A dam is a barrier that stops or restricts the flow of water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption , industrial use ,
aquaculture, and navigability.
Hydropower is often used in conjunction with dams to generate electricity. A dam can also be used to collect water or for storage of water which can be evenly distributed between locations. 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.
The word dam can be traced back to
Middle English and before that, from Middle Dutch , as seen in the names of many old cities. The first known appearance of dam occurs in 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". [ citation needed ] 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.
Selection of Dam Site 👉
The selection of Dam site for constructing a dam should be governed by the following factors.
1. Suitable foundation must be available.
2. For economy, the length of the dam should be as small as possible, and for a given height, it should store the maximum volume of water.
3. The general bed level at dam site should preferably be higher than that of the river basin. This will reduce the height of the dam.
4. A suitable site for the spillway should be available in the near vicinity.
5. Materials required for the construction of dam should be easily available, either locally or in the near vicinity.
6. The value of land and property submerged by the proposed dam should be as low as possible.
7. The dam site should be easily accessible, so that it can be economically connected to important towns and cities.
8. Site for establishing labor colonies and a healthy environment should be available near the site.
- Topography.
- Geology and Foundation Conditions.
- Availability of materials.
- Spillway size and location.
- Earthquake zone.
- Height of the Dam.
- Other factors such as cost of construction and maintenance, life of dam, aesthetics etc.
Types Of Dams Based On Design 👉
Based on design we will classify dams on following basis
Gravity Dams
A gravity dam is an enormous sized dam fabricated from concrete or stone masonry. They’re designed to hold massive volumes of water. By utilizing concrete, the load of the dam is actually in a position to withstand the horizontal thrust of water pushing towards it. Because of this it’s known as a gravity dam. Gravity primarily holds the dam right down to the bottom, stopping water from toppling it over.
Examples of Gravity dam : Grand Coulee Dam (USA), ( Nagarjuna Sagar Dam (India) and Itaipu Dam ( Between Brazil and Paraguay).
Earth Dams
An earth dam is product of earth (or soil) constructed up by compacting successive layers of earth, utilizing essentially the most impervious materials to form a core and inserting extra permeable substances on the upstream and downstream sides. A facing of crushed stone prevents erosion by wind or rain, and an ample spillway, normally of concrete, protects against catastrophic washout of water overtoping the dam.
Earth dam resists the forces exerted upon it primarily because of shear strength of the soil. Though the load of the earth dam also helps in resisting the forces, the structural behavior of an earth dam is entirely different from that of a gravity dam. The earth dams are normally built in broad valleys having flat slopes at flanks (abutments).The foundation requirements are much less stringent than those of gravity dams
Examples of earthfill dam : Rongunsky dam (Russia) and New Cornelia Dam (USA).
Rockfill Dams
A rockfill dam is constructed of rock fragments and boulders of huge size. An impervious membrane is positioned on the rockfill on the upstream side to reduce the seepage by means of the dam. The membrane is often manufactured from cement concrete or asphaltic concrete. In early rockfill dams, steel and timber membrane were also used, however now they’re out of date.
Mohale dam, Lesoto Africa
A dry rubble cushion is positioned between the rockfill and the membrane for the distribution of water load and for offering a assistance to the membrane. Generally, the rockfill dams have an impervious earth core within the center to check the seepage instead of an impervious upstream membrane. The earth core is positioned against a dumped rockfill. It’s essential to supply adequate filters between the earth core and the rockfill on the upstream and downstream sides of the core in order that the soil particles will not be carried by water and piping doesn’t occur.
Examples of rockfill dam : Mica Dam (Canada) and Chicoasen Dam (Mexico)
Arch Dams
An arch dam is curved in plan, with its convexity in the direction of the upstream side. An arch dam transfers the water stress and other forces primarily to the abutments by arch action. An arch dam is sort of appropriate for narrow canyons with sturdy flanks that are able to resist the thrust produced by the arch action.
Hoover Dam, USA
The part of an arch dam is roughly triangular like a gravity dam however the part is relatively thinner. The arch dam may have a single curvature or double curvature within the vertical plane. Typically, the arch dams of double curvature are economical and are used in practice.
Examples of Arch dam : Hoover Dam (USA) and Idukki Dam (India)
Buttress Dams
Buttresses are triangular concrete partitions which transmit the water pressure from the deck slab to the foundation. Buttresses are compression members usually spaced throughout the dam site every 6 to 30 meter, relying upon the size and design of the dam.
Buttress dams are generally referred to as hollow dams because the buttresses don’t form a solid wall stretching throughout a river valley.The deck is often a reinforced concrete slab supported between the buttresses, which are often equally spaced.
Functions and uses of Dam 👉
A dam is constructed mainly for power generation, irrigation/water supply or flood prevention. However in most of cases dams have multiple functions. The main uses of a dam are presented below.
