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26 August 2011

Dams

A dam is a structure that blocks or impounds water for the purpose of retaining the water or preventing or diverting the flow of water. Dams can serve many purposes, including water storage, irrigation, and flood prevention. A hydroelectric dam uses moving water to generate electricity. Of the 80,000 dams found throughout the United States, only about 2,388 dams can be used to create electricity.

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BACKGROUND

A dam is a structure that blocks or impounds water for the purpose of retaining the water or preventing or diverting the flow of water. Dams can serve many purposes, including water storage, irrigation, and flood prevention. A hydroelectric dam uses moving water to generate electricity. Of the 80,000 dams found throughout the United States, only about 2,388 dams can be used to create electricity.

A hydroelectric dam forces water to pass through turbines, which are engines that are powered by water moving blades that rotate around a spindle. The turbine then turns generators, which change the water's kinetic energy (energy of the moving object or body) into electrical energy (energy created by the movement of electrons). The electricity created by hydroelectric dams may be distributed to homes and businesses through power lines. Depending on the size, type, and location of a dam, its effect on the surrounding environment may be significant and may include the disruption of local wildlife or human populations, erosion of soil, and destruction of surrounding habitats.

Many ancient cultures, including the Mesopotamians, Egyptians, and Chinese, built dams for water storage or irrigation. More than 2,000 years ago, the Greeks used water to turn wheels that ground wheat into flour. The modern-day hydroelectric dam was introduced in the late 19th Century.

In 1880, a water turbine was connected to a generator that powered lamps at a chair factory in Grand Rapids, Michigan. By 1886, there were 45 hydroelectric plants in the United States and Canada. This number grew to 200 by 1889. In 1886, the first hydroelectric plant was built in the western United States. By the beginning of the 20th Century, 15% of the electricity in the United States was created by hydroelectric power plants. This increased to 25% by 1920 and 40% by 1940. Better turbines brought about the construction of mega-dams in the mid-1900s. The 1930s saw the beginning of the hydropower era in the United States when several mega-dams were constructed. In 1936, the Hoover Dam, the largest dam in the world at the time, was built on the Colorado River. Nine more dams were built along the Colorado River, including the Glen Canyon Dam, sending water and power to various parts of the southwestern United States. The construction of the Grand Coulee Dam in Washington state, the largest hydropower producer in the United States with 6,809 megawatts (MW) generating capacity, was completed in 1941.

Dam development in the United States and Europe hit its peak in the middle of the 19th Century. As the environmental movement gained momentum in 1960s, perceptions about dams, especially large dams like the Hoover Dam, changed. Instead of being symbols of modernity, some began to consider dams as sources of social and environmental destruction. Opponents to dam building, called dam busters, argue that dams: damage aquatic ecosystems, killing or altering the migratory patterns of fish and other aquatic life; affect water quality; and are detrimental to the health of local human populations as reservoirs become breeding grounds for vector-borne diseases transmitted by organisms such as mosquitoes.

As opposition to dams became more prevalent in the United States and Europe, dam construction moved to developing countries. American mega-dams became the models for large dam construction projects around the world. Currently, dams with the most generating capacity are the Three Gorges Dam in China at 20,000MW, Guri Dam in Venezuela at 10,000MW, and the Itaipu Dam on the border of Paraguay and Brazil with 12,500MW.

Today, about seven percent of the electricity produced in the United States, enough for 28 million households, comes from hydroelectric power. As of the year 2000, the United States was second only to Canada in electricity produced from hydropower plants, generating over 300 billion kilowatt-hours.

According to the U.S. Department of Energy (DOE), the undeveloped hydropower potential is significant. The DOE claims another 30,000MW of electricity could come from hydropower. Although the percentage of U.S. electricity that comes from hydropower has decreased since its height in the middle of the 20th Century, the net generation of electricity from hydropower has increased 5.2% in the United States since 2007. As of 2006, 2,388 licensed hydropower plants existed in the United States.

Unlike fossil fuels, hydropower is a renewable and sustainable resource. Hydropower is the largest renewable source of energy in the United States. Some environmental groups support the use of hydroelectricity as a renewable resource, as long as the environmental impact of dams, hydroelectric power plants, and reservoirs of stored water is minimized. Government agencies such as the DOE and the U.S. Department of the Interior (DOI) consider hydroelectricity to be a component of a clean, efficient, and safe energy strategy.

