Dam

Man-made dams are structures built to restrict or prevent the flow of water that comes from large rivers and streams. Dams are built for many reasons: for irrigation purposes and to prevent rivers and streams from overflowing due to excessive rain or snowfall, as well as to generate hydroelectricty. Dams hold a body of water that can be used to generate electricity as well as provide a water supply for towns and cities that may require it.

Hydroelectricity is a renewable resource that can supply constant and efficient energy. It provides at least 20 percent of the world’s electricity. Dams constructed to supply hydroelectricity can provide many benefits to the environment, wildlife, and obviously, to mankind. If dams are constructed with care and precision, they can be used to improve the quality of life for fish and wildlife. However, there are increasing concerns about the damage that dams convey on the environment.^[1]

The most famous dams include the Hoover Dam, one of the largest at the time of its construction; the Nurek Dam, located on the Vakhsh river in the Asian nation of Tajikistan, is currently the tallest in the world; the Three Gorges Dam in the Yangtze River in China; and the Rogun Dam, also on the Vakhsh river, is currently undergoing construction and due to be the tallest dam in the world.^[2]

[edit] History

[edit] Early Dams

Dams have been in existence since 2950-2975 B.C., when the ancient Egyptians constructed gravity dams made out of stone bricks. Known as the Sadd el Kafara, Arabic for “Dam of the Pagans,” the dam towered at 37 feet (11.3 m) tall and was 348 feet (106 m) wide and 265 feet (81 m) at the bottom. Using 100,000 tons of gravel and stone, the dam was constructed. The dam was eventually listed as a failure because it lacked the watertight component necessary to keep it standing.

The second known dam was Nimrod’s Dam in Mesopotamia in 2000 B.C. Built north of Baghdad, Nimrod’s dam was a type of earth dam, similar to the gravity dam except made with soil. The dam was constructed in an attempt to prevent dangerous flooding.

The Romans built the Ponte di San Mauro Dam in 100 A.D., a gravity dam constructed out of concrete. Its remnants are still present today.

Another type of dam, the arch dam, was first conceived when the Kebar Dam was constructed in 1280 A.D in Quam, Mongolia. Built of limestone, it differed from other types of dams because it utilized the arch shape to support the structure of the dam and to keep it intact. The dam stands at 85 feet (26 m) high and is 180 feet (55 m) long. The Mongolians used the rock formations to help support the arch of such a large dam.

Arch dams were typically used when solid rock habitats were present and the gap to be bridged was generally narrower than those of gravity dams, which consisted of more shallow gaps and less solid rock or soil foundations. With the publication of Spaniard Don Pedro Bernado’s book on designing dams in the 1736, he suggested an alternative design referred to as the multiple arch dam, a dam that required buttresses to act as a support. This later became known as the buttress dam. The first dams to follow this form are the Almendralejo Dam, which were used to store water and generate hydroelectricity, and the Allum Dam.

In the mid 1800s, dams were becoming more of a necessity as a rush in gold mining led to a demand for more water resources. After time, dams became more than just practical structures, but ones that could contribute to the architecture of design.

The buttress dams, as introduced by Bernado, became frequent in the designs of dams in Spain and gradually this type of dam made its way around the world, including the U.S. The Old Mission Dam was constructed across the San Diego River as one of the first dams in the U.S.

The first dams were small -- only five feet (1.5 m) high -- and were built with mortar blocks, but soon bigger and more buttress dams were built. Concrete and rock developed a reputation for being sturdier, more solid building components and dams were eventually built with concrete as opposed to mortar. The Bear Valley Dam was initially built with mortar in 1884 but replaced with concrete in 1910.^[3]

The oldest dam still in operation is the Kofini Dam. Located on the Lakissa River in Greece, the dam was constructed 1260 B.C.^[4]

[edit] Modern Dams in the U.S.

In the U.S., the Bureau of Reclamation is responsible for initiating many of the dam and reservoir projects that are undertaken throughout the U.S., including the world famous Hoover Dam, located between the states of Nevada and Arizona. Under the Bureau's instruction, more than 600 dams throughout the U.S. have been constructed.

Others responsible for the construction of dams in the U.S. are the U.S. Army Corporation of Engineers and the Tennessee Valley Authority, the largest power company in the country.

In the beginning of the 1900s, dozens of dam projects were planned for the country -- the Truckee Carson Dam and the Roosevelt Dam were among these. In the 1930s a trend emerged that saw every major river obstructed by dams, hence the Hoover Dam in 1936, a dam so astronomical that it can be viewed from space.

Building projects such as the Glen Canyon Dam in the 1950s revealed that the curved feature of arch dams provide walls with the structured support to withstand the force exerted by water streams.^[5]

Today, dams continue to get bigger, as will be evident with the completion of the Three Gorges Dam and the Rogun Dam. Some critics have raised concerns about the damage that dams can do to the environment.

[edit] World’s Biggest Dams

[edit] Nurek Dam

Located in Vahksh, Tajikistan, the Nurek Dam stands 985 feet (300 m) tall.

[edit] Rogun Dam

Also located in Tajikistan, in the city of Vahksh, the Rogun Dam 1,100-feet tall.

[edit] Three Gorges Dam

This gravity dam is currently under construction in the Yangtze River in China. Due to be completed in 2009, the dam is expected to reach 607 feet (185 m) in height and be 7,575 feet ( 2,309 m) wide. It will be the world’s largest hydroelectricity plant.

[edit] Types 

[edit] Arch Dams

Arch dams are usually formed around solid rock formations. An arch shape is used to squeeze the dam together when the force of water is pushing against it. This type of dam is successful because it also exerts pressure towards the ground, which keeps it stable and intact. Typical examples include the El Atazar Dam in Spain.

