DAMS

DAMS

With the exception of the Great Wall of China, dams are the largest structures ever built. Throughout history, big dams have prevented flooding, irrigated farmland, and generated tremendous amounts of electricity. Without dams, modern life as we know it would simply not be the same.

Since the first large-scale dam was built in Egypt more than 5,000 years ago, engineers have devised various types of dams to withstand the forces of a raging river.

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, like the El Atazar Dam in Spain, are thin and require less material than any other type of dam.

Buttress dams...
may be flat or curved, but one thing is certain: a series of supports, or buttresses, brace the dam on the downstream side. Most buttress dams, like the Bartlett Dam in Arizona, are made of reinforced concrete.

Embankment dams...
are the most commonly built dams in the United States. They are massive dams made of earth and rock. Like gravity dams, embankment dams rely on their heavy weight to resist the force of the water. But embankment dams are also armed with a dense, waterproof core that prevents water from seeping through the structure. Tailings dams -- large structures that hold back mining waste -- are a type of embankment dam.

Gravity dams...
are massive dams that resist the thrust of water entirely by their own weight. Most gravity dams, 13513k1018n like the Grand Coulee Dam in Washington, are expensive to build because they require so much concrete. Still, many people prefer its solid appearance to the thinner arch and buttress dams.

Take at look at the forces that affect gravity dams.

All dams -- whether they're embankment, buttress, arch, or gravity -- must be maintained as they get older. Without proper maintenance, spillways can clog and concrete can crack. Some dams are even removed because they block the migration of fish.

When should dams be taken down? When should they be repaired? Engineers must consider the services that each dam provides and the environmental impact that each dam creates before they make this decision -- and this isn't easy. Oftentimes, there is no right answer.

Now that you know more about different types of dams, make some of your own decisions about troubled dams in the Dam Challenge!

Dams don't last forever. Hot and cold weather makes them crack. Water erodes their foundations. They create environmental problems. Eventually, every dam must be repaired, removed, or replaced.

Gravity Dam: Forces
Water pushes against the gravity dam, but the heavy weight of the dam pushes down into the ground and prevents the structure from falling over.

Embankment Dam: Forces
Water pushes against the embankment dam, but the heavy weight of the dam pushes down into the ground and prevents the structure from falling over.

Buttress Dam: Forces
Water pushes against the buttress dam, but the buttresses push back and prevent the dam from toppling over. The weight of the buttress dam also pushes down into the ground.

Arch Dam: Forces
The arch squeezes together as the water pushes against it. The weight of the dam also pushes the structure down into the ground.

Aswan High Dam

Vital Statistics:
Location: Aswan, Egypt
Completion Date: 1970
Cost: $1 billion
Reservoir Capacity: 5.97 trillion cubic feet
Type: Embankment
Purpose: Flood control, hydroelectric power, irrigation
Reservoir: Lake Nasser
Materials: Rock, clay
Engineer(s): planned by a team of British engineers; built by a team of Soviet engineers

In the middle of the arid Egyptian desert lies one of the largest embankment dams in the world. It is called the Aswan High Dam, or Saad el Aali in Arabic, and it captures the mighty Nile River in the world's third largest reservoir, Lake Nasser. Before the dam was built, the Nile River overflowed its banks once a year and deposited four million tons of nutrient-rich silt on the valley floor, making Egypt's otherwise dry land productive and fertile. But there were some years when the river did not rise at all, causing widespread drought and famine. In 1952, Egyptian president Gamal Abdal-Nasser pledged to control his country's annual flood with a giant new dam across the Nile River. His plan worked.

The Aswan High Dam captures floodwater during rainy seasons and releases the water during times of drought. The dam also generates enormous amounts of electric power -- more than 10 billion kilowatt-hours every year. That's enough electricity to power one million color televisions for 20 years!

Unfortunately, the dam has also produced several negative side effects. In order to build the dam, 90,000 Egyptian peasants had to move. To make matters worse, the rich silt that normally fertilized the dry desert land during annual floods is now stuck at the bottom of Lake Nasser! Farmers have been forced to use about one million tons of artificial fertilizer as a substitute for natural nutrients that once fertilized the arid floodplain.

Fast Facts:

• About 95 percent of Egypt's population lives within 12 miles of the Nile River.
• Since the dam was completed in 1970, the fertility of Egypt's farmland has gradually decreased. Today, more than half of Egypt's soil is rated medium to poor.
• Enough rock was used in the construction of the Aswan High Dam to build 17 Great Pyramids at Giza, one of the Seven Wonders of the Ancient World.

Grand Coulee Dam

Vital Statistics:
Location: Grand Coulee, Washington, USA
Completion Date: 1942
Cost: $300 million
Reservoir Capacity: 421 billion cubic feet
Type: Gravity
Purpose: Flood control, hydroelectric power, irrigation
Reservoir: Franklin D. Roosevelt Lake
Materials: Concrete
Engineer(s): Bureau of Reclamation

The Grand Coulee Dam, located on the Columbia River in central Washington, is the largest single producer of electricity in the United States. Made from 12 million cubic yards of concrete, the Grand Coulee Dam is also one of the largest concrete structures in the world. But engineers were confronted with a unique problem when building such a massive concrete dam.