TECHNIQUE

Hydroelectric dams: Hydroelectric dams use the planet's hydrologic, or water cycle, to create electricity. The hydrologic cycle is driven by the sun. It begins as water evaporates from lakes and oceans creating moisture that forms clouds. Precipitation, such as rain or snow, then falls back to the earth replenishing bodies of water.

Hydroelectric dams use these continuously replenished bodies of water to create electricity. A dam can block a river, creating a reservoir or lake behind it. Sometimes a dam is built on a large river with a significant drop in elevation. The water is released through the dam and falls through a shaft or penstock. The falling water turns the rotating blades of turbines found at the base of the dam. The turbines' blades turn a shaft attached to the generator's rotor. As the shaft turns, the rotor causes electromagnets to move past conductors, which causes the flow of electricity. Power lines connected to the generator carry electricity to households and businesses.

Other types of dams: Many dams serve more than one purpose. Of the 80,000 dams found throughout the United States, only about 2,388 dams can be used to create electricity. Dams may also store water for use by surrounding communities and for livestock, the irrigation of crops, or recreation. Thirty-five percent of dams are used for recreation. Diversion dams may transfer water from one area to another. When rainfall is high, dams can control the flow of water and may prevent flooding.

Structural categories: The type of dam constructed on a river or stream depends on the location, materials available, and the dam's purpose. All dams are built to resist the force of water. A dam usually falls into one of four structural categories: arch, buttress, gravity, or embankment. Arch dams are concrete arches facing upstream and anchored in rock. The force of the water presses against the arch, holding back the water in the reservoir. They are usually built in narrow canyons. A buttress dam typically has a steel framework and downstream supports. It is usually constructed in wider valleys and may be very costly. A gravity dam has a solid, stable design and can be made of masonry or cement dug into a rock foundation. An embankment dam may be less expensive to build because it uses local materials. These materials, however, might be permeable to water. An embankment dam may have an impermeable core to prevent water from seeping through it.

Impoundment dam: A storage or impoundment dam can be built on a river, blocking the flow of water, raising the water level, and creating a reservoir or lake behind the dam. The stored water can be released through the hydroelectric dam to create electricity when needed. The water can be released in short or long cycles, depending on the season or electricity needs. The environmental impact of impoundment dams may be significant. The large reservoirs they create may disrupt the migration and reproduction of fish and wildlife, displace human populations, or flood surrounding areas. Earthquakes may affect the structural integrity of dams and the weakened dams may pose a threat to the surrounding communities.

Pumped storage: Pumped-storage plants are not necessarily a true energy source because they may not produce electricity. A pumped-storage plant usually pumps water from a lower to higher reservoir. The water can be released to the lower reservoir through a hydroelectric dam when more electricity is needed.

Run-of-river: Run-of-river plants do not impede rivers or streams like other power plants that change the rivers' water levels, course, or flow. Although some run-of-river plants have no dams, these plants usually consist of low dams that allow the rivers or streams to flow naturally through the plant, which in turn generates electricity. The amount of electricity generated depends on the amount of water flowing in the river.

Examples of dams: Many large-scale dams were constructed under challenging conditions and are now internationally renowned as engineering marvels. The Hoover Dam stands 726.4 feet (221.3 meters) high. It is a national historic landmark and has been called one of America's seven modern civil engineering wonders. Over 21,000 workers built the Hoover Dam during the Great Depression. At the time it was built, the Dam was the top hydropower producer in the world. It now produces 2,074MW of electricity, or the same amount as two nuclear power plants.

When completed, the Three Gorges Dam was the world's largest hydropower construction project. The Three Gorges Dam is located in the Xilingxia gorge of the Yangtze River in China. This hydroelectric dam generates 20,000MW of electricity, the most in the world. It is constructed from 14.86 million cubic meters of concrete. The reservoir created by the dam floods 632 kilometers (km) of land and displaced 1.13 million people, according to the China Three Gorges Project.

Plant sizes: Hydropower plants range in size and capacity, depending on their location and use. The U.S. Department of Energy defines a large hydropower plant as having a capacity of 30MW. A small hydropower plant has a capacity of 100kW to 30MW.

The smallest hydropower plant, often called a micro hydropower plant, has a capacity of about 100kW and may be used by an individual home, farm, or village.

Cost: The cost of construction and operation of a dam depends on the location, size, and type of dam. According to the U.S. Department of Energy, the average operating cost of a hydropower plant is four cents per kilowatt-hour. Average maintenance cost is three cents per kilowatt-hour. The average lifespan is over 50 years and generating capacity is 31MW.

