What is Gravity Dam? |Advantages & Disadvantages Of Gravity Dam | Forces Acting on a Gravity Dam

What is Gravity Dam? | Advantages & Disadvantages Of Gravity Dam | Uses of Gravity Dam | Forces Acting on a Gravity Dam |Types of Gravity Dams

What is Gravity Dam?

A gravity dam can be defined as a can be solid concrete or masonry structure constructed across a river to create a reservoir on its upstream, and it is designed to hold water back by using only the weight of the used material and the dam’s resistance against the foundation to oppose the horizontal pressure of water pushing against it.

The gravity dam’s shape is triangular in shape and has an apex at its top with the maximum width at the bottom. When constructed, the section is proportioned such that it can resist the external force acting on the dam by its own weight.

This type of structure is durable and requires very little maintenance. Currently, concrete gravity dams are most prefer as compared to other dams.

One of the reasons is that they can be constructed with ease, and on any dam site where a solid rock foundation is strong enough to bear the vast weight of the dam can be found. 

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A gravity dam is an essentially solid concrete structure that resists imposed forces principally by its own weight. Although usually straight in the plan, these dams are sometimes curved or angled to accommodate site topography. They are the simplest of the concrete types and the most typical to design and build.

A buttress dam is a gravity structure that, in addition to its own weight, utilizes the weight of water over the upstream face to provide stability.

Two of the most common gravity-buttress dams are the flat-slab type “and the massive-head type. The design and analysis of the multiple-arch type of buttress dam are closely akin to concrete arch dam design and analysis.

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Usually, the base of a concrete gravity dam is equal to approximately 0.7 times the height of the dam:

base = 0.7 * height

The shape of the gravity dam resembles a triangle. This is because of the triangular distribution of water pressure. The deeper the water, the more horizontal pressure it exerts on the dam.

So at the reservoir’s surface, the water is exerting no pressure, and at the bottom of the reservoir, the water is exerting maximum pressure. The shape can vary slightly; any of these shapes can be used to simulate a gravity dam if the exact shape is unknown.

Advantages & Disadvantages Of Gravity Dam

Advantages of Gravity Dam

• · A gravity dam requires minimal maintenance compared to other dams.
• · A Gravity dam is relatively stronger and more stable than an earth dam.
• · A Gravity dam can build any height compared to an earth dam.
• · Gravity dam does no fall suddenly; it gives enough indication of the fall over time, whereas earth dams generally can fail suddenly.

Gravity Dam Disadvantages 

• · Gravity dams can be done/built only on a solid sound rock foundation. In comparison, earth dams can be built anywhere on weak foundations or permeable foundations with good foundation treatment.
• · The initial cost of building a gravity dam is always higher when compared to an earth dam.
• · When mechanized plants, such as manufacturing, curing concrete, and transporting mass concrete, are not available, then the gravity dams may take more time to construct.
• · A Gravity dam requires highly-skilled labor for its construction.

Gravity Dams Construction

A gravity dam is a type of dam that use the weight or force of water to push against the ground, thus creating an artificial lake.

Gravity dams are often used for hydroelectric power production and as reservoirs for drinking water and irrigation.

Gravity dams can be built of concrete, earth, wood, or rock.  The construction process typically begins with installing a steel sheet pile wall where you want the bottom of your reservoir to be on one side of a river valley.

The other side will have dirt pushed into it so that when the water fills up in between them they create an impenetrable barrier downstream from it stopping any more flow from coming through.

A sluice gate is also installed at this point which allows operators to regulate the amount of water that goes through.

A gravity dam is a wall made to hold back water on a river or even underground. It is usually made out of concrete but sometimes it could be made out of dirt or wood.

It has a mechanism for the opening and closing of the spillway and the gate that regulates how much water is allowed to go through.

Components of Gravity Dam

The dam is developed around the reservoir with the spillway in between. In this case, it is a sluice gate that allows the excess water to flow out of the river once it fills up.

This mechanism can be made of concrete, wood or metal all of which help keep the river from overflowing.

The gate can be manual or automatic depending on how much maintenance is needed with each one of them they will have differences that are decided by their unique design.

