Dams

UNIT - III

Chapter – 8

DAMS

 

DAMS

 

1. COMPONENTS OF A DAM (Fig. 1)

1. Dam: A Dam is a heavy hydraulic structure, constructed across a river. It stores water on one side up to a certain level. Dam is an impervious barrier (obstruction), built across a natural stream of water so that a Reservoir is formed.

2. Reservoir: The total impounded water collected on the upstream side is called Reservoir.

3. Upstream Side: Side of the dam on which water gets collected is called Upstream Side.

4. Downstream Side: It is the stretch of the stream below or down the dam.

5. Catchment Area: Catchment Area is the rainfall area on the upstream side of the dam. It is feeding water to the reservoir of the dam. The catchment area decides the capacity of the reservoir as well as the height of the dam.

6. Spillway: Spillway is the waterway for the overflow of a dam. It acts as a safety valve for the dam. Its function is to let out the excess water from the dam to the downstream side. Otherwise, the excess water in the dam may cause breach of the dam due to over-storage.

7. Crest: Crest is the top surface of the dam. It may be used as road way.

8. Sluice or Gate: It is the component of a reservoir with regulating arrangement to control the quantity of water. Through the sluice, water is let out from the upstream side of the dam.

9. Heel: Heel of the dam is the upstream end of the base of the dam.

10. Toe: Toe of the dam is the down stream end of the base of the dam.

11. Full Reservoir Level (F.R.L.):

Full Reservoir Level is the level up to which water can be stored in the reservoir of the dam.

12. Gross Storage Capacity of the Reservoir: It is the total quantity of water stored in the reservoir up to the full reservoir level.

13. Live Storage: Live Storage is the quantity of water stored in the dam from the level of the lowest set of sluices to the full reservoir level.

14. Dead Storage: Dead Storage is the quantity of water stored below the level of the lowest set of sluices in the dam. This water cannot be released out and used for irrigation, etc.

15. Mean Sea Level (M.S.L.): Mean Sea Level means the height of the dam above the sea level.

16. Maximum Water Level (M.W.L.) or High Flood Level (H.F.L.): During floods, water may be allowed to rise above the full reservoir level to a certain maximum level, called Maximum Water Level (M.W.L.) or High Flood Level (H.F.L.). Top surface of the dam is always constructed above the Maximum Water Level of the reservoir.

17. Free Board: Free Board is the difference in height between the top surface of the dam (crest) and the Maximum Water Level of the reservoir.

18. Drainage Gallery: It is a passage left inside the dam, provided along the length of the dam above the river bed. Its purposes are: (i) To drain the water seeping through the foundation and upstream masonry and (ii) To serve as a longitudinal inspection chamber.

19. Anacut: Anacut is the area to be irrigated by a dam.

20. Stability of Dam Structure: The load of the heavy structure of the dam acts vertically downwards on the foundation. Due to the storing of water in the reservoir in the upstream side, a lateral thrust of high magnitude acts on the dam. This thrust tends to push the dam to the downstream side.

 

2. PURPOSES OF A DAM

1. Irrigation

Stored water in the dam is mainly used for irrigation purposes. Rain fall is not evenly distributed throughout the year. Dam is used to store the surplus water available during rainy season which would otherwise flow into the sea. The water thus stored is used for irrigation purposes during dry months.

2. Drinking Water Supply

In areas of ground water shortage, the stored water in the dam can be used as a very good source for drinking water supply in towns and cities.

3. Hydro-Electric Power Generation

If adequate Head of Water (Potential Energy) is available in the reservoir of the dam, it is used for power generation in hydel power plants.

4. Industrial Purposes

Dams are used to supply water for industrial purposes.

5. Flood Control

If heavy flood water in a river is left un-obstructed, it will result in heavy loss of human lives and cause heavy damages to bridges, roads, etc. The dam across the river acts as a good flood control measure by letting out the excess quantity of water.

6. Fish Farming

Dam forms an excellent base for breeding of fish, because of continuous availability of water.

7. Recreational Purposes

The dam serves as an excellent place for recreations such as boating, swimming, etc. Thus, the dam makes it a place of tourism importance.

8. Storage of Water for Navigation

In some places, reservoirs are constructed and the backwater of reservoir provides a very deep channel for navigation.

Multi-purpose Reservoir: A reservoir planned, designed and constructed to serve multipurposes such as Irrigation, Drinking Water Supply, Hydro-Electric Power Generation, Industrial Purposes, Flood Control, Fish Farming, Recreational Purposes and Navigation.

 

3. FACTORS GOVERNING SELECTION OF SITE FOR DAM

1. Catchment Area: Catchment Area should be able to contribute adequate supply of water to the reservoir of the proposed dam.

2. Foundation Soil: Sound foundation soil should be available at the site to carry heavy load. For Earth Dams, any type of foundation is suitable. However, for Gravity or Concrete or Masonry Dams, sound rocks at surface or within a reasonable depth, are important.

3. Ecology: The balance of ecology (means Natural Environment) should not be disturbed.

4. River Cross-Section at the Site: River cross-section at the site should have a narrow gorge (valley) to allow large storage space and to reduce the length of the dam.

5. Height of the Dam: The dam cost is proportional to the square of the dam height. . Therefore, the height should be minimum for the intended storage of water.

6. Storage Capacity: The dam should provide adequate storage capacity for the reservoir.

7. Costs: The dam should have minimum construction and maintenance costs.

8. Reservoir Silting: The site should be such that reservoir silting is minimum.

9. Spillway: Suitable location for Spillway should be available in the near vicinity.

10. Submergence: Value of the land submerged by the proposed dam should be as low as possible. It should be less than the benefits expected from the dam. The rehabilitation of the people displaced due to submergence is a problem to be tackled in the case of large dams.

11. Bed Level: The bed level of the dam should preferably be on a higher level than that of the river basin to facilitate drainage.

12. Water-Tight: The reservoir should be water-tight. Otherwise, the stored water may escape through its bed and banks.

13. Topography: The topography of the site should be such that the length of the dam should be as small as possible. Also, for a given height, the storage capacity should be as high as possible. Therefore, the river valley at the site should be as narrow as possible for maximum storage capacity.