1} 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. The reservoir water is stored at a higher level than the turbines, which are housed in a power station. Sometimes, the power station is directly in front of a dam, and pipes through the dam feed water directly to the turbines. In other cases, the power station is some distance downhill from the reservoir, and the water is fed to it through long pipes or tunnels called penstocks.
2} Stabilize water flow / irrigation
Dams are often used to control and stabilize water flow, often for
agricultural purposes and irrigation.
Plants will grow naturally on fertile soil that is watered by rain. However, in regions of the world where the climate is very dry for some seasons, the soil becomes so dry that it restricts the growth of vegetation. This problem can be overcome by irrigation, a man-made system for watering the land.
Irrigation water can be stored in
reservoirs during the rainy season, then in the drier seasons it can be released from the reservoir and distributed over the land through a system of canals. Usually, the water flows under the influence of gravity to the areas requiring it, or the water can be pumped out of the canals onto the land.
3} Flood prevention
Water from snow and rainfall will find its way into stream and rivers and eventually into the sea. After severe storms, or heavy snow or rain over a number of days, the water level in rivers can rise dramatically. Sometimes, the water flows over riverbanks or walls causing flooding of farmland, property, and in the worst cases, loss of life.
A reservoir can be used to control the amount of water flowing in a river after heavy rain. The water level in the reservoir is kept low during the rainier periods of the year. When heavy rain occurs, it is stopped by the dam and held back in the reservoir. When the reservoir gets too full, the floodwater can be passed downstream over a spillway. Sometimes, floodgates are used on top of spillways, and they can be fully or partly opened to control the amount of water let out into the river downstream. Dams such as the Blackwater dam of Webster, New Hampshire and the Delta Works are created with flood control in mind.
4} 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.
5} Water supply of urban areas/ Industrial usage
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.
6} Navigation
The construction of a dam across a river forms a reservoir that raises the water level upstream, stores the water, and slows down its rate of flow. This improves the navigation conditions upstream of the dam for ships and boats. Dangerous areas of rocks and sandbanks, previously in shallow water, become well covered, and rapids in the river disappear. Also, water from the reservoir can be released into the river
downstream during the drier seasons of the year to make sure that it is deep enough for navigation all the year round.
7} Water diversion
A dam can be used to divert all or a portion of the flow of a river from its natural course. Diversion dams do not generally impound water in a reservoir. Instead, the water is diverted into an artificial water course or canal, which may be used for irrigation or return to the river after passing through hydroelectric
generators, flow into a different river or be itself dammed forming a reservoir.
8} To contain and store waste (tailings) from mines ; eg Omai Tailings Dam, Guyana, South America which stored waste from a gold mining operation.
In addition to aforesaid functions, a raw water reservoir doesn't simply hold water until it is needed. It can be regarded as the first part of the water treatment process . The time the water is held for before it is released is known as the retention time, and is a design feature that allows larger particles and silts to settle out as well as time for the biological treatment of algae and bacteria by plankton-like creatures that naturally live within the water. In general the water quality during storage is improved especially regarding turbidity and there are indications for system’s self purification.
ADVANTAGES: 👉
1. Once a dam is constructed, electricity can be produced at a constant rate.
2. If electricity is not needed, the sluice gates can be shut, stopping electricity generation. The water can be saved for use another time when electricity demand is high.
3. Dams are designed to last many decades and so can contribute to the generation of electricity for many years / decades.
4. The lake that forms behind the dam can be used for water sports and leisure / pleasure activities. Often large dams become tourist attractions in their own right.
5. The lake's water can be used for irrigation purposes.
6. The build up of water in the lake means that energy can be stored until needed, when the water is released to produce electricity.
7. When in use, electricity produced by dam systems do not produce green house gases. They do not pollute the atmosphere.
DISADVANATGES:👉
1. Dams are extremely expensive to build and must be built to a very high standard.
2. The high cost of dam construction means that they must operate for many decades to become profitable.
3. The flooding of large areas of land means that the natural environment is destroyed.
4. People living in villages and towns that are in the valley to be flooded, must move out. This means that they lose their farms and businesses. In some countries, people are forcibly removed so that hydro-power schemes can go ahead.
5. The building of large dams can cause serious geological damage. For example, the building of the Hoover Dam in the USA triggered a number of earth quakes and has depressed the earths surface at its location.
6. Although modern planning and design of dams is good, in the past old dams have been known to be breached (the dam gives under the weight of water in the lake). This has led to deaths and flooding.
7. Dams built blocking the progress of a river in one country usually means that the water supply from the same river in the following country is out of their control. This can lead to serious problems between neighbouring countries.
8. Building a large dam alters the natural water table level. For example, the building of the Aswan Dam in Egypt has altered the level of the water table. This is slowly leading to damage of many of its ancient monuments as salts and destructive minerals are deposited in the stone work from rising damp caused by the changing water table level.
Questions and Answers About Large Dams 👉
Q. What is a large dam? How many large dams are there?
Ans: A large dam is defined by the dam industry as one higher than 15 metres (taller than a four-story building). There are more than 57,000 large dams worldwide. There are more than 300 major dams - giants which meet one of a number of criteria on height (at least 150 metres), dam volume and reservoir volume.
