Dams

A dam is an artificial obstruction constructed across a stream or river for impounding water on the side of it. The term dam refers to both the obstruction and the water stored. Reservoir refers to the basin where water is stored.

Purpose of dam

>To store and control the water for irrigation.

>To store and divert the water for domestic uses.

>To supply water for industrial uses.

>To develop hydroelectric power plant to produce electricity.

>To increase water depths for navigation.

>To increase storage space for flood control.

>To preserve and cultivate the useful aquatic life.

Selection of site for dam

The dam is constructed at the place where the width of the river is minimum. The river site flanked on either side by hillocks would be ideal. Impervious stratum should be available at a reasonable depth for resting the dam foundation. It is desirable to construct the dam on high ground compared to the river basin. The reservoir basin should be reasonably water tight; the stores water should not escape out through its side walls and bed. The material required for dam construction should be available easily and cheaply. The site should be near highway and railway. This will facilitate easy transportation of men and materials. The site should be suitable for locating spillways to sufficient size and number. The site should e allowed for easy diversion of the river, during dam construction. The site should be disturbed much by natural environment.

Classification of dams

(a) Based on the structural behavior

(i) Gravity dam

(ii) Arch dam

(iii) Buttress dam

(b) Based on materials

1) Rigid dam

(i)Timber dam

(ii) R.C.C dam

(iii) Steel dam

(iv) Composite dam

2) Non-Rigid dam

(i) Earth dam

(ii) Rock fill dam

(c) Based on hydraulic design

(i) Non-over flow dam

(ii) Over flow dam

(d) Based on storage capacity

(i) Large dam

(ii) Small dam

(e) Based on the height

(i) High dam

(ii) Low dam

(f) Based on the purpose

(i) Storage dam

(ii) Detention dam

(iii) Diversion dam

Dam terminology

(a) Catchment area

A reservoir gets water mainly from its catchment area. It is defined as that area which always contributes the surplus water present over it to the natural drainage grid present in the area.

(b) Full reservoir level (F.R.L)

The level up to which the water is stored in a reservoir is called full reservoir level.

Maximum water level (M.W.L)

During the floods the water may be allowed to rise above the F.R.L to a certain maximum level called maximum water level (or) high flood level. (H.F.L)

(d) Crest

The top surface of the dam is called the crest of the dam. The crest may be used as roadway.

(e) Free Board

The level difference between the top of the bottom (crest) and the maximum water level is called free board.

(f) Drainage gallery

It is an opening provided long the length of the dam, above river bed, for the purpose of drainage the water seeping through the foundation bed under pressure.

(g) Toe

The downstream end’s at the base of the dam.

(h) Heel

The upstream end at the base of the dam

(i) Ayacut

It is defined as the area to be irrigated by dam.

(j) Upstream

The stretch of the stream above the dam site is called the upstream. Water is retained on the upstream side of the dam only.

(k) Down stream

The stream of the stream below the dam site is called stream. The surplus water is discharged to the downstream only.

Types of dam

(a) Gravity dam

Self weight of the dam resists the water pressure and other external forces. These dams are made of concrete and masonry. The gravity dam is always built on impervious foundation. The elementary profile dam is a triangle. Self weight of the dam is going to resist all other disturbing forces acting on the dam. Therefore t needs a good foundation soil, preferably rocky strata. This type of dam is Therefore is more durable and has maximum rigidity. Most of the gravity dams due to inadequate or ill designed foundation

(b) Arch Dam

An arch dam is curved in plan with its convex face holding the water. This structure is less massive when compared to the gravity dam. Te force exerted by the stored water on the upstream side will be transferred to the abutments by arch reaction. Arch dams are of two types:

(i) Constant radius type in which the radius of the arch is constant.

(ii) Constant angle arch dam (Central angle of the arch is constant)

Example of the arch dam – Idukki dam in kerala.

(c) Timber dam

Timber dam is used up to 10 m height. It is used where good timber is available in plenty. Leakage through timber is heavy. Timber dam may be buttress type of crib type.

(d) Steel dam

Steel dam is used for medium heights.

It is water tight. It is cheap. It can be constructed and repaired easily. It requires periodic painting. It may be strutted directly or it may be cantilever type dam.

(e) Earth dam

Earth dam are made of locally available soil and gravels. Earth dams are adopted if the height of the dam required is small and the foundation soil is having low bearing capacity. It can be used in the regions where is heavy rainfall and the foundation soil is having low bearing capacity. It can be used in the regions where there is heavy rainfall and the banks of the river area are steep. It cannot be used at sites where there is heavy rainfall and the banks of the river area are steep. It cannot be used at sits where there is no possibility of having spillways. These dams are cheapest. It may be constructed with or without a core wall. Core wall is well constructed at the centre of section of earth dam along the length of the dam for reducing the percolation of water through the body of the earth. Core wall is constructed of masonry or concrete. The dam without core or heart suffers from unequal settlement.

Conventional types of earth dams

(i) Homogeneous earth dam

It is the Earth dam’s simplest type. It consist o single materials throughout the structures. Sometimes, stones are pitched over the upstream face of the dam to safeguard the dam. It is made only of earth with horizontal filter near the toe.

(ii) Zoned type earth dam

In this dam, the outer zone is made of pervious soil and the inner zone is made of impervious soil. Normally, the inner zone will be clay and the outer zone is of locally available soil.

(iii) Diaphragm type earth dam: (or) Core type earth dam

In this dam, a thin impervious wall of masonry, or cement concrete is constructed at the middle of the section along the length of the dam. This wall arrests leakage to a large extent.

(f) Rock fill dam

A dam made up of rocks and boulders for the outer zone, with the inner – zone made of impervious materials like clay is called a rock-fill dam. Rock fill dam should be constructed on rock foundation so that the settlement is minimum. Rock fill dam adopted in places where good rock is available in plenty. This type of dam is used for medium height.