Types of dams

Types of dams are classified into

Storage dams

A storage dam is constructed for storing the surplus water available in the river during the rainy season. Otherwise, the excess water from the river flows into the sea. the water thus stored is used for irrigation during dry months.

1. Gravity dam
2. Earth dam
3. Arch dam
4. Rock fill dam

Diversion dams

A Diversion dam is built across a river to raise the water level which can be diverted into canals to the place of use

1. Weir dam
2. Barrage dam

Overflow dams

An overflow dam is constructed to carry surplus discharge of water, including flood water, over its crest.

1. Spillways

Non-overflow dams

in a non-overflow dam, the top surface of the dam (crest) is kept at a higher level than the maximum expected high flood level (H.F.L)

1. Gravity dam
2. Earth dam
3. Rockfill dam

Rigid dams

As the name implies, a rigid dam is constructed using rigid construction materials such as stone masonry, R.C.C. or P.C.C., steel and timber. The basic profile of a rigid dam is a triangle. The rigid types of dams are,

1. Solid masonry or concrete gravity dam
2. Arched masonry or concrete dam
3. Buttress dam
4. Steel dam and timber dam

Non-rigid dams

A non-rigid dam is constructed of non-rigid materials such as earth soil or rock fill. It has a trapezoidal basic profile. these dams are flexible structures. they can deform slightly corresponding to the deflection of the foundation.

1. Earth dam
2. Rock fill dam

Gravity dam

A gravity types of dams is a rigid type of dam. It is constructed of concrete or masonry. masonry gravity dams are constructed only at small heights. But, all major important gravity dams are nowadays constructed of concrete only.

Profile of gravity dam

The cross-section of the gravity dam is a rigid angled triangle in elevation with upstream water on the vertical side. However, the triangular profile cannot be adopted in practice.

1. Water pressure (W.P.)

Water stored on the upstream side exerts pressure on the dam. The intensity of pressure varies in a triangular fashion. The intensity of water pressure at the maximum water level (M.W.L) is nil. Water pressure is maximum at the heel.

2. Uplift pressure (U.P.)

The upstream side and downstream side of the dam have a difference in water head. this water head causes seepage of water through the pores ( porous concrete ) of the dam and its foundation.

3. Pressure due to earthquake forces

Earthquake forces may set up waves in the crust of the earth. These waves develop high stresses, causing instability to the dam.

4. Wind pressure

Its acts on the exposed area of the dam. It is rather a minor force.

5. Self-weight of the dam (W)

Self-weight of the dam is the main resisting force of the dam. It is designed such that it can resist the total disturbing external forces acting on it.

Components of gravity dam

It consists of Reservoir, Upstream face, Downstream face, crest, M.W.L., free board, Upper supply sluice, lower supply sluice, Heel, Toe, Drainage gallery and Hard foundation rock (Impervious strata).

1. Upper supply and lower supply sluices

Supply sluices, namely, upper supply and lower supply sluices are openings provided through the body of the dam from its upstream side to the downstream side.

2. Spillway

During floods, the quantity of incoming water will be heavy. The surplus water beyond the storage capacity of the reservoir has to be immediately diverted safely

3. Drainage Gallery

It is an opening provided in the dam, a few meters away from the upstream vertical face. It may be in the longitudinal or transverse direction.

Advantages

• A gravity dam is most suited at sites with narrow gorges or valleys and steep side slopes. It is adapted for use as an overflow dam.
• Gravity dam has maximum rigidity, due to its heavy and solid self-weight. hence, it is also named a solid gravity dam.

Disadvantages

• A gravity dam can be constructed only on the sound rock foundation
• the initial cost is higher than the earth dam
• Skilled labour is required for its construction

Arch dam

An arch types of dams is a rigid type dam, curved in the plan as shown. It has a constant horizontal radius on the upstream side. Its convex face is facing the reservoir (upstream) side. Strong abutments are constructed on either side of the dam.

Stability

The arch dam depends on the arch action for its stability. The water pressure on the upstream side is transferred horizontally to the abutment on either side of the dam by arch action. The forces involved are Water pressure and a slight uplift pressure, which may lead to the yielding of the abutment. Therefore, abutments should be very strong to withstand these forces.

Advantages

• The arch dam is suitable for narrow valleys or gorges
• Saving in the quantity of concrete is around 50% compared to a gravity dam
• When the height of the dam is more than its length, an arch dam is economical therefore, this dam is preferable for very great heights.