Q. Which countries have the most large dams?
Ans: China has over 23,000 large dams. The US is the second most dammed country with some 9,200 large dams, followed by India, Japan, and Brazil.
Q. How many are being built today?
Ans: The rate at which large dams are completed has declined from around 1,000 a year from the 1950s to the mid-1970s to around 260 a year during the early 1990s. More than 1000 large dams were under construction at the beginning of 1994. The countries with the most large dams under construction are currently China, Turkey, South Korea and Japan.
Q. Why is there so much opposition to large dams?
Ans: Large dams have provoked opposition for numerous social, environmental, economic and safety reasons. The main reason for opposition worldwide are the huge numbers of people evicted from their lands and homes to make way for reservoirs. The livelihoods of many millions of people also suffer because of the downstream effects of dams: the loss of fisheries, contaminated water, decreased amounts of water, and a reduction in the fertility of farmlands and forests due to the loss of natural fertilizers and irrigation in seasonal floods. Dams also spread waterborne diseases such as malaria, leishmaniasis and schistosomiasis. Opponents also believe that the benefits of dams have frequently been deliberately exaggerated and that the services they provide could provided by other more efficient and sustainable means.
Q: How many people have been displaced by dams?
Ans: Between 40 and 80 million, the majority of them in China and India.
Q: Aren’t people displaced by dams fairly compensated?
Ans: In nearly every case which has been studied the majority of people evicted - usually poor farmers and indigenous people - are further impoverished economically and suffer cultural decline, high rates of sickness and death, and great psychological stress. In some cases people receive no or negligible compensation for their losses. Where compensation is given, cash payments are very rarely enough to compensate for the loss of land, homes, jobs and businesses and replacement land for farmers is usually of poorer quality and smaller than original holdings.
Q: What happens when people refuse to move to make way for dams?
Ans: In many cases people have been forced out of their homes at gunpoint, in others they have simply been flooded out when the dam authorities started to fill the reservoir. In Guatemala in 1982, 369 Mayan Indians, mainly women and children, were murdered after their community refused to accept the inadequate compensation offered for the loss of their homes to the Chixoy Dam.
Q: How much land has been flooded under reservoirs?
Ans: More than 400,000 square kilometres - the area of California - have been inundated by reservoirs worldwide. This represents 0.3 per cent of the world’s land area, but the significance of the loss is greater than the figure suggests as river valley land provides the world’s most fertile farmland, and most diverse forests and wetland ecosystems.
Q: Have many people been killed in dam collapses?
Ans: More than 13,500 people have been swept to their deaths by the roughly 200 dams outside China which have collapsed or been overtopped during the 20th century. Two large dams which burst when a massive typhoon hit the Chinese province of Henan in August 1975 left an estimated 80,000 to 230,000 dead. This disaster was kept secret by the Chinese government and was only revealed to the outside world in 1995. People have also died in earthquakes caused by the great weight of water in large reservoirs. A magnitude 6.3 earthquakes caused by Koyna Dam in India in 1967 killed around 180 people.
Q: What are the benefits provided by large dams?
Ans: The majority of large dams are built for irrigation; almost all major dams are built for hydropower. Nearly one-fifth of the world’s electricity is generated by dams. Dams also provide flood control, supply water to cities, and can assist river navigation. Many dams are multipurpose, providing two or more of the above benefits.
Q. What is a large dam? How many large dams are there?
Ans: A large dam is defined by the dam industry as one higher than 15 metres (taller than a four-story building). There are more than 57,000 large dams worldwide. There are more than 300 major dams - giants which meet one of a number of criteria on height (at least 150 metres), dam volume and reservoir volume.
Q. Which countries have the most large dams?
Ans: China has over 23,000 large dams. The US is the second most dammed country with some 9,200 large dams, followed by India, Japan, and Brazil.
Q. How many are being built today?
Ans: The rate at which large dams are completed has declined from around 1,000 a year from the 1950s to the mid-1970s to around 260 a year during the early 1990s. More than 1000 large dams were under construction at the beginning of 1994. The countries with the most large dams under construction are currently China, Turkey, South Korea and Japan.
Q. Why is there so much opposition to large dams?
Ans: Large dams have provoked opposition for numerous social, environmental, economic and safety reasons. The main reason for opposition worldwide are the huge numbers of people evicted from their lands and homes to make way for reservoirs. The livelihoods of many millions of people also suffer because of the downstream effects of dams: the loss of fisheries, contaminated water, decreased amounts of water, and a reduction in the fertility of farmlands and forests due to the loss of natural fertilizers and irrigation in seasonal floods. Dams also spread waterborne diseases such as malaria, leishmaniasis and schistosomiasis. Opponents also believe that the benefits of dams have frequently been deliberately exaggerated and that the services they provide could provided by other more efficient and sustainable means.
