Why build dams?
Water is a source of life and a force of nature. Like fire, being able to control water lets humans use its power for food, energy and protection. Dams are some of the earliest civil engineering projects. Engineers are the people who figure out how to direct water to the right place, at the right time and in the right amount. Dams are structures built to block the flow of water and help hold or direct water as it's needed.
Dams make modern life possible.
Without dams, life as we know it would not exist. Dams protect people by controlling flooding in low-lying areas. They help us get the water we need for drinking and growing crops. They also control the flow of water energy that makes hydro-electric power. The lakes created by dams are sometimes places where people can go swimming, boating or fishing.
Dams control water and have many important jobs: to control flooding, supply water, generate electricity and create recreational areas.
Two main parts of a dam make it possible:
- A structure or barrier to stop the flow of water
- A spillway to control the release of water to the downstream area
What materials are dams made of?
- Most dams are earthen dams.
- Reinforced concrete dams are expensive, but sometimes needed.
- Dams are also made of masonry - a building material made of stone, clay, brick or concrete.
Forces of Nature
Water has one job – to get past anything in its way!
Flowing water has energy!
How do you hold the water back while you are building a dam? Building anything in a strong current of water is difficult and dangerous. Cofferdams and diversion channels make it possible. A cofferdam is a temporary dam built to change the path of a river to go around a construction site so the dam can be built on dry ground. A diversion channel is a way for water to get around the construction site while the dam is being built.
Dams are under great pressure – water pressure!
Water presses on dams with force that is measured in pounds of force per inch. The pressure at the bottom of a lake 33 ft. deep is twice the pressure at the top! Engineers design dams to stand up to differences in water pressure.
Will nature eventually win?
Dams don't last forever. Dams must be maintained to survive the forces of nature. Dam maintenance and dam removal are part of the life cycle of every dam.
Large teams of engineers design and build dams. Managing and coordinating so many workers means engineers have to be people who are skilled, smart and like teamwork!
Geotechnical engineers are needed to build dams that are sturdy and strong from the ground up. Geotechnical engineers study rocks, soil and underground water around the location of the dam to make sure dams are designed with the condition of the earth and soil in mind.
Structural engineers design and build dams to be strong and safe for a long time. They know how to build structures that stand up to manmade and natural forces – especially water – that impact a dam’s structure.
Environmental engineers, construction engineers and mechanical engineers also have an important role in the design, construction and maintenance of a working dam.
Questions to ponder
Dams change the environment. Dams are large structures that make a lot of changes to the area around them when they are built. Communities, policy makers, and engineers have to think about how building such large structures would help and hurt the nearby people, towns, land and animals. One of the questions they ask is how would the dam change how well nearby people live and how will the power it supplies help those people.
South Forks Dam
The site of the Johnstown flood and the United State's largest dam disaster.
There are over 84,000 dams in the United States! Nearly 2,000 of those are taller than an 8-story building.
Aswan High Dam
This embankment dam has a mile-wide base.
Grand Coulee Dam
The Grand Coulee Dam in Washington State was built with 12 million cubic yards of concrete: enough to build a sidewalk four feet wide and four inches think and wrap it twice around the equator! It is also the largest single power producer in the US.
As of 2010, the tallest dam in the world is Nurek Dam in Tajikistan. Finished in 1980, it reaches 300 meters (984 feet) tall.
Type of Dams
Every dam is unique
For each dam, engineers decide what materials and design features to use that match how that dam will be used and the limits of the place where it will be built. There are four main types of dams.
Arch Dams are good for narrow rocky locations. They are curved, and the natural shape of the arch holds back the water in the reservoir. Arch dams are thin and require less material than any other type of dam. The arch squeezes together as the water pushes against it. The weight of the dam also pushes the structure down into the ground.
Buttress Dams can be flat or curved. A series of supports, or buttresses hold up the dam on the downstream side. When water pushes against the buttress dam, the buttresses push back and keep the dam from falling over. The weight of the buttress dam also pushes down into the ground, which also helps keep it stable. Most buttress dams are made of reinforced concrete.
Embankment Dams are the most common dams in the United States. They are huge dams made of earth and rock. Like gravity dams, embankment dams use their heavy weight to stand up to the force of water. When water pushes against the embankment dam, the heavy weight of the dam pushes down into the ground and keeps the dam from falling over. Embankment dams also have a dense, waterproof core that keeps water from seeping through the structure.
Gravity Dams are gigantic dams that stand up to the force of water by only using their own weight. When water pushes against the gravity dam, the heavy weight of the dam pushes down into the ground and keeps the structure from falling over. Even though most gravity dams are expensive to build because they need so much concrete, lots of people want to use them because they like how much more solid they look than the thinner arch and buttress dams.
Photos courtesy of: Hoover Dam - Photo, U.S. Bureau of Reclamation; Folsom Dam - uvw916a on FlickR; Folsom Gate - US BureaOfReclamation(#); Folsom Team - USACEpublicaffairs on FlickR; Nature at Folsom - sporkwrapper on FlickR; South Forks - Johnstown Area Heritage Association Archives; O'Shanessy Dam - ______________; Aswan High Dam - © 2000 WGBH; Grand Coulee Dam - Photo, U.S. Bureau of Reclamation; Nurek Dam - © David Trilling/Corbis; Glen Canyon - lazytom on FlickR; arch dam diagram - © 2000 WGBH; buttress dam diagram - © 2000 WGBH; embankment dam diagram - © 2000 WGBH; gravity dam diagram - © 2000 WGBH