beginning of a town or city
because it was easy to cross the river at such a place, and often gave rise to
the respective place's names in Dutch. For instance the Dutch capital Amsterdam
(old name Amstelredam) started with a dam through the river Amstel in the late
12th century, and Rotterdam started with a dam through the river Rotte, a minor
tributary of the Nieuwe Maas. The central square of Amsterdam, covering the
original place of the 800 year old dam, still carries the name Dam Square or
simply the Dam.
The age of hydropower and large dams emerged following the development of the
turbine. French engineer Benoît Fourneyron perfected the first water turbine in
1832. The era of mega-dam building was initiated after Hoover Dam was completed
on the Colorado River in 1936. By 1997, there were an estimated 800,000 dams
worldwide, some 40,000 of them over fifteen meters high.
Types of dams
Dams can be formed by human agency, natural causes, or even by the intervention
of wildlife such as beavers. Man-made dams are typically classified according
to their size (height), intended purpose or structure.
By size
International standards (including International Commission on Large Dams,
ICOLD) define large dams as higher than 15 meters and major dams as over 150
meters in height.[26] The Report of the World Commission on Dams also includes
in the large category, dams, such as Barrages, which are between 5 and 15
meters high with a reservoir capacity of more than 3 million cubic meters.
The tallest dam in the world is the 300-meter-high Nurek Dam in Tajikistan.
Intended purposes include providing water for irrigation to a 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, retaining wet season flow to minimise downstream
flood risk and containing effluent from industrial sites such as mines or
factories. Some dams can also serve as pedestrian or vehicular bridges across
the river as well. When used in conjunction with intermittent power sources
such as wind or solar, the reservoir can serve as pumped water storage to
facilitate base load dampening in the power grid. Few dams serve all of these
purposes but some multi-purpose dams serve more than one.
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 over topped. A weir is a type of small
overflow dam that are often used within a river channel to create an
impoundment lake for water abstraction purposes and which can also 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.
A diversionary dam is a structure designed to divert all or a portion of the
flow of a river from its natural course.
By structure
Based on structure and material used, dams are classified as timber dams,
arch-gravity dams, embankment dams or masonry dams, with several subtypes.
Masonry and concrete dams
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, as for example the Daniel-Johnson Dam,
Québec, Canada. 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.
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, gravity dams can prove to be a better
alternative to other types of dams. When built on a carefully studied
foundation, the gravity dam probably represents the best developed example of
dam building. Since the fear of flood is a strong motivator in many regions,
gravity dams are being built in some instances where an arch dam would have
been more economical.
Gravity dams are classified as "solid" or "hollow" and are
generally made of either concrete or masonry. This is called
"Zoning". The core of the dam is zoned depending on the availability
of locally available materials, foundation conditions and the material
attributes. 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.
A gravity dam can be combined with an arch dam, an arch-gravity dam, for areas
with massive amounts of water flow but less material available for a purely
gravity dam.
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
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. To prevent internal erosion of clay into the rock fill due to
seepage forces, the core is separated using a filter. Filters are specifically
graded soil designed to prevent the migration of fine grain soil particles.
When suitable material is at hand, transportation is minimized leading to cost
savings during construction. Rock-fill dams are resistant to damage from
earthquakes. However, inadequate quality control during construction can lead
to poor compaction and sand in the embankment which can lead to liquefaction of
the rock-fill during an earthquake. Liquefaction potential can be reduced by
keeping susceptible material from being saturated, and by providing adequate
compaction during construction. An example of a rock-fill dam is New Melones
Dam in California.
Earth-fill dams
Earth-fill dams, also called earthen, rolled-earth or simply earth dams, are
constructed as a simple embankment 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.
Tarbela Dam is a large dam on the Indus River in Pakistan. It is located about
50 km (31 mi) northwest of Islamabad, and a height of 485 ft (148 m) above the
river bed and a reservoir size of 95 sq mi (250 km2) makes it the largest earth
filled dam in the world. The principal element of the project is an embankment
9,000 feet (2743 meters) long with a maximum height of 465 feet (143 meters).
The total volume of earth and rock used for the project is approximately 200
million cubic yards (152.8 million cu. Meters) which makes it the largest man
made structure in the world , except for the Great Chinese Wall which consumed
somewhat more material.
