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A dam is an obstruction constructed across a river or stream to impound water upstream. Humans mastered the art of damming the rivers as early as perhaps 5000 B.C. Many believe, reasonably, that Beavers, small rodent like creatures, might have inspired early human dam builders. In India and other ancient civilizations ancient dam like structures were discovered by archaeologists.
Beaver built dams were a nature’s marvel. They cut large trees with their teeth to bring them down across rivers, put logs, pieces of wood, branches and twigs to build a crib like structure and fill them with stones and earth! Beaver dams some times turn out to be useful to humans and other animals too.
The Aswan Dam across the River Nile in Egypt, is the first modern dam to be constructed. It was completed in 1902 for controlling the floods of Nile River and irrigating fields. It was followed by many other dams across the world. Roosewelt dam is the first major dam in USA. It was constructed in 1911 across Salt River, Arizona.
There are many kinds of dams and there are various technical classifications. However, the following are the important types of dams.
Earth Dams: Earth dams are constructed by laying compacted earthen bunds across the river. The type of soil used for the bund and compaction efficiency hold key to the safety of dam. Its upstream sides are usually protected by a surface layer flat rock, called riprap.
There are three sub-types of earthen dams. Homogeneous type dam is constructed of a same type of soil throughout the cross section. Zoned type dams, has an impervious core surrounded by a relatively pervious zone. Diaphragm type dam has a tall impervious wall of less than 10 m thickness inside the cross section.
Ex: Green Mountain dam on Colorado River, US,
Trinity dam in California, US and
Hirakud Dam on Mahanadi River, India.
Rock-Fill dams: These are constructed by making a compact mass of obstruction using loose rocks and boulders across the river. Often, an impervious RCC slab is laid upstream.
Ex: The Salt Springs dam in California, US and
The Cougar dam on McKenzie river, Oregon, US.
Solid Masonry Gravity dams: A Gravity dam resists the pressure of water by its sheer weight. As such it is very heavy and need strong rocky foundation. Masonry gravity dams are more stable and stronger than earth and rockfill dams.
Ex: Aswan Dam (Old) Egypt and
Roosewelt Dam, USA.
All the above types of dams are known from ancient times. Other types are modern, came into being only in 20th century.
Gravity dams, now a days, are also constructed of Concrete. And known as Concrete Gravity Dams.
Ex: Hoover Dam, USA
Bhakra Dam, India and
Three Gorges dam, China.
Hollow Masonry dams: They are similar to solid gravity type dams but have 35 to 40% less concrete or masonry. They consists of an inclined RCC slab on upstream side which transmits the water weight to supporting buttresses (vertical pier like walls) or arches built below it. It is complex in design and construction and is labour intensive.
Uniqueness of this type is extendibility. The upstream wall can be extended by laying extra supports, to raise the height of dam. Power houses and other ancillary structures can be constructed in the open space below the slab. These are also called as Buttress Dams.
Ex: Itaipu Dam on Parana River, Brazil – Paraguay.
Arch dams: These are RCC dams much thinner than gravity dams. They are curved in plan, with concave face downstream. It retains most water pressure by transmitting it to the sides of dam. As such it is built only when the river valley is narrow and solid rocky abutments are available to anchor the dam. It is aesthetically most pleasing type of all dams.
Ex: Sautet dam on Drac River, France,
Daniel Johnson dam, Canada (Multi-Arch),
Mauvoisin Dam on Drause River, Switzerland and
Idukki Dam on Periyar River, Kerala, India.
Steel dams: These are not used currently for major projects. Their use as minor and coffer dams is prevalent.
Ex: Redridge Steel Dam, Michigan, US
Timber dams: These are also temporary in nature and small in size. These are constructed of well seasoned timber, in the form of cribs with planking on upstream. Its life is generally limited to 25 to 30 years.
Rubber dam: This most recent type of dam used huge special rubber cylindrical shells. They are inflated by compressed air (or some times water), to store water and pressure is released to deflate it and release water, which simply overflows the shell.
They need extreme care in design and erection but are very easy and quick to erection. They are limited to small projects.
Ex: Janjhavathi Project, Andhra Pradesh, India.
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