Q: How many people have been displaced by dams?
Ans: Between 40 and 80 million, the majority of them in China and India.
Q: Aren’t people displaced by dams fairly compensated?
Ans: In nearly every case which has been studied the majority of people evicted - usually poor farmers and indigenous people - are further impoverished economically and suffer cultural decline, high rates of sickness and death, and great psychological stress. In some cases people receive no or negligible compensation for their losses. Where compensation is given, cash payments are very rarely enough to compensate for the loss of land, homes, jobs and businesses and replacement land for farmers is usually of poorer quality and smaller than original holdings.
Q: What happens when people refuse to move to make way for dams?
Ans: In many cases people have been forced out of their homes at gunpoint, in others they have simply been flooded out when the dam authorities started to fill the reservoir. In Guatemala in 1982, 369 Mayan Indians, mainly women and children, were murdered after their community refused to accept the inadequate compensation offered for the loss of their homes to the Chixoy Dam.
Q: How much land has been flooded under reservoirs?
Ans: More than 400,000 square kilometres - the area of California - have been inundated by reservoirs worldwide. This represents 0.3 per cent of the world’s land area, but the significance of the loss is greater than the figure suggests as river valley land provides the world’s most fertile farmland, and most diverse forests and wetland ecosystems.
Q: Have many people been killed in dam collapses?
Ans: More than 13,500 people have been swept to their deaths by the roughly 200 dams outside China which have collapsed or been overtopped during the 20th century. Two large dams which burst when a massive typhoon hit the Chinese province of Henan in August 1975 left an estimated 80,000 to 230,000 dead. This disaster was kept secret by the Chinese government and was only revealed to the outside world in 1995. People have also died in earthquakes caused by the great weight of water in large reservoirs. A magnitude 6.3 earthquakes caused by Koyna Dam in India in 1967 killed around 180 people.
Q: What are the benefits provided by large dams?
A: The majority of large dams are built for irrigation; almost all major dams are built for hydropower. Nearly one-fifth of the world’s electricity is generated by dams. Dams also provide flood control, supply water to cities, and can assist river navigation. Many dams are multipurpose, providing two or more of the above benefits.
Q: Surely we need dams to produce cheap and clean electricity?
Ans : Hydroelectricity is cheap to produce -- once dams are built. The problem is the huge costs of building dams and the long time it takes to build them. Itaip� Dam, for example, cost $20 billion and took 18 years to build. Actual costs for hydropower dams are also almost always far higher than estimated costs - on average around 30 per cent higher. Dam designers are often very optimistic about how much power their dams will produce and often fail to account for the impacts of droughts meaning that dams often produce less power than promised. Itaip� generates around 20 per cent less electricity than predicted.
When these high costs, delays and risks of low river flows are factored into calculations of the costs of electricity it can be seen that hydropower is now an expensive form of power generation. Hydropower should not be considered as clean power because of the destruction of river ecosystems and its many social impacts. Internationally private investors in power projects are largely avoiding large dams and prefer to invest in cheaper and less risky gas-fired power plants.
Q: What forms of power generation do large dam critics support?
Ans: Electricity use in most parts of the world is extremely wasteful. The priority before building new power plants should always be to improve the efficiency of existing energy supply and use. When new power plants are clearly needed, most environmentalists favor the use of solar and wind power, which are now on the verge of becoming commercially viable. Until these renewables are viable, gas-fired generation is cost-effective and has a far lower environmental impact than coal or oil-generation. Small dams can be a sustainable and economic source of electricity, especially in rural areas.
Q: Are dams an effective method of stopping flood damage?
Ans: Dams can stop regular annual floods but often fail to hold back exceptionally large floods. Because dams lead people to believe that floods are controlled, they lead to increased development of floodplains. When a large flood does come, damages caused are often greater than they would have been without the dam.
Q: Are there other ways of supplying water to farmers and cities?
Ans: Most water from large dams goes to farmers - only a very small percentage goes to cities. Irrigation systems around the world are in general very wasteful of water. The cheapest and most effective way of providing more water to cities is therefore to increase the efficiency of irrigated agriculture. The benefits of irrigated agriculture have in any case been seriously overstated - many large irrigation schemes have displaced huge numbers of small landholders and replaced traditional farming systems with agribusiness plantations producing expensive crops for cities and for export, increasing landlessness and rural hunger. Improving leakage and waste in urban water supply systems is also important.
Q. Do critics of large dams oppose all dams?
Ans: In general, opponents of large dams do not believe that no dam should ever be constructed. They do believe that dams (and other development projects) should only be built after all relevant project information has been made public, the claims of project promoters of the economic, environmental and social benefits and costs of projects are verified by independent experts, and when affected people agree that the project should be built.p and clean electricity?
