What is Dam ?

Itaipu Dam (Brazil)
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.

The appearance is similar to a single-arch dam but with a distinct vertical curvature to it as well lending it the vague appearance of a concave lens as viewed from downstream.

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.

The multiple-arch dam consists of a number of single-arch dams with concrete buttresses as the supporting abutments, as for example the Daniel-Johnson Dam, Québec, Canada.

Impressively, the immense volume of concrete used in constructing the dam was properly cured using large refrigeration units equivalent to 50,000 deep freezers.

The multiple-arch dam does not require as many buttresses as the hollow gravity type, but requires good rock foundation because the buttress loads are heavy.

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.

In a gravity dam, the force that holds the dam in place against the push from the water is Earth's gravity pulling down on the mass of the dam. The water presses laterally (downstream) on the dam, tending to overturn the dam by rotating about its toe (a point at the bottom downstream side of the dam).

The dam's weight counteracts that force, tending to rotate the dam the other way about its toe. The designer ensures that the dam is heavy enough that the dam's weight wins that contest. In engineering terms, that is true whenever the resultant of the forces of gravity acting on the dam and water pressure on the dam acts in a line that passes upstream of the toe of the dam.

The Guaira Falls stood twice the height of Niagara falls and surged twice as much water.

Furthermore, the designer tries to shape the dam so if one were to consider the part of dam above any particular height to be a whole dam itself, that dam also would be held in place by gravity. i.e. there is no tension in the upstream face of the dam holding the top of the dam down.

Atatürk Dam (Turkey)
Located on the Euphrates river, the Atatürk Dam is the largest in Turkey and ranks sixth from the largest earth-and-rock filled embankment dams in the world

The designer does this because it is usually more practical to make a dam of material essentially just piled up than to make the material stick together against vertical tension.

It is the centerpiece of the 22 dams that exist on the Euphrates and the Tigris, which comprise the integrated sectors of the Southeastern Anatolia Project, or GAP in Turkish (Güney Doğu Anadolu Projesi).

Note that the shape that prevents tension in the upstream face also eliminates a balancing compression stress in the downstream face, providing additional economy.

The Atatürk reservoir has a capacity of 48.7 cubic kilometers of water and equipped with 8x300 MW Francis turbines, which generate 8,900 GWh of electrical power per annum.

The designer also ensures that the toe of the dam is sunk deep enough in the earth that it does not slide forward.

For this type of dam, it is essential to have an impervious foundation with high bearing strength.

The construction of the dam wiped out many important historical sites like the birthplace of the Ancient Greek poet Lucian.

When situated on a suitable site, a gravity dam can prove to be a better alternative to other types of dams. When built on a carefully studied foundation, the gravity dam probably represents the best developed example of dam building.

Since the fear of flood is a strong motivator in many regions, gravity dams are being built in some instances where an arch dam would have been more economical.

Kariba Dam (Zimbabwe)
One of the largest in Africa, the Kariba Dam supplies 1,626 MW of power to the Copperbelt parts of both Zambia and Zimbabwe.

Impounding the Zambezi river, Kariba dam is outfitted with 10 types of Francis turbines capable of outputting an average of 6,400 GWh of electrical power per year.

Gravity dams are classified as "solid" or "hollow" and are generally made of either concrete or masonry. This is called "zoning".

The core of the dam is zoned depending on the availability of locally available materials, foundation conditions and the material attributes.