The components of gravity dams include;

1. Sluice way:

This is an opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side.

2. Face:

It is the concrete part of the dam adjoining to the ground, which acts as a vertical wall in the river.

3. Embankment:

The embankment is built up in front of the face to run parallel with it, and is used to connect it to the shoreline.

4. Crest:

The crest is mainly a horizontal slab over the embankment and face, which rises above their level and provides a passage for water flow over and through it.

5. Crest wall:

This wall is constructed above the top of the sluice way along the crest, and is used to carry the load from water level of reservoir side.

6. Gate House:

It is a facility located at gate head or in a corner to accommodate operators and gears required for operation of the gate.

7. Spillway:

This is a passage for excess water to go through after the reservoir fills up, and can be either an overflow channel or an outlet tunnel.

8. Pump House:

It is a structure built to operate the facility for pumping out water from the reservoir into the river, in case of any emergency situation.

9. Sub Gate Chamber:

It is constructed below dam for installing machinery and electrical equipment required for operating the sluice way and gate head.

10. Abutment:

This is a vertical wall over the crest and is built to support the weight of an embankment.

11. Approach:

It is the approach at the lowest level for construction of the dam, from which all main elements such as abutment and spillway are constructed.

12. Support Wall:

It is a wall that provides support for an embankment along with the crest. It is used to hold back water during high water conditions, and also structures like piers or towers for specific purposes.

13. Heel:

It is a low embankment in the river bed, which prevents water from escaping through the low lying areas. It also directs flow of water into the main spillway.

14. Abutment Dam:

It is a dam constructed on one or both side of an embankment to support it and prevent it from slipping towards the river.

15. Galleries:

It is a small rooms like structure left within the dam for checking operations.

16. Diversion tunnel:

Tunnels are constructed for diverting water before the construction of dam. This helps in keeping the river bed dry.

Uses of Gravity Dam

Gravity dams are a type of dam that use the weight of water to resist pressure caused by gravity.  This weight is generated by constructing an arch on the upstream side and then filling it with concrete or some other material.

Gravity dams are often used in areas where there is not enough space for earth fill dams, like narrow gorges or steep slopes.

They can also be used as weirs, which create strong currents downstream that slow down flooding during heavy rains and help prevent erosion along rivers banks.

Gravity dams are often used in areas where there is not enough space for earth fill dams, like narrow gorges or steep slopes.

They can also be used as weirs, which create strong currents downstream that slow down flooding during heavy rains and help prevent erosion along rivers banks.

Gravity dams have many uses but they must be built carefully because if they fail, people living downstream could drown when their homes flood from a sudden rush of water flowing downhill into town centers or villages near rivers banks.

A gravity dam is a type of dam that use the weight or force of water to push against the ground, thus creating an artificial lake.

Gravity dams are often used for hydroelectric power production and as reservoirs for drinking water and irrigation.

Forces Acting on a Gravity Dam

The forces that act on the gravity dams include;

1. Weight of the dam

The weight of the dam is transferred from the center of gravity of the dam to the foundation, where it bears on that point. In general, if the weight of a dam is evenly distributed, then the total horizontal force (due to weight) acting on a section of a dam is equal to its weight divided by its width.

2. Pressure (or Head)

It has been noted that pressure is measured in terms of height or head. Otherwise, pressure is measured in terms of force or pressure (or stress) acting per unit area.

3. Geographical effects

The geographical conditions are very important in deciding how much head is required for a particular gravity dam. The geographical factors include; topography (slopes, valleys, hills), rainfall distribution, climate (intensity and distribution) and others.

The intensity of the force is determined by various factors such as the angle of inclination, mode of support, type of operation and others etc.

The variation in head among different parts of the dam can be attributed to various factors such as soil conditions, type and position of the foundation, soil liquefaction and other geological features etc.

4. Earthquake forces

Earthquake ground motion consists of uniform-slip waves that travel at a constant rate over the surface of the Earth. Although this is a very small effect, it can cause failure in dams.

