In this article  we are going to cover the topic about Dam, Types of dams, selection criteria required for the site of the dam and on what basis the type of dam is selected for a particular site and the factors effecting the selection of site of dam.

A Dam is an obstruction or barrier built across a river or natural stream to create a reservoir for impounding water. The side on which water gets collected is called as upstream side and the other side of the barrier is known as downstream side.

Types of Dams

1. Earth Dams :

Earth dams are made up of soil that is pounded down solidly. It is built in areas where foundation are not strong enough to withstand the weight of concrete dam and where the earth is easily available. some of the examples of Earth dams are given below :

• Green Mountain Dam  in U.S.A which is located on Colorado river.
• Swift dam in Washington, U.S.A.

2. Rock-fill Dam :

Rock-fill dams are constructed from loose rocks and boulders piled in river bed. A slab of R.C.C is often laid across the upstream side to make the dam water tight. Examples of this types of dams are given below :

• The salt spring dam, California, U.S.A..
• San Gabriel dam in U.S.A.

3. Gravity Dam :

Gravity dams are one of the most expensive dams to be constructed. They are more durable and solid than rock-fill and earth dam. They can be constructed on site, where the natural foundation is strong enough to bear the self-weight of dam.

• Bhakra dam in Himachal Pradesh, India.
• Hoover dam in Nevada, U.S.A.

4. Hollow Masonry Dam :

The construction of hollow masonry dam is similar to that of gravity dam. They contain less concrete or masonry about 30% to 40%. Generally the weight of water is carried by deck of R.C.C or by arch action. These types of dams are difficult to built are are adopted only when the skilled labour is available.

5. Steel Dams :

These types of dams are not used for major works. Steel dams are often used as temporary coffer dams needed for the construction of permanent dams. Steel cofferdams are usually reinforced with timber or earth fill.

6. Timber Dam :

Timber dam is basically constructed with framework of timber struts and beams. These types of dams are one kind of a temporary dam, although a well designed timber dam may exist for more than 20 years. This types of dams needs to be regularly maintained.

7. Arch Dam :

This types of dams are curved in plan and carries at major part of its water load horizontally to the abutment by arch action. Arch dams are very complicated in design and even for construction. It is preferred when the site is extremely high and narrow valley portion is available.

8. Buttress Dam :

This types of dams are built across number of piers or buttresses. This types of dams requires skilled labours for its construction.

Site Selection Criteria for Dam

• Suitable foundation must be available.
• For economy, the length of the dam should be as small as possible and for given height it should store maximum amount of water.
• A suitable site for the spillway should be available in or nearby vicinity.
• The bed level at the dam site should preferably be higher than that of the river basin. This will reduce the height of the dam and will facilitate the drainage problem.
• The reservoir basin should be reasonably water-tight.
• Material required for the construction of the dam should be easily available locally.
• The value of the land and property submerged by the proposed dam should be as low as possible.
• The dam site should be easily accessible, such that it can be economically connected to important towns.
• Site for establishing labour colonies and healthy environment should be available near by vicinity.

Selection of Type of Dam

1. Topography :

• A narrow U-shaped valley flowing between high rocky walls would suggest a concrete dam.
• A low-rolling plane country would suggest an earth fill dam.
• A narrow V-shaped valley indicates choice of an arch dam.

2. Geology and Foundation Conditions :

The foundation strata should be strong enough to carry weight of dam. Hence the dam site must be surveyed by geologists so as to detect the thickness of foundation strata, presence of faults, fissured material etc. The various types of foundations generally encountered are :-

• Solid-rock foundation :

Similar like granite, gnesis etc. Almost every type of dam can be built as it is strong enough to withstand the self weight of the dam and external forces acting on the dam.

• Gravel foundation :

This type of foundation is suitable for earth and rock fill dams. For this foundation, low concrete gravity dams upto height of 15 meter  may also be suggested.

• Silt and fine sand foundation :

This type of foundation suggests the adoption of earth or very low gravity dam, upto height of 8 meter.

• Clay foundation :

This foundation may be accepted for earthen dam after special treatment.

3. Availability of Materials :

In order to achieve economy in the dam, the materials required for its construction must be available locally at short distance from the construction site.

4. Spillway size and location :

The cost of constructing a separate spillway may be enormous or sometimes suitable site for construction of spillway may not be available. In such cases combining the spillway and dam into one structure may be desirable.

5. Earthquake zone :

If dam is located in earthquake zone, its design must include the earthquake forces.

6. Height of Dam :

Earthen dams are usually not provided for height more than 30 meter. Hence for greater height, gravity dams are generally preferred.

7. Other consideration :

It includes life of dam, the width of roadway to be provided over the dam, overall cost of the construction and maintenance.