Ownership: Non-federal hydropower plants are licensed by the U.S. Federal Energy Regulatory Commission. Licenses are usually reviewed after every 30-50 years. The private sector (utility, cooperative, industrial owners) owns 69% of the 2,388 hydropower plants in the United States. However, the public sector owns 73% of the 74,872MW of electricity generating capacity. Seven federal agencies own the largest percentage (51%) of the total U.S. hydroelectric capacity.

Usage: All but a few states use hydropower. Most hydroelectric power plants are located in California and New York, although plants in California and Washington have the most capacity for electricity generation, according to the U.S. Department of Energy (DOE).

Environmental impact: Many hydroelectric dams were built in the late 19th and early 20th Centuries when little was known about the environmental impact of dams. Large dams may disrupt the natural flow of rivers; in fact, they may change a river habitat to a lake habitat. The changes in water flow may affect the water temperature and oxygen levels, which, in turn, may harm aquatic life.

Fish mortality: Dams may control the flow of water, causing water levels to rise and fall dramatically, depending on the need for electricity, recreation, etc. Fish may become stranded in low water or soil may be washed away in high waters. Dams may disrupt the migration and reproduction of aquatic life such as fish, which may swim into the penstock and get caught in the blades of the turbine.

Human displacement: Reservoirs or lakes created by impoundment dams may displace human populations. The Three Gorges Dam in China, the largest dam in the world, reportedly displaced more than 1.2 million people.. Wildlife habitats may be destroyed by flooding. The U.S. Department of Energy is sponsoring research into technologies that will improve turbines, optimize hydropower operations, and reduce the harmful effects of hydropower plants on the environment.

THEORY/EVIDENCE

History: Ancient civilizations created dams to improve the quality of human lives and expand human civilization. Dams allowed for the storage of water and the irrigation of crops. In the late 18th Century, dams took on a new purpose. With the creation of turbines and generators, hydroelectric dams began using the natural flow of water to create electricity. Unlike fossil fuels, this energy source is renewable, meaning it is never used up.

By the 1900s, large dams were being built across the United States and other countries. The Hoover Dam, once the largest dam in the world, is now dwarfed by mega-dams being built around the world. Dams cross most of the globe's largest rivers. The Yellowstone River is the only major river in the United States that is not dammed.

Hydroelectric power: Hydroelectric power plants are renewable sources of energy. They may be inexpensive and cause little pollution with the proper technology. Many environmental groups support the use of hydropower instead of fossil fuels or nuclear power for electricity generation. However, dams may have serious negative effects on the surrounding areas.

Impact: Concern began to grow over the environmental impact of dams in the 1980s and 1990s. Evidence was mounting that dams may have a harmful effect on wildlife, human populations, habitats, and water and land quality. Some reservoirs created by dams have flooded towns and displaced local people.

Dam opponents: "Dam busters" are opponents to short-sighted dam-building projects that do not consider the dam's negative impact on local wildlife and human populations. They charge government agencies, utilities, and international loan corporations like the World Bank with failing to recognize the high environmental and human cost of many dam projects.

Support: Some government agencies, such as the U.S. Department of Energy (DOE) and the United Nations, and environmental groups, like the Union of Concerned Scientists, support reducing the environmental impact of dams. Over the last 10 years, many groups have assessed the impact of dams on people, animals, and the environment and have considered how dams might be improved. According to the DOE, hydropower represents an important renewable source of energy, but it may also have a negative effect on the environment. As a result of these assessments, the DOE created the Advanced Hydropower Technology (AHT), which supports the development of new hydropower technologies that have less environmental impact.

The Low Impact Hydropower Institute created a program called Low Impact Hydropower Certification. The program awards certificates to dams that meet certain criteria for protecting the environment. The dams are rated based on river flows, water quality, fish passage and protection, watershed protection, threatened and endangered species protection, cultural resource protection, recreation, and facilities recommended for removal. Thirty-seven facilities are currently certified by the institute.

HEALTH IMPACT/SAFETY

General: Some government agencies, such as the U.S. Department of Energy (DOE) and the United Nations, and environmental groups, like the Union of Concerned Scientists, support reducing the environmental impact of dams. Over the last 10 years, many groups have assessed the impact of dams on people, animals, and the environment and have considered how dams might be improved. According to the DOE, hydropower represents an important renewable source of energy, but it may also have a negative affect on the environment. For instance, dams may change water quality and aquatic habitats, and turbines in the water may kill or injure fish and other wildlife. As a result of these assessments, the DOE created the Advanced Hydropower Technology (AHT), which supports the development of new hydropower technologies that have less environmental impact.