5. Water pressure

There is no perfect analogy to the head but it is thought that the magnitude of water pressure at any point on a dam is equal to the area of the face multiplied by its depth.

6. Wave pressure

It is noticed that the wave pressure acting on the dam is not uniform but there are differences in pressure at different times.

7. Wind pressure

The wind pressure acting on a gravity dam is usually very low (less than 10 psi) because of the resistance of the caprock. However, if the caprock is absent or damaged, then there is potential for significant pressures to be measured.

8. Uplift pressure

The uplift pressure is the pressure that tends to lift a dam. This pressure is known to be proportional to the area of the water surface above; so, as the surface increases, so does the uplift force.

9. The hydrostatic pressure

In any rivers, there are mainly two types of water flows; free flow and confined flow. In a gravity dam with two converging walls, we get free flow at the center line whereas in an earth dam there is no free flow but confined flow at center line.

9. Silt pressure

In a gravity dam, large quantities of silt will get suspended in the dam. This will have an effect on the height and also the stability of the dam.

A small amount of silt is not going to have any effect on the height and stability of a gravity dam, but if many tons of silt are getting suspended in the reservoir, then this may cause a lot of litter in all parts of the dam and might lead to its failure.

10. Ice pressure

Ice pressure is a pressure created as a result of ice formation due to water freezing. The ice forms at the base of the dam and the pressure is exerted upwards through the foundation. This has caused many dams to fail.

11. Buoyancy force

The buoyancy force is a phenomenon where air becomes trapped in the dam during construction. The air becomes trapped due to its less density compared to water and so it floats on the water’s surface.

Types of Gravity Dams

1. Conventional concrete dams

This type of gravity dam is built from concrete and has concrete at the water contact. A concrete gravity dam with a hydrostatic pressure is known as a curved gravity dam.

2. Masonry Dam

A masonry gravity dam is built from concrete block or natural rock. The blocks are placed in horizontal courses of stone and mortar.

The blocks are not much thinner than the bedding joints. A masonry gravity dam with a hydrostatic pressure is known as an arch dam.

 Gravity Dams FAQs

What are the benefits of gravity dam?

Gravity dams are very effective in their operation. It has a number of advantages, including low maintenance costs, low pressure and small footprint.

What are the disadvantages of gravity dam?

Gravity dams of great height can be constructed only on sound rock foundations. Again, Initial cost is more than earth dam and takes longer time in construction. It require more skilled labor than earth dam

What is the difference between gravity and an earth dam?

A gravity dam is a type of dam that stands because of its weight (or the force of gravity). An earth dam is a type of dam that stands because it has piers or bedrock.

The main difference between the two is that an earth dam is supported by piers and bedrock, whereas a gravity dam stands because of its own weight.

Can the foundation of a gravity dam be damaged?

Because of its sheer weight, gravity dams can be damaged by earthquakes or other forces that cause movement on hillsides or when the gypsum rock is disturbed. This type of damage can cause the foundation to fail.

 What kind of design is required for a gravity dam?

The design for a gravity dam depends on the width and height of the reservoir, as well as its gradient (slope), and amount of head required at any given location. Most dams are designed using a combination of numerical analysis and field testing.

What is the best soil for a gravity dam foundation?

Normally the best soil for a gravity dam foundation is fast-draining, firm, well-drained earth or bedrock with minimum water content.

What are some common characteristics of a gravity dam?

• Gravity dams normally have an impervious bottom that prevents the leakage of water into the subsoil and also protection against erosion.
• A gravity dam is normally built either from concrete or masonry, and it has a large number of piers (or abutments).

What are the most important considerations for a gravity dam?

The most important considerations involve the design of the foundation, the selected material of construction and the proper selection of hydraulic parameters.

What is a gabion wall?

A gabion is a basket filled with rocks and cement that can be used in place of concrete blocks. Gabions have the advantages of reduced cost, easy assembly and quick construction compared to concrete masonry and conventional gravity dams; however, they have an increased risk of leakage compared to a conventional gravity dam.

Where can I use the gravity dam?

Gravity dams can be used where the stream has relatively low capacity or uniform flow characteristics.