Hydroelectricity is cheap to produce -- once dams are built. The problem is the huge costs of building dams and the long time it takes to build them. Itaip� Dam, for example, cost $20 billion and took 18 years to build. Actual costs for hydropower dams are also almost always far higher than estimated costs - on average around 30 per cent higher. Dam designers are often very optimistic about how much power their dams will produce and often fail to account for the impacts of droughts meaning that dams often produce less power than promised. Itaip� generates around 20 per cent less electricity than predicted.
When these high costs, delays and risks of low river flows are factored into calculations of the costs of electricity it can be seen that hydropower is now an expensive form of power generation. Hydropower should not be considered as clean power because of the destruction of river ecosystems and its many social impacts. Internationally private investors in power projects are largely avoiding large dams and prefer to invest in cheaper and less risky gas-fired power plants.
Q: What forms of power generation do large dam critics support?
Ans: Electricity use in most parts of the world is extremely wasteful. The priority before building new power plants should always be to improve the efficiency of existing energy supply and use. When new power plants are clearly needed, most environmentalists favor the use of solar and wind power, which are now on the verge of becoming commercially viable. Until these renewables are viable, gas-fired generation is cost-effective and has a far lower environmental impact than coal or oil-generation. Small dams can be a sustainable and economic source of electricity, especially in rural areas.
Q: Are dams an effective method of stopping flood damage?
Ans: Dams can stop regular annual floods but often fail to hold back exceptionally large floods. Because dams lead people to believe that floods are controlled, they lead to increased development of floodplains. When a large flood does come, damages caused are often greater than they would have been without the dam.
Q: Are there other ways of supplying water to farmers and cities?
Ans : Most water from large dams goes to farmers - only a very small percentage goes to cities. Irrigation systems around the world are in general very wasteful of water. The cheapest and most effective way of providing more water to cities is therefore to increase the efficiency of irrigated agriculture. The benefits of irrigated agriculture have in any case been seriously overstated - many large irrigation schemes have displaced huge numbers of small landholders and replaced traditional farming systems with agribusiness plantations producing expensive crops for cities and for export, increasing landlessness and rural hunger. Improving leakage and waste in urban water supply systems is also important.
Q. Do critics of large dams oppose all dams?
Ans : In general, opponents of large dams do not believe that no dam should ever be constructed. They do believe that dams (and other development projects) should only be built after all relevant project information has been made public, the claims of project promoters of the economic, environmental and social benefits and costs of projects are verified by independent experts, and when affected people agree that the project should be built.
Functions and uses of Dam 👉
A dam is constructed mainly for power generation, irrigation/water supply or flood prevention. However in most of cases dams have multiple functions. The main uses of a dam are presented below.
1} 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. The reservoir water is stored at a higher level than the turbines, which are housed in a power station. Sometimes, the power station is directly in front of a dam, and pipes through the dam feed water directly to the turbines. In other cases, the power station is some distance downhill from the reservoir, and the water is fed to it through long pipes or tunnels called penstocks.
2} Stabilize water flow / irrigation
Dams are often used to control and stabilize water flow, often for
agricultural purposes and irrigation.
Plants will grow naturally on fertile soil that is watered by rain. However, in regions of the world where the climate is very dry for some seasons, the soil becomes so dry that it restricts the growth of vegetation. This problem can be overcome by irrigation, a man-made system for watering the land.
Irrigation water can be stored in
reservoirs during the rainy season, then in the drier seasons it can be released from the reservoir and distributed over the land through a system of canals. Usually, the water flows under the influence of gravity to the areas requiring it, or the water can be pumped out of the canals onto the land.
3} Flood prevention
Water from snow and rainfall will find its way into stream and rivers and eventually into the sea. After severe storms, or heavy snow or rain over a number of days, the water level in rivers can rise dramatically. Sometimes, the water flows over riverbanks or walls causing flooding of farmland, property, and in the worst cases, loss of life.
A reservoir can be used to control the amount of water flowing in a river after heavy rain. The water level in the reservoir is kept low during the rainier periods of the year. When heavy rain occurs, it is stopped by the dam and held back in the reservoir. When the reservoir gets too full, the floodwater can be passed downstream over a spillway. Sometimes, floodgates are used on top of spillways, and they can be fully or partly opened to control the amount of water let out into the river downstream. Dams such as the Blackwater dam of Webster, New Hampshire and the Delta Works are created with flood control in mind.
4} 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.
5} Water supply of urban areas/ Industrial usage
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.
6} Navigation
The construction of a dam across a river forms a reservoir that raises the water level upstream, stores the water, and slows down its rate of flow. This improves the navigation conditions upstream of the dam for ships and boats. Dangerous areas of rocks and sandbanks, previously in shallow water, become well covered, and rapids in the river disappear. Also, water from the reservoir can be released into the river
downstream during the drier seasons of the year to make sure that it is deep enough for navigation all the year round.
7} Water diversion
A dam can be used to divert all or a portion of the flow of a river from its natural course. Diversion dams do not generally impound water in a reservoir. Instead, the water is diverted into an artificial water course or canal, which may be used for irrigation or return to the river after passing through hydroelectric
generators, flow into a different river or be itself dammed forming a reservoir.
