What is Dam ?

For achieving the above objects, thorough and systematic investigations of following geological characters of the areas in general and of the preliminarily selected site in particular would have to be carried out.

Dams have fragmented two thirds of the world’s large rivers and flooded a land area the size of California. Their reservoirs contain three times as much water as all the world’s rivers, and constantly lose close to four Niagara Falls to evaporation. Dams trap 40 cubic kilometers of sediments every year, and starve deltas of the silt that protects them against the intruding sea.

It is an enclosure constructed around the construction site to exclude water so that the construction can be done in dry. A cofferdam is thus a temporary dam constructed for facilitating construction.

Such a study when interpreted properly would rule out some areas for the dam placement and help in identifying the locations that are most suitable topographically and economically, where further detailed geological investigations could be carried out. For obtaining the above information, conventional geological and geophysical surveys need to be conducted.

Dams are built for purposes such as generating hydroelectricity, creating recreational areas, minimizing flood risks, diverting water for irrigation, and providing water for towns and cities.

That was the first time in history when the Colorado River was under the control of man. The dam impounds the Colorado River which consequently forms Lake Mead, the largest reservoir by volume (when full) in the United States.

A coffer dam is usually constructed on the upstream of the main dam to divert water into a diversion tunnel (or channel) during the construction of the dam

Dams generate 16% of the world’s electricity and irrigate food crops for 12-15% of the world’s population. To a lesser extent, dams have also been built for water supply, flood protection, navigation and tourism purposes. Most dams have been built for irrigation, but 80% of the water they store is used for hydropower.

single most important feature that must be known thoroughly at the site and all around and below the valley up to a reasonable depth is the Lithology, i.e. types of the rocks that make the area. Surface and subsurface studies using the conventional and latest techniques of geological and geophysical investigations are carried out.

The type of dam built in any given situation depends on factors such as the intended purpose, environmental considerations, available finances and the location.

Known as the world's largest hydropower dam, the Three Gorges dam stretches out 2.3 km to span and impounds the Yangtze river in the Hubei province in China. Capable of producing 87 TWh of electricity per annum, this hydropower dam uses 32x700 MW and 2x50 MW of Francis turbines. Its structural profile is designed with a large base width of 115 m and tapers to 40 m at the crest.

When the flow in the river during construction of the dam is not much, the site is usually enclosed by the coffer dam and pumped dry. Sometimes a coffer dam on the downstream of the dam is also required.

studies would reveal the type, the composition and textures of the rocks exposed along the valley floor, in the walls and up to the required depth at the base. Rocks are inherently anisotropic materials, showing variation in properties in different directions.

Dams block the migration of fish, deplete rivers of oxygen, and interfere with the biological triggers that guide fish. They also reduce the ability of rivers to clean themselves. Due to dam building and other factors, the population of freshwater species declined by 37% between 1970-2008 - more than the populations of any other ecosystems. Tropical freshwater populations declined by a stunning 70%.

Intended not only to produce electricity, the three Gorges Dam was also constructed to increase the shipping capacity of the Yangtze river and mitigate the chances of flooding downstream by providing large water storage space.

Different types of dams include masonry dams, embankment dams and arch-gravity dams.

A gravity dam is a massive sized dam fabricated from concrete or stone masonry. They are designed to hold back large volumes of water. By using concrete, the weight of the dam is actually able to resist the horizontal thrust of water pushing against it. This is why it is called a gravity dam.

Yet, it is of great significance to know what class of rocks make up the area – igneous, sedimentary or metamorphic; and also which type and sub-type is more prevalent; and whether it is only one class of the rock existing there, or more types of the same or different classes of the rocks are found.

Dams are not climate-neutral. Particularly in the tropics, organic matter rotting in their reservoirs emits methane, an aggressive greenhouse gas. Scientists have estimated that reservoirs account for 4% of all human-made climate change, equivalent to the climate impact of aviation. The floods and droughts caused by climate change in turn make dams less safe and less economic.

Gravity essentially holds the dam down to the ground, stopping water from toppling it over.

Beavers create their own dams from sticks and mud as protection from predators and also to help provide easy access to food.

Considered as the largest earth-filled dam in the world, the Tarbela Dam in Pakistan impounds the Indus river to serve as irrigation supply, flood control, and to produce hydroelectric power. In order to properly divert the Indus river, the dam's construction had to be done in three stages where large tunnels had to be constructed to act as diversion channels.

Dams are huge engineering projects that require vast financial resources. For example, the construction of the Itaipu Dam on the border of Brazil and Paraguay (completed in 1984) cost around US$20 billion.

Dams have displaced an estimated 80 million people, with 23 million alone in China. Displacement robs people who are already poor and marginalized of their resources, skills and cultural identity, and impoverishes them further. Dams have also negatively impacted about 500 million people living downstream. The benefits of dams often bypass the people who sacrifice their livelihoods for them.

It is possible that the entire site may be made up of one type of rock, say, for example, fine textured sandstones; it is also possible that it may have alternate layers of sandstones, shales and clays, all of varying types. Complex lithology definitely poses challenging design problems.