The Low Impact Hydropower Institute created a program called Low Impact Hydropower Certification. The program awards certificates to dams that meet certain criteria for protecting the environment. The dams are rated based on river flows, water quality, fish passage and protection, watershed protection, threatened and endangered species protection, cultural resource protection, recreation, and facilities recommended for removal. Thirty-seven facilities are currently certified by the institute.

Aquatic life: Altering the natural flow of rivers or changing river habitats into lake habitats may disrupt aquatic life. Fish may be killed by turbines found in hydroelectric dams. Migration and reproduction of fish may also be disrupted by dams.

Dams may affect the migration of local fish populations. Many environmental groups have argued that dams may disrupt the migration of fish in the Pacific Northwest, including salmon, coho, chinook, and steelhead fish. Since the construction of four dams on the Snake River, the migrating salmon population has declined 90%. A recent study, however, showed that migration of some endangered steelhead and chinook through dammed waters was as high as those fish migrating in rivers without dams.

The U.S. Department of Energy's "Large Turbine Testing Project" is currently testing new turbines that operate more efficiently, generate more electricity, and improve water quality. The new turbines may also allow fish to pass safely as opposed to being caught in the turbines, which can cause injury or death. Improvements to fish ladders and other technologies that may help migrating fish pass through dams are also being developed and tested.

Cultural heritage: The creation of reservoirs by dams may cause flooding of surrounding areas. Local communities or towns may be affected, and people may be forced to relocate. Local artifacts, such as temples, shrines, burial sites, or archeological artifacts, may be destroyed.

Flooding land may disrupt the practices and traditions of native people. According to the Cree people of Quebec, Canada, the construction of dams in their communities destroyed sources of food and age old patterns of life. Native tribes and ethnic minorities are more likely to be displaced, according to the World Commission on Dams.

Diseases: Dams may affect the health of people living in nearby areas. Small and large dams may help spread disease by creating reservoirs whose stagnant waters serve as breeding grounds for pests like mosquitoes that may carry diseases, such as malaria, schistosomiasis, filariasis, onchocerciasis, and dracunculosis.

One study found that children living close to small dams in Ethiopia were more likely to have malaria than children living in areas far from dams. Another study in Ethiopia found that dams may create good conditions for spreading schistosomiasis and malaria by providing the still water necessary for the breeding of parasites that spread these infections. Schistosoma is spread by a parasite (called Schistosoma mansoni) that lives in freshwater snails and has become a public health problem at dam sites such as Kariba, Aswan, and Akosombo in Africa. The same is true of malaria, which is spread by mosquitoes that breed in stagnant water. The World Commission on Dams states that there is an increase of malaria transmission and disease at most reservoir projects in malaria-endemic areas.

However, building dams away from populated areas may help control the spread of disease. Flushing streams, controlling water levels and salinity, and encouraging local communities to use mosquito nets may improve water quality and reduce the risk of waterborne-illnesses.

Displacement: The World Commission on Dams states that 40-80 million people worldwide have been displaced by dams. Resettlement may be crippling for an individual or family. Displaced people often have trouble making a living after they are resettled. The employment, housing, or compensation they were promised may not meet their needs after resettlement. People living downstream of dams often have trouble making a living after local fisheries are disrupted by the dams. Native tribes and ethnic minorities are more likely to be displaced, according to the World Commission on Dams.

In India, about 44,000 people are displaced each year by dam projects. The Three Gorges Dam was built on the Yangtze River in China and reportedly displaced more than 1.2 million people, submerged many rare artifacts and cultural sites, and destroyed the habitat of rare plants and animals.

Earthquakes: There is some evidence that heavy reservoirs created by dams may cause earthquakes. According to the World Commission on Dams, the reservoir water exerts pressure on cracks and fissures underground. When the pressure is great enough, the water lubricates faults that are otherwise prevented from slipping by the friction of the rocks. The commission identified more than 90 cases of earthquakes triggered by reservoirs.

Energy: Hydroelectric dams generate a sustainable and renewable source of electricity.

Flooding: Many dams are used to help prevent flooding. They can be used to regulate river levels by temporarily storing flood water and slowly releasing it over time.