8} To contain and store waste (tailings) from mines ; eg Omai Tailings Dam, Guyana, South America which stored waste from a gold mining operation.
In addition to aforesaid functions, a raw water reservoir doesn't simply hold water until it is needed. It can be regarded as the first part of the water treatment process . The time the water is held for before it is released is known as the retention time, and is a design feature that allows larger particles and silts to settle out as well as time for the biological treatment of algae and bacteria by plankton-like creatures that naturally live within the water. In general the water quality during storage is improved especially regarding turbidity and there are indications for system’s self purification.
ADVANTAGES: 👉
1. Once a dam is constructed, electricity can be produced at a constant rate.
2. If electricity is not needed, the sluice gates can be shut, stopping electricity generation. The water can be saved for use another time when electricity demand is high.
3. Dams are designed to last many decades and so can contribute to the generation of electricity for many years / decades.
4. The lake that forms behind the dam can be used for water sports and leisure / pleasure activities. Often large dams become tourist attractions in their own right.
5. The lake's water can be used for irrigation purposes.
6. The build up of water in the lake means that energy can be stored until needed, when the water is released to produce electricity.
7. When in use, electricity produced by dam systems do not produce green house gases. They do not pollute the atmosphere.
DISADVANATGES:👉
1. Dams are extremely expensive to build and must be built to a very high standard.
2. The high cost of dam construction means that they must operate for many decades to become profitable.
3. The flooding of large areas of land means that the natural environment is destroyed.
4. People living in villages and towns that are in the valley to be flooded, must move out. This means that they lose their farms and businesses. In some countries, people are forcibly removed so that hydro-power schemes can go ahead.
5. The building of large dams can cause serious geological damage. For example, the building of the Hoover Dam in the USA triggered a number of earth quakes and has depressed the earths surface at its location.
6. Although modern planning and design of dams is good, in the past old dams have been known to be breached (the dam gives under the weight of water in the lake). This has led to deaths and flooding.
7. Dams built blocking the progress of a river in one country usually means that the water supply from the same river in the following country is out of their control. This can lead to serious problems between neighbouring countries.
8. Building a large dam alters the natural water table level. For example, the building of the Aswan Dam in Egypt has altered the level of the water table. This is slowly leading to damage of many of its ancient monuments as salts and destructive minerals are deposited in the stone work from rising damp caused by the changing water table level.
Questions and Answers About Large Dams 👉
Q. What is a large dam? How many large dams are there?
Ans: A large dam is defined by the dam industry as one higher than 15 metres (taller than a four-story building). There are more than 57,000 large dams worldwide. There are more than 300 major dams - giants which meet one of a number of criteria on height (at least 150 metres), dam volume and reservoir volume.
Q. Which countries have the most large dams?
Ans: China has over 23,000 large dams. The US is the second most dammed country with some 9,200 large dams, followed by India, Japan, and Brazil.
Q. How many are being built today?
Ans: The rate at which large dams are completed has declined from around 1,000 a year from the 1950s to the mid-1970s to around 260 a year during the early 1990s. More than 1000 large dams were under construction at the beginning of 1994. The countries with the most large dams under construction are currently China, Turkey, South Korea and Japan.
Q. Why is there so much opposition to large dams?
Ans: Large dams have provoked opposition for numerous social, environmental, economic and safety reasons. The main reason for opposition worldwide are the huge numbers of people evicted from their lands and homes to make way for reservoirs. The livelihoods of many millions of people also suffer because of the downstream effects of dams: the loss of fisheries, contaminated water, decreased amounts of water, and a reduction in the fertility of farmlands and forests due to the loss of natural fertilizers and irrigation in seasonal floods. Dams also spread waterborne diseases such as malaria, leishmaniasis and schistosomiasis. Opponents also believe that the benefits of dams have frequently been deliberately exaggerated and that the services they provide could provided by other more efficient and sustainable means.
Q: How many people have been displaced by dams?
Ans: Between 40 and 80 million, the majority of them in China and India.
Q: Aren’t people displaced by dams fairly compensated?
Ans: In nearly every case which has been studied the majority of people evicted - usually poor farmers and indigenous people - are further impoverished economically and suffer cultural decline, high rates of sickness and death, and great psychological stress. In some cases people receive no or negligible compensation for their losses. Where compensation is given, cash payments are very rarely enough to compensate for the loss of land, homes, jobs and businesses and replacement land for farmers is usually of poorer quality and smaller than original holdings.
Q: What happens when people refuse to move to make way for dams?
Ans: In many cases people have been forced out of their homes at gunpoint, in others they have simply been flooded out when the dam authorities started to fill the reservoir. In Guatemala in 1982, 369 Mayan Indians, mainly women and children, were murdered after their community refused to accept the inadequate compensation offered for the loss of their homes to the Chixoy Dam.
Q: How much land has been flooded under reservoirs?
