What is Dam ?

Gravity dams are well suited for blocking rivers in wide valleys or narrow gorge ways. Since gravity dams must rely on their own weight to hold back water, it is necessary that they are built on a solid foundation of bedrock.

The dam's main wall was built with earth and rock fill that spans 2,743.2 m from the island to the right-hand side of the river. Two concrete auxiliary dams span the river from the island to the left-hand-side.

Built along the Colorado River between 1931 and 1936, The Hoover Dam reaches 726 feet in height (221 metres).

Most dams that displace large populations are being built by authoritarian governments. In Burma, China, Colombia, Ethiopia, Guatemala, Sudan and other countries, dam builders have often responded to opposition with serious human rights violations. In the worst dam-related massacre, more than 440 indigenous people were killed to make way for Guatemala’s Chixoy Dam in 1982.

Examples of Gravity dam: Grand Coulee Dam (USA), ( Nagarjuna Sagar Dam (India) and Itaipu Dam ( Between Brazil and Paraguay).

Along with lithology, the structural features of rocks of the site are also thoroughly investigated. This involves detailed mapping of planes of weakness like bedding planes, schistosity, foliation, cleavage, joints, shear zones, faults and fault zones, folding and the associated features.

Equipped with 10x175 MW and 4x432 MW of turbines, Tarbela Dam is capable of producing 14.959 billion kWh of electricity per annum.

When the Hoover Dam was completed it was both the world’s largest concrete structure and largest hydroelectric power station.

Large dams belong to the most expensive investments many governments have ever made. An estimated 2000 billion dollars has been spent on dams since 1950. Due to planning errors, technical problems and corruption, dams experience average delays of 44 percent and cost overruns of 96 percent. Such massive overruns make them uneconomic.

An earth dam is made of earth (or soil) built up by compacting successive layers of earth, using the most impervious materials to form a core and placing more permeable substances on the upstream and downstream sides

It is because each one of these features modifies the engineering properties of the rocks to a great extent. While mapping these features, special attention is given to recording their attitude, spacing and nature. Joints, for instance, may not be as harmful when sparsely developed and of a closed nature, but these may render the same rock very weak and permeable when profusely developed and of open type.

The Hoover Dam is made of enough concrete to make a two lane highway from New York to San Francisco, that’s around 4000 miles (2500 kilometres).

One of Spain's tallest structures, the Almendra Dam, literally translates to almond, is located in the country's province of Salamanca. Impounding the Tormes river, this concrete gravity arch dam is part of the hydroelectric system known as the Duero Drops. The Duero Drops system is composed of five dams from Spain and three other dams nearby Portugal. The spillway seen from the photo below can disperse water at a rate of 3,039 cubic meters per second.

A facing of crushed stone prevents erosion by wind or rain, and an ample spillway, usually of concrete, protects against catastrophic washout should the water overtop the dam

Sooner or later reservoirs silt up, and the cost of maintaining dams becomes bigger than their benefits. In the United States, more than 1000 dams have been removed at great cost. When dams are not properly built or maintained, they can break. In the world’s biggest dam disaster, the failure of China’s Banqiao Dam killed an estimated 171,000 people in 1975.

Shear zones have to be searched, mapped and treated with great caution. In some cases, these may be developed to such an extent that the rock may necessitate extensive and intensive rock treatment (e.g. excavation, backfilling and grouting etc.). In still other cases their development may be to an unmanageable scale. In such cases cost factors may demand the abandoning of the site for a better alternative.

This fascinating hydroelectric dam stretches 7, 919 m from Brazil to Paraguay impounding the Parana river. It beats the Three Gorges Dam in terms of power output at an average of 89.5 TWh per annum by using 20x700 MW Francis turbines. Ten of the turbines generate power for Paraguay, while the other ten brings power to Brazil. Itaipu dam is, in fact, a series of four dams: a concrete wing dam, a main concrete dam, a rock-fill dam, and an earth-fill dam.

As of 2010, the tallest dam in the world is Nurek Dam in Tajikistan. Completed in 1980, it reaches 300 metres (984 feet) in height.

In 2012, governments and businesses installed 75 gigawatt of wind and solar power, compared with 30 gigawatt of hydropower. Such alternatives fare even better when social and environmental impacts and transmission costs are included. The International Energy Agency has proposed that 60 percent of the funds needed to achieve energy access for all should go to local renewable energy projects.

Earth dam resists the forces exerted upon it mainly due to shear strength of the soil. Although the weight 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.

Impressively, the immense volume of concrete used in constructing the dam was properly cured using large refrigeration units equivalent to 50,000 deep freezers. Another mind-blowing fact about this dam is that the Guaira Falls, once known as the world's most amazing water feature, was submerged under water when the Itaipu reservoir was filled. The Guaira Falls stood twice the height of Niagara falls and surged twice as much water.

Many dam failures have occurred in the past, these disasters can cause significant damage and threaten lives.

The strength of sound, unfractured stratified rock is always greater when the stresses are acting normal to the bedding planes than if applied in other directions. This being so, horizontal beds should offer best support for the weight of the dam. The resultant force (due to weight of the dam and thrust of the impounded water) is always inclined downstream. As such, gently upstream dipping layers offer best resistance to the resultant forces in a dam. They

The earth dams are usually built in wide valleys having flat slopes at flanks (abutments).The foundation requirements are less stringent than those of gravity dams, and hence they can be built at the sites where the foundations are less strong. They can be built on all types of foundations.

In 1889 a failure of the South Fork Dam near Johnstown, Pennsylvania, USA, killed over 2200 people.