Because earthen dams can be constructed from materials found on-site or nearby,
they can be very cost-effective in regions where the cost of producing or
bringing in concrete would be prohibitive.
Asphalt-concrete core
A third type of embankment dam is built with asphalt concrete core. The
majority of such dams are built with rock and/or gravel as the main fill
material. Almost 100 dams of this design have now been built worldwide since
the first such dam was completed in 1962. All asphalt-concrete core dams built
so far have an excellent performance record. The type of asphalt used is a
viscoelastic-plastic material that can adjust to the movements and deformations
imposed on the embankment as a whole, and to settlements in the foundation. The
flexible properties of the asphalt make such dams especially suited in
earthquake regions.
Cofferdams
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. Common uses for cofferdams include construction and repair of off shore
oil platforms. In such cases the cofferdam is fabricated from sheet steel and
welded into place under water. Air is pumped into the space, displacing the
water allowing a dry work environment below the surface. Upon completion the
cofferdam is usually deconstructed unless the area requires continuous
maintenance.
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 because of 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. Splash dams
were timber crib dams used to help float logs downstream in the late 19th and
early 20th centuries.
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.
Steel dams
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.
Beaver dams
Beavers create dams primarily out of mud and sticks to flood a particular
habitable area. By flooding a parcel of land, beavers can navigate below or
near the surface and remain relatively well hidden or protected from predators.
The flooded region also allows beavers access to food, especially during the
winter.
Construction elements
Power generation plant
As of 2005, hydroelectric power, mostly from dams, supplies some 19% of the
world's electricity, and over 63% of renewable energy. Much of this is
generated by large dams, although China uses small scale hydro generation on a
wide scale and is responsible for about 50% of world use of this type of power.
Most hydroelectric power comes from the potential energy of dammed water
driving a water turbine and generator; to boost the power generation
capabilities of a dam, the water may be run through a large pipe called a
penstock before the turbine. A variant on this simple model uses pumped storage
hydroelectricity to produce electricity to match periods of high and low
demand, by moving water between reservoirs at different elevations. At times of
low electrical demand, excess generation capacity is used to pump water into
the higher reservoir. When there is higher demand, water is released back into
the lower reservoir through a turbine
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 floodgates designed to
control the flow through the spillway. Types of spillway include: 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 over
topped and washed away in the event of a large flood. Fusegate elements are
independent free-standing block set side by side on the spillway which work
without any remote control. They allow to increase the normal pool of the dam
without compromising the security of the dam because they are designed to be
gradually evacuated for exceptional events. They work as fixed weir most of the
time allowing overspilling for the common floods.
The spillway can be gradually eroded by water flow, including cavitation or
turbulence of the water flowing over the spillway, leading to its failure. It
was the inadequate design of the spillway which led to the 1889 over-topping of
the South Fork Dam in Johnstown, Pennsylvania, resulting in the infamous
Johnstown Flood (the "great flood of 1889").
Erosion rates are often monitored, and the risk is ordinarily minimized, by
shaping the downstream face of the spillway into a curve that minimizes
turbulent flow, such as an ogee curve.
Dam creation
Common purposes
Function
Example
Power generation
Hydroelectric power is a major source of electricity in the world. Many
countries that have rivers with adequate water flow, that can be dammed for
power generation purposes. For example, the Itaipu on the Paraná River in South
America generates 14 GW and supplied 93% of the energy consumed by Paraguay and
20% of that consumed by Brazil as of 2005.
Water supply
Many urban areas of the world are supplied with water abstracted from rivers
pent up behind low dams or weirs. Examples include London - with water from the
River Thames and Chester with water taken from the River Dee. Other major
sources include deep upland reservoirs contained by high dams across deep
valleys such as the Claerwen series of dams and reservoirs.
Stabilize water flow / irrigation
Dams are often used to control and stabilize water flow, often for agricultural
purposes and irrigation.[31] Others such as the Berg Strait dam can help to
stabilize or restore the water levels of inland lakes and seas, in this case
the Aral Sea.[32]
Flood prevention
Dams such as the Blackwater dam of Webster, New Hampshire and the Delta Works
are created with flood control in mind.[33]
Land reclamation
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