(Redirected from Dams)
Hydroelectric dam in cross section
A 'dam' is a barrier across flowing water that obstructs, directs or slows down the flow, often creating a
reservoir,
lake or impoundments. In
Australian and
South African English, the word "dam" can also refer to the
reservoir as well as the structure. Most dams have a section called a ''
spillway or
weir'' over which, or through which, water flows, either intermittently or continuously.
Dams generally serve the primary purpose of retaining water, while other structures such as
levees and
dikes are used to prevent water flow into specific land regions. The tallest dam in the world is the 300 meter high
Nurek Dam in
Tajikistan.
[ Guinness Book of Records 1997 Pages 108-109 ISBN 0-85112-693-6]
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.
[2]
Most 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 affected the
Tigris and
Euphrates rivers, and could be quite unpredictable. The earliest recorded dam is believed to have been on the Sadd Al-Kafara at Wadi Al-Garawi, which is located about 25 kilometers south of Cairo, and built around 2600 B.C.
[ overview of the hystory of water resources and irrigation management in the near east region Mohamed Bazza ] It was destroyed by heavy rain shortly afterwards.
The oldest surviving and standing dam in the world is believed to be the
Grand Anicut, also known as the
Kallanai, an ancient dam built on the
Kaveri River in the state of
Tamil Nadu located in southern
India. It was built by the
Chola king
Karikalan, and dates back to the 2nd century AD.
[ Dikes and Dams, Thick with Politics, , Bijker, Wiebe, Isis, ]
The
Kallanai is a massive dam of unhewn stone, over 300 meters long, 4.5 meters high and 20 meters (60 feet) wide,
across the main stream of the Cauvery. The purpose of the dam was to divert the waters of the Cauvery across the fertile Delta region for irrigation via canals. The dam is still in excellent repair, and served as a model for later engineers, including the Sir Arthur Cotton's 19th-century dam across the Kollidam, the major tributary of the Cauvery. The land area irrigated by the ancient irrigation network, of which the dam was the centerpiece, was 69,000
acres (280 square kilometers). By the early 20th century the irrigated area had been increased to about 1,000,000 acres (4,000 square kilometers).
In
ancient China, the
Prime Minister of
Chu (state),
Sunshu Ao, is the first known
hydraulic engineer of China. He served
Duke Zhuang of Chu during the reign of
King Ding of Zhou (
606 BC-
586 BC), ruler of the
Eastern Zhou Dynasty. His large earthen dam 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.
[3]
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, believed to be the original place of the 800 year old dam, still carries the name ''
Dam Square''.
Types of dams
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 their size (height), intended purpose or structure.
By size
International standards define ''large dams'' as higher than 15 meters and ''major dams'' as over 150 meters in height.
[4]
By purpose
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.
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 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.
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,
embankment dams or
masonry dams, with several subtypes.
Masonry dams
Arch dams
Main articles: Arch dam
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.
[ Arch Dam Forces ]
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.
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 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.
Embankment dams
The
San Luis Dam near Los Banos, California is an embankment dam
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.
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 embankment dam type is built with
asphalt concrete core. The majority of such dams are built with rock and or gravel as main fill material. Almost 100 dams of this design have now been built world- wide since the first dam was completed in 1962. All dams built have an excellent performance record. This type of asphalt 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
A timber crib dam in
Michigan, photographed in 1978.
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.
Steel dams
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.
Beaver dams
Main articles: 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
Main articles: Hydroelectricity
As of 2005, hydroelectric power, mostly from dams, supplies some 19% of the world's electricity, and over 63% of
renewable energy.
[5] 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.
5
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
Main articles: Spillway
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.
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 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. |
|---|
| Stabilize water flow / irrigation | Dams are often used to control and stabilize water ''flow'', often for agricultural purposes and irrigation.[6] 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.[7] |
|---|
| Flood prevention | Dams such as the Blackwater dam of Webster, New Hampshire and the Delta Works are created with flood control in mind.[8] |
|---|
| Land reclamation | Dams (often called dykes or levees in this context) are used to prevent ingress of water to an area that would otherwise be submerged, allowing its reclamation for human use. |
|---|
| Water diversion | See: diversion dam. |
|---|
Siting (location)
One of the best places 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 also
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
Impact assessment
Impact is assessed in several ways: the benefits to human society arising from the dam (agriculture, water, damage prevention and power), the harm or benefits to nature and wildlife (especially fish and
rare species), the impact on the geology of an area - whether the change to water flow and levels will increase or decrease stability, and the disruption to human lives (relocation, loss of
archeological or cultural matters underwater).
Environmental impact
Dams affect many ecological aspects of a river. Rivers depend on the constant disturbance of a certain tolerance. Dams slow the river and this disturbance may damage or destroy this pattern of ecology. Temperature is also another problem that dams create. Rivers tend to have fairly homogeneous temperatures. Reservoirs have layered temperatures, warm on the top and cold on the bottom; in addition often it is water from the colder (lower) layer which is released downstream, and this may have a different
dissolved oxygen content than before. Organisms depending upon a regular cycle of temperatures may be unable to adapt; the balance of other
fauna (especially
plant life and
microscopic fauna) may be affected by the change of oxygen content.
Water exiting a turbine usually contains very little suspended sediment, which can lead to scouring of river beds and loss of riverbanks; for example, the daily cyclic flow variation caused by the
Glen Canyon Dam was a contributor to
sand bar erosion.
Older dams often lack a
fish ladder, which keeps many fish from moving up stream to their natural breeding grounds, causing failure of breeding cycles or blocking of migration paths.