R.C.C. Buttress dam

R.C.C. Buttress types of dams is constructed

1. When the river is wide,
2. Where the foundation soil is pervious and
3. Where the height of the dam required is less.

A buttress dam consists of a series of R.C.C. buttress, a Foundation slab or base slab, a sloping R.C.C. face slab or deck slab, lateral beams called braces and cut-off wall

A buttress dam has a continuous upstream face, inclined at about 45% to the horizontal.

Cut-off wall

A cut-off wall is constructed below the ground level. Due to the difference in water head between the upstream and downstream sides, the water may seep through the foundation and body of the dam. The seeping water exerts an uplift pressure on the dam. Cut-off walls reduce the uplift pressure on the foundation.

Advantages

• Stability: weight of water on the inclined face slab helps the stability of the dam
• The dead weight of the dam is less
• The height of the buttress dan can be easily increased by simply extending the buttresses and the face slab
• The braces add to the strength of the buttresses
• Economical: In the gravity dam, the full strength of concrete is not utilized at all points. This is uneconomical. But, in a buttress dam, the total volume of concrete is reduced by concentrating the materials where stresses are more. Thus, the buttress dam has a relatively thin face slab section.

Steel Dam

Steel types of dams are constructed only in small sizes for low-capacity reservoirs. However, it requires periodical painting. These are not in use nowadays. The steel dam consists of a steel framework with a thick steel plate called a flat slab.

Timber Dam

A timber types of dams is constructed only in small size for low-capacity reservoirs. However, it Is also used only for temporary purposes such as to divert the flow of river water for the construction of the main dam. After the construction of the main dam, the timber dam cannot be made water-tight. height of the timber dam is restricted to 9 meters only.

Earth dams

Earth types of dams is a non-rigid type of dam with a trapezoidal profile. It is made of locally available soils and gravels with minimum processing. This makes the cost of construction cheaper than a rigid dam. Therefore, it is the most common type of dam, constructed in areas where the foundation is not sufficiently strong enough to bear the weight of a gravity dam. Earth dams can be constructed for moderate height.

An embankment with a core arrangement to prevent water seepage is an embankment dam.

Types of Earth dam

Based on the construction materials used, earth dams are classified as:

1. Homogeneous Embankment Earth dam
2. Zoned Embankment dam
3. Diaphragm Type Embankment Earth dam

Homogeneous Embankment Earth dam

It is the simplest type of earth dam of trapezoidal profile. It consists of a single kind of earth materials ( excluding the materials used for slop protection ). It is homogeneous throughout the structure and hence the name Homogeneous Earth dam

Use: Homogeneous Embankment Earth types of dams is structurally not strong. It is not frequently used, the height varies 6 to 8 meters.

Stability: A purely homogeneous type of non rigid construction results in seepage of water. Also, stones are pitched called stone pitching on the gravel bed, over the inclined upstream face of the dam.

Zoned Embankment Type Earth dam

This types of dams consists of two-zone, viz., inner zone and outer zone, and hence the name zoned type. The inner zone is made of impervious soil. Therefore, the inner zone is called the inner impervious core, formed along the length of the dam. The outer zone is made of previous soil. Hence, the outer zone is called the outer pervious shell or casing. The inner core is supported and protected by the outer shell.

The crest is about 5 meters wide ( top width ) with a paved road to enable access for inspection and maintenance

Use: Zoned embankment dams can be used for dams of moderate or great heights

Stability: The inner impervious core zone gives added strength to the structure of the dam.

Diaphragm Type Embankment Earth dam

This types of dams consists of a thin impervious wall of masonry, cement concrete, impervious soils, steel or timber, known as the core. The core is constructed in the middle of the section along the length of the dam. This core is called the diaphragm. The diaphragm is surrounded by earth fill. It is joined to a strong impervious foundation bed. The diaphragm core arrests leakage or seepage of water to a great extent.

Advantages of embankment earth dams

1. Embankment earth types of dams are suited to a wide variety of foundation conditions. They are suitable for both steep gorges (narrow valleys) and wide valleys
2. Local materials can be made use of
3. Reduced cost of construction
4. About 80% of the total number of dams in the world are earth dams

Failures of Earth dams

Failures of earth types of dams are of three types

1. Seepage failures
2. Structural failures
3. Hydraulic failures

Rock fill Earth dams

Rock fill types of dams are made up of various sizes of loose rocks (hence the name rock fill dam) and boulders at the downstream side to provide stability

The dam is made of dry rubble stone masonry on the upstream side. Impervious R.C.C. member (covering) is laid on the sloping side of the stone masonry for water tightness

A cut-off wall is provided at the upstream side to check the seepage in the foundation soil.

Stability: The stability of the rock fill types of dams is better than earth dams. it may also be subjected to uneven settlement failure, causing cracks in the R.C.C. membrane.

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