Ans: More than 400,000 square kilometres - the area of California - have been inundated by reservoirs worldwide. This represents 0.3 per cent of the world’s land area, but the significance of the loss is greater than the figure suggests as river valley land provides the world’s most fertile farmland, and most diverse forests and wetland ecosystems.
Q: Have many people been killed in dam collapses?
Ans: More than 13,500 people have been swept to their deaths by the roughly 200 dams outside China which have collapsed or been overtopped during the 20th century. Two large dams which burst when a massive typhoon hit the Chinese province of Henan in August 1975 left an estimated 80,000 to 230,000 dead. This disaster was kept secret by the Chinese government and was only revealed to the outside world in 1995. People have also died in earthquakes caused by the great weight of water in large reservoirs. A magnitude 6.3 earthquakes caused by Koyna Dam in India in 1967 killed around 180 people.
Q: What are the benefits provided by large dams?
Ans: The majority of large dams are built for irrigation; almost all major dams are built for hydropower. Nearly one-fifth of the world’s electricity is generated by dams. Dams also provide flood control, supply water to cities, and can assist river navigation. Many dams are multipurpose, providing two or more of the above benefits.
Q. What is a large dam? How many large dams are there?
Ans: A large dam is defined by the dam industry as one higher than 15 metres (taller than a four-story building). There are more than 57,000 large dams worldwide. There are more than 300 major dams - giants which meet one of a number of criteria on height (at least 150 metres), dam volume and reservoir volume.
Q. Which countries have the most large dams?
Ans: China has over 23,000 large dams. The US is the second most dammed country with some 9,200 large dams, followed by India, Japan, and Brazil.
Q. How many are being built today?
Ans: The rate at which large dams are completed has declined from around 1,000 a year from the 1950s to the mid-1970s to around 260 a year during the early 1990s. More than 1000 large dams were under construction at the beginning of 1994. The countries with the most large dams under construction are currently China, Turkey, South Korea and Japan.
Q. Why is there so much opposition to large dams?
Ans: Large dams have provoked opposition for numerous social, environmental, economic and safety reasons. The main reason for opposition worldwide are the huge numbers of people evicted from their lands and homes to make way for reservoirs. The livelihoods of many millions of people also suffer because of the downstream effects of dams: the loss of fisheries, contaminated water, decreased amounts of water, and a reduction in the fertility of farmlands and forests due to the loss of natural fertilizers and irrigation in seasonal floods. Dams also spread waterborne diseases such as malaria, leishmaniasis and schistosomiasis. Opponents also believe that the benefits of dams have frequently been deliberately exaggerated and that the services they provide could provided by other more efficient and sustainable means.
Q: How many people have been displaced by dams?
Ans: Between 40 and 80 million, the majority of them in China and India.
Q: Aren’t people displaced by dams fairly compensated?
Ans: In nearly every case which has been studied the majority of people evicted - usually poor farmers and indigenous people - are further impoverished economically and suffer cultural decline, high rates of sickness and death, and great psychological stress. In some cases people receive no or negligible compensation for their losses. Where compensation is given, cash payments are very rarely enough to compensate for the loss of land, homes, jobs and businesses and replacement land for farmers is usually of poorer quality and smaller than original holdings.
Q: What happens when people refuse to move to make way for dams?
Ans: In many cases people have been forced out of their homes at gunpoint, in others they have simply been flooded out when the dam authorities started to fill the reservoir. In Guatemala in 1982, 369 Mayan Indians, mainly women and children, were murdered after their community refused to accept the inadequate compensation offered for the loss of their homes to the Chixoy Dam.
Q: How much land has been flooded under reservoirs?
Ans: More than 400,000 square kilometres - the area of California - have been inundated by reservoirs worldwide. This represents 0.3 per cent of the world’s land area, but the significance of the loss is greater than the figure suggests as river valley land provides the world’s most fertile farmland, and most diverse forests and wetland ecosystems.
Q: Have many people been killed in dam collapses?
Ans: More than 13,500 people have been swept to their deaths by the roughly 200 dams outside China which have collapsed or been overtopped during the 20th century. Two large dams which burst when a massive typhoon hit the Chinese province of Henan in August 1975 left an estimated 80,000 to 230,000 dead. This disaster was kept secret by the Chinese government and was only revealed to the outside world in 1995. People have also died in earthquakes caused by the great weight of water in large reservoirs. A magnitude 6.3 earthquakes caused by Koyna Dam in India in 1967 killed around 180 people.
Q: What are the benefits provided by large dams?
A: The majority of large dams are built for irrigation; almost all major dams are built for hydropower. Nearly one-fifth of the world’s electricity is generated by dams. Dams also provide flood control, supply water to cities, and can assist river navigation. Many dams are multipurpose, providing two or more of the above benefits.
Q: Surely we need dams to produce cheap and clean electricity?