[9] Even the presence of a fish ladder does not always prevent a reduction in fish reaching the
spawning grounds upstream. In some areas, young fish ("smolt") are transported downstream by
barge during parts of the year. Turbine and power-plant designs that have a lower impact upon aquatic life are an active area of research.
A large dam can cause the loss of entire
ecospheres, including
endangered and
undiscovered species in the area, and the replacement of the original environment by a new inland lake.
Depending upon the circumstances, a dam can either reduce or increase the net production of
greenhouse gases. An 'increase' can occur if the reservoir created by the dam itself acts as a source of substantial amounts of potent
greenhouse gases (
methane and
carbon dioxide) due to plant material in flooded areas decaying in an
anaerobic environment. According to the
World Commission on Dams report, when the reservoir is relatively large and no prior clearing of forest in the flooded area was undertaken, greenhouse gas emissions from the reservoir could be higher than those of a conventional oil-fired thermal generation plant.
[10] A 'decrease' can occur if the dam is used in place of traditional power generation, since electricity produced from hydroelectric generation does not give rise to any
flue gas emissions from fossil fuel combustion (including
sulfur dioxide,
nitric oxide,
carbon monoxide, dust, and
mercury from
coal).
Human social impact
The impact on human society is also significant. For example, the
Three Gorges Dam on the
Yangtze River in
China, is more than five times the size of the
Hoover Dam (
USA) and will create a reservoir 600 km long, to be used for hydro-power generation. Its construction required the loss of over a million people's homes and their mass relocation, the loss of many valuable archaeological and cultural sites, as well as significant ecological change.
[ Three Gorges dam wall completed ]
Economics
Construction of a hydroelectric plant requires a long lead-time for site studies, hydrological studies, and environmental impact assessment, and are large scale projects by comparison to traditional power generation based upon
fossil fuels. The number of sites that can be economically developed for hydroelectric production is limited; new sites tend to be far from population centers and usually require extensive
power transmission lines. Hydroelectric generation can be vulnerable to major changes in the
climate, including variation of rainfall, ground and surface
water levels, and glacial melt, causing additional expenditure for the extra capacity to ensure sufficient power is available in low water years.
Once completed and provided it is well designed and maintained, a hydroelectric power source is broadly speaking, comparatively cheap and reliable. It has no fuel and low escape risk, and as a
renewable energy source it is cheaper than both nuclear and wind power. as well as being more easily regulated to store water as needed and generate high power levels on demand, compared to
wind power.
Dam failure
The reservoir emptying through the failed
Teton Dam.
International special sign for works and installations containing dangerous forces
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.
During an armed conflict, a dam is to be considered as an "installation containing dangerous forces" due to the massive impact of a possible destruction on the civilian population and the environment. As such, it is protected by the rules of
International Humanitarian Law (IHL) and shall not be made the object of attack if that may cause severe losses among the civilian population. To facilitate the identification, a
protective sign consisting of three bright orange circles placed on the same axis is defined by the rules of IHL.
The main causes of dam failure include spillway design error (
South Fork Dam), geological instability caused by changes to water levels during filling or poor surveying (
Vajont Dam,
Malpasset), poor maintenance, especially of outlet pipes (
Lawn Lake Dam,
Val di Stava Dam Collapse), extreme rainfall (
Shakidor Dam), and human, computer or design error (
Buffalo Creek Flood,
Dale Dike Reservoir,
Taum Sauk pumped storage plant).
A notable case of deliberate dam failure (prior to the above ruling) was the
British Royal Air Force Dambusters raid on
Germany in
World War II (codenamed ''"
Operation Chastise"''), in which three German dams were selected to be breached in order to impact on German infrastructure and manufacturing and power capabilities deriving from the
Ruhr and
Eder rivers. This raid later became the basis for several films.
Notes
1. The American Heritage® Dictionary of the English Language, Fourth Edition
2. Source: Tijdschrift voor Nederlandse Taal- en Letterkunde (Magazine for Dutch Language and Literature), 1947. The first known appearance of the word ''dam'' stems from 1165. However, there is one village, Obdam, that is already mentioned in 1120. The word seems to be related to the Greek word ''taphos'', meaning ''grave'' or ''grave hill''. So the word should be understood as ''dike from dug out earth''. The names of more than 40 places (with minor changes) from the Middle Dutch era (1150 - 1500 CE) such as Amsterdam (founded as 'Amstelredam' in the late 12th Century) and Rotterdam, also bear testimony to the use of the word in Middle Dutch at that time.
3.
Needham, Joseph (1986). ''Science and Civilization in China: Volume 4, Part 3''. Taipei: Caves Books, Ltd.
4. Methodology and Technical Notes
5. Renewables Global Status Report 2006 Update, ''REN21'', published 2006, accessed 2007-05-16
6. The Impact of Agricultural Development on Aquatic Systems and its Effect on the Epidemiology of Schistosomes in Rhodesia
7. Kazakhstan
8. Blackwater Dam
9. http://bataviansforahealthyriver.org/dam_fact.htm
10. [1]
See also
★
Delta Works
★
Zuiderzee works
★
List of reservoirs and dams
★
Canal lock
★
Beaver a dam-building rodent
★
Megaprojects
★
Dam Busters
★
List of world's tallest dams
External links
★
International Commission on Large Dams (ICOLD)
★
Structurae: Dams and Retaining Structures
★
★
"Design of Small Dams", US Bureau of Reclamation, 65MB pdf
★
"Dam science" Canadian Geographic
★
International Rivers Network (IRN)
★
University of Washington Freshwater and Marine Image Bank Collection
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