When concrete is made, it produces a chemical reaction that gives off heat. As concrete cools, it gradually shrinks. If the shrinkage is not controlled, cracks can form -- and cracks are disastrous in dams. The solution? Engineers pumped cold water through an intricate network of pipes in the concrete to help cool the concrete as it hardened. It's a good thing they did this, because it would have taken 200 years for the concrete to cool naturally, and many cracks would have formed!

The Grand Coulee Dam is the largest producer of hydroelectric power in the United States and the third largest hydroelectric facility in the world. With its 28 generators producing up to 23,860,944,469 kilowatt-hours annually, it is the primary source of electric power to states in the Northwest.

Fast Facts:

• The base of the Grand Coulee Dam is almost four times as large as the base of the Great Pyramid of Giza.
• If all of the pipes used to cool the cement in the Grand Coulee Dam were laid end to end, they would stretch from New York City to the Grand Canyon!
• There is enough concrete in the Grand Coulee Dam to build a highway across the United States.

Itaipu Dam

Vital Statistics:
Location: Brazil and Paraguay
Completion Date: 1991
Cost: $18 billion
Reservoir Capacity: 1.02 trillion cubic feet
Type: Gravity
Purpose: Hydroelectric power
Reservoir: Itaipu Reservoir
Materials: Concrete
Engineer(s): International Engineering Company; Itaipu Binacional

Eighteen was a lucky number for engineers working on the Itaipu Dam. The 4.8-mile-long complex of concrete and rockfill dams on the Upper Parana River at the Brazil-Paraguay border has 18 generators, and it took 18 years and $18 billion to build. The main structure, a hollow, concrete gravity dam, has a powerhouse capable of generating 12,600 megawatts of electricity. That's enough to power most of the state of California. In fact, the enormous dam provides 25 percent of Brazil's energy supply and 78 percent of neighboring Paraguay's energy supply. But building one of the largest hydroelectric dams in the world was not easy.

Engineers actually had to shift the course of the seventh largest river in the world, the Parana River, around the construction site before building the Itaipu Dam. It took almost three years for workers to carve a 1.3-mile-long, 300-foot-deep, 490-foot-wide diversion channel for the river. Fifty million tons of earth and rock were removed in the process. The American Society of Civil Engineers recognized this amazing feat and named the Itaipu Dam one of the "Seven Wonders of the Modern World."

Fast Facts:

• Engineers chose a hollow gravity dam because it required 35 percent less concrete than a solid gravity dam. The hollow dam is still heavy and sturdy enough to resist the thrust of water entirely by its own weight.
• The volume of iron and steel used in the dam would be enough to build 380 Eiffel Towers.
• The dam is a major tourist attraction. More than nine million visitors from 162 countries have visited the structure since it was completed in 1991.

Three Gorges Dam

Vital Statistics:
Location: Three Gorges, China
Completion Date: 2009
Cost: $17-$100 billion
Reservoir Capacity: 1.39 trillion cubic feet
Type: Gravity
Purpose: Flood Control, hydroelectric power, irrigation, navigation
Reservoir: not named
Materials: Concrete
Engineer(s): Changjiang Water Resources Commission; Zhongnan Investment & Design Institute; Huadong Investment & Design Institute

Construction is now under way in China on what will be the world's largest hydroelectric dam. When it is completed in 2009, Three Gorges Dam will stretch more than one mile across the Yangtze River and soar 600 feet above the valley floor. It will be the largest concrete dam in the world, and will produce 18,000 megawatts of electrical energy, nudging Brazil's Itaipu Dam to second place.

China's Three Gorges Dam is years from completion, but environmentalists and human rights advocates are already concerned about the consequences of such a huge structure. To make way for the enormous project, more than one million people living on the banks of the Yangtze River will have to move to higher ground. The 350-mile-long reservoir will submerge villages, ancient temples, burial grounds, and the spectacular canyons that tourists from all over the world come to see. Environmentalists also argue that the dam will wipe out a number of rare species, including the Yangtze River dolphin, and that the reservoir will trap millions of tons of raw pollutants spewing from China's largest industrialized city, Chongqing.

When finished, Three Gorges Dam will generate one-ninth of China's power. Unfortunately, the dam may be remembered not for its hydroelectric power, but for its drastic social and environmental impact.

• About 20,000 people are working nearly round the clock to complete the 1.24-mile-wide structure by 2009.
• The lake that will form behind Three Gorges Dam will stretch for about 350 miles -- the distance from San Francisco to Los Angeles.
• When it is completed in 2009, the enormous Three Gorges Dam reservoir will actually be visible from the moon!