Ans : Hydroelectricity is cheap to produce -- once dams are built. The problem is the huge costs of building dams and the long time it takes to build them. Itaip� Dam, for example, cost $20 billion and took 18 years to build. Actual costs for hydropower dams are also almost always far higher than estimated costs - on average around 30 per cent higher. Dam designers are often very optimistic about how much power their dams will produce and often fail to account for the impacts of droughts meaning that dams often produce less power than promised. Itaip� generates around 20 per cent less electricity than predicted.
When these high costs, delays and risks of low river flows are factored into calculations of the costs of electricity it can be seen that hydropower is now an expensive form of power generation. Hydropower should not be considered as clean power because of the destruction of river ecosystems and its many social impacts. Internationally private investors in power projects are largely avoiding large dams and prefer to invest in cheaper and less risky gas-fired power plants.
Q: What forms of power generation do large dam critics support?
Ans: Electricity use in most parts of the world is extremely wasteful. The priority before building new power plants should always be to improve the efficiency of existing energy supply and use. When new power plants are clearly needed, most environmentalists favor the use of solar and wind power, which are now on the verge of becoming commercially viable. Until these renewables are viable, gas-fired generation is cost-effective and has a far lower environmental impact than coal or oil-generation. Small dams can be a sustainable and economic source of electricity, especially in rural areas.
Q: Are dams an effective method of stopping flood damage?
Ans: Dams can stop regular annual floods but often fail to hold back exceptionally large floods. Because dams lead people to believe that floods are controlled, they lead to increased development of floodplains. When a large flood does come, damages caused are often greater than they would have been without the dam.
Q: Are there other ways of supplying water to farmers and cities?
Ans: Most water from large dams goes to farmers - only a very small percentage goes to cities. Irrigation systems around the world are in general very wasteful of water. The cheapest and most effective way of providing more water to cities is therefore to increase the efficiency of irrigated agriculture. The benefits of irrigated agriculture have in any case been seriously overstated - many large irrigation schemes have displaced huge numbers of small landholders and replaced traditional farming systems with agribusiness plantations producing expensive crops for cities and for export, increasing landlessness and rural hunger. Improving leakage and waste in urban water supply systems is also important.
Q. Do critics of large dams oppose all dams?
Ans: In general, opponents of large dams do not believe that no dam should ever be constructed. They do believe that dams (and other development projects) should only be built after all relevant project information has been made public, the claims of project promoters of the economic, environmental and social benefits and costs of projects are verified by independent experts, and when affected people agree that the project should be built.p and clean electricity?
Hydroelectricity is cheap to produce -- once dams are built. The problem is the huge costs of building dams and the long time it takes to build them. Itaip� Dam, for example, cost $20 billion and took 18 years to build. Actual costs for hydropower dams are also almost always far higher than estimated costs - on average around 30 per cent higher. Dam designers are often very optimistic about how much power their dams will produce and often fail to account for the impacts of droughts meaning that dams often produce less power than promised. Itaip� generates around 20 per cent less electricity than predicted.
When these high costs, delays and risks of low river flows are factored into calculations of the costs of electricity it can be seen that hydropower is now an expensive form of power generation. Hydropower should not be considered as clean power because of the destruction of river ecosystems and its many social impacts. Internationally private investors in power projects are largely avoiding large dams and prefer to invest in cheaper and less risky gas-fired power plants.
Q: What forms of power generation do large dam critics support?
Ans: Electricity use in most parts of the world is extremely wasteful. The priority before building new power plants should always be to improve the efficiency of existing energy supply and use. When new power plants are clearly needed, most environmentalists favor the use of solar and wind power, which are now on the verge of becoming commercially viable. Until these renewables are viable, gas-fired generation is cost-effective and has a far lower environmental impact than coal or oil-generation. Small dams can be a sustainable and economic source of electricity, especially in rural areas.
Q: Are dams an effective method of stopping flood damage?
Ans: Dams can stop regular annual floods but often fail to hold back exceptionally large floods. Because dams lead people to believe that floods are controlled, they lead to increased development of floodplains. When a large flood does come, damages caused are often greater than they would have been without the dam.
Q: Are there other ways of supplying water to farmers and cities?
Ans : Most water from large dams goes to farmers - only a very small percentage goes to cities. Irrigation systems around the world are in general very wasteful of water. The cheapest and most effective way of providing more water to cities is therefore to increase the efficiency of irrigated agriculture. The benefits of irrigated agriculture have in any case been seriously overstated - many large irrigation schemes have displaced huge numbers of small landholders and replaced traditional farming systems with agribusiness plantations producing expensive crops for cities and for export, increasing landlessness and rural hunger. Improving leakage and waste in urban water supply systems is also important.
Q. Do critics of large dams oppose all dams?
Ans : In general, opponents of large dams do not believe that no dam should ever be constructed. They do believe that dams (and other development projects) should only be built after all relevant project information has been made public, the claims of project promoters of the economic, environmental and social benefits and costs of projects are verified by independent experts, and when affected people agree that the project should be built.
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