However, if a dam breaks, it, may affect the safety of surrounding communities. In 1889, the South Fork Dam in Johnstown, Pennsylvania, broke after a heavy rain, resulting in the deaths of more than 2,200 people. Earthquakes may damage the structural integrity of dams in the affected area. However, it is uncommon for dams to break in the United States.

Greenhouse gases: Dams may cause the creation of reservoirs or lakes that flood the surrounding areas. If the area has a lot of vegetation, the plants and trees may rot underwater. The rotting plants can give off large amounts of methane gas, which may contribute to global climate change. The World Commission on Dams found that the flooding caused by a dam in Brazil produced greenhouse gases similar to those of a thermal power plant. The level of greenhouse gas emission from reservoirs or lakes may be highly variable depending on the location.

Livelihood: The diverting of water from rivers or creation of reservoirs or lakes by dams may impact local human populations. Although dams may improve irrigation, they may also divert water from fields and watering holes, negatively impacting crops and livestock. Water released by dams may erode the soil, making it unfit for farming.

Mercury: Mercury poisoning may also be a concern in some areas. Impounded water created by dams floods the surrounding land and may dissolve mercury located in the soil. The mercury then gets into local fish. Humans who eat this fish may suffer from mercury poisoning, which may cause neurological and other health problems. Some of the Cree people of Quebec, Canada, who ate contaminated fish suffered from mercury poisoning after a dam was built near their community.

Recreation: Dams create reservoirs, which are often used for recreational purposes, such as boating, swimming, and fishing. As a result, these reservoirs can potentially enhance local tourism.

Water supply: Properly constructed and maintained dams help provide consistent water supplies. This is particularly important when there are droughts.

FUTURE RESEARCH OR APPLICATIONS

The basic design of hydroelectric dams has not changed since mid-1900s, but advances are being made in turbine technology. Turbines are found at the end of the penstock, or shaft, which runs from the top of the dam to the bottom carrying water. As water flows through the penstock, the blades of the turbine turn a shaft connected to the rotor of a generator.

The U.S. Department of Energy's "Large Turbine Testing Project" is currently developing new turbines that operate more efficiently, generate more electricity, and improve water quality. The new turbines may also allow fish to pass safely as opposed to being caught in the turbines, which can cause injury or death. Improvements to fish ladders and other technologies that may help migrating fish pass through dams are also being developed and tested.

Two new types of renewable resources may be able to create electricity using moving water without impoundment dams or diverting water flows. Hydrokinetic and wave energy technologies may be able to use streams, estuaries, ocean currents, constructed waterways, and devices that capture the energy of ocean waves, according to the U.S. Department of Energy. These technologies are in the preliminary stages of development and several projects are underway to test their effectiveness. The Federal Energy Regulatory Commission issued 36 permits in 2008 researching the impact of hydrokinetic technologies along the lower Mississippi river. The Wave Energy Systems and Renewables Facility (WESRF) at Oregon State University has deployed two different types of devices called wave energy conversion buoys, one that harnesses wave motion to create an electric current and the other that uses pressurized seawater to turn turbines. In September 2008, WESRF and Columbia Power Technologies successfully tested a prototype of a waver energy generator. Tidal energy harnessed by turbines in New York's East River delivered hydrokinetic energy to a supermarket and parking garage in 2006 and 2007. Verdant Power, the installers of the project, aims to increase the number of turbines and supply more homes with electricity by 2010.

AUTHOR INFORMATION

This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

  • Brewster D. Environmental management for vector control. Is it worth a dam if it worsens malaria? BMJ. 1999 Sep 11;319(7211):651-2. View abstract
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  • Idaho National Laboratory. www.inl.gov
  • Low Impact Hydropower Institute. http://lowimpacthydro.org
  • Natural Standard: The Authority on Integrative Medicine. www.naturalstandard.com
  • Power Scorecard. www.powerscorecard.org
  • Science Encyclopedia, vol. 2. www.science.jrank.org
  • Union of Concerned Scientists. www.ucsusa.org
  • U.S. Department of Energy (DOE). www.energy.gov
  • U.S. Fish and Wildlife Service. www.fws.gov
  • U.S. Geologic Survey. www.usgs.gov
  • Welch DW, Rechisky EL, Melnychuk MC, et al. Survival of migrating salmon smolts in large rivers with and without dams. PLoS Biol. 2008 October28;6(10): e265. View abstract
  • World Commission on Dams. www.dams.org


Copyright © 2011 Natural Standard (www.naturalstandard.com)
 
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