[edit] Buttress Dams

Buttress dams are similar to arch dams in that they also contain an arched design, but they rely on the support of pillars or buttresses. Some are supported by concrete, such as Arizona’s Bartlett Dam. When water pressure is exerted against the dam, the buttresses positioned on the other side and stabilized into the ground provide a support for the dam while also pushing the water’s weight into the ground.

[edit] Embankment Dams

Embankment dams are built with a combination of earth and rock with a nucleus that is triangular, dense, and watertight. When water pushes against the embankment the force transfers to the dam’s core and is pushed downwards. Tailings, which are structures used in mining, are also a type of embankment dam. This type of dam accounts for approximately 80 percent of dams that are constructed.

[edit] Gravity Dams

Gravity dams use their own weight to resist the force exerted by water streams. They are usually large and costly to construct due to the amount of concrete they require. An example of a gravity dam is the Grand Coulee Dam in Washington.

[edit] Constructing a Dam

When any successful dam is completed, it is the result of careful planning and consideration. The site of the dam is chosen in a location that poses the least possible disturbance to the environment and its inhabitants, whether human or wildlife. The type of dam being built is in large part due to its surrounding environment and geology. This also helps determine what materials are going to be used to build the dam: whether it is concrete, timber, or reinforced steel.^[6]

Prior to the construction phase onsite, some work is prepared in advance, usually away from the dam. Some dams require access roads and tunnels before the construction on the actual dam can commence.

As dams are used to restrict and contain a water source, a lot of construction occurs on the bed of the river. Workers use tunneling machines and excavators to prepare for the dam construction. Cofferdams, airtight work sites for constructing in rivers or high depths of water, are also built to help advance the construction of the dam wall and clear the riverbed.

Generally dams require a good deal of excavation, calling on equipment such as excavators and shovels to remove rock, aggregate, and soil from the earth. Extracted materials are needed at a later date for backfill of the dam, so they are loaded into a dump truck until further use of them is required. Dams may also require tunneling machines, depending on the design of the dam, to provide easy access to the dam. Access roads are also sometimes built, such as when the Glen Canyon was constructed. In fact, not only were access roads constructed in this remote location, but also several diversion tunnels, a bridge, and an entire town for the workmen and their families.^[7]

Before the wall of the dam is constructed and once the overburden is removed by shovels, a foundation is laid to provide a support for the dam. This is an extremely crucial stage as most dam types require the force of the water to be received and pushed down the dam wall. During the foundation stage, workers grout the foundation and ensure air pockets and holes are carefully filled in to prevent as much corrosion as possible. Drills are used to penetrate the concrete and produce holes for which can be grouted. Some holes are also drilled to allow for bolts to be installed.

The construction of the wall takes place upon completion of the previous stages referred to. Some dams call for long periods of concrete pouring. Articulated dump trucks are used to return the backfill to the site while compaction is completed by pavers and rollers and other compaction equipment.

Once the wall is completed, the tunnels are closed and workers prepare the dam wall with features such as an outlet valve and a spillway, to help water escape from the dam. The spillway also acts as a safety measure in case of flooding or excess water spillage from the dam.^[8]

[edit] Maintaining Dams

Dams require a good deal of maintenance long after they have been constructed to avoid water leaks and clogs. Concrete risks cracking and must be repaired immediately, if not proactively. Dams have to be taken down for a variety of reasons, such as the blockage of fish migration, as well as irreperable damage.^[9]

[edit] How Dams Work

A dam requires a wall that is watertight so that it can fulfill its primary purpose, which is to provide a storage space for water that will eventually be utilized, whether for generating hydroelectricity or for some other purpose. The dam must also consist of a strong wall that can resist the force that is exerted by water. Outlet valves provide a means for controlled water to escape from the dam. In the event of a major flood, a spillway must be developed for the dam so that the water has a safe way of escaping that is not harmful to the structure of the dam.

[edit] Failed Dams

When dams do not succeed or collapse, they pose obvious hazards to people and the environment. Flooding occurs with the sudden collapse of a dam and contractors can best learn from dams that have not stood the test of time for various reasons. Tailing dams can not only cause flooding concerns but also some make a home for toxic materials that can make them a dual hazard in the event of a collapse. Examples of such failed dams include the Stava Tailings Dam in Italy and the Omai Tailings Dam in Guyana, which released toxic cyanide slurries into the environment.

Dams fail for reasons including poor construction or the wrong materials used. The Teton Dam in Idaho experienced a series of cracks and corrosion of concrete, which led to its collapse in 1976. Some dams, such as the St. Francis Dam and the Malpasset Dam in France experience problems with geology or the formation of rocks that support the dams. Other factors that play into the failure of dams include natural disasters such as earthquakes and landslides. ^[10]

[edit] References

1. ↑ The History of Dams. Water Project. 2008-09-09.
2. ↑ Dams. Water Encyclopedia. 2008-09-09.
3. ↑ History. UC Davis. 2008-09-09.
4. ↑ Dams. Water Encyclopedia. 2008-09-09.
5. ↑ Dams. Water Encyclopedia. 2008-09-09.
6. ↑ Dams. Water Encyclopedia. 2008-09-09.
7. ↑ Dams. Richard Woodward. 2008-09-09.
8. ↑ Dams. Richard Woodward. 2008-09-09.
9. ↑ Dam Basics. PBS. 2008-09-09.
10. ↑ Dams. Richard Woodward. 2008-09-09.