THINKING BIG : Building DAMS : Constructing a Concrete Giant

Water has always been a crucial resource. In almost everything we do, water plays a significant role. It is an essential element in food production--both in growing plants and sustaining livestock. It is also a source of hydroelectric energy. And as communities grow, so does the need for water. The greater the growth, the more complicated becomes the task of obtaining water. Since not everyone lives next to rivers or lakes, dams were built to help collect and distribute it.

Large water reclamation projects such as dams require tremendous amounts of materials and manpower to construct. For example, it took more than 5,000 people four years to complete Hoover Dam. The massive project required 5 million barrels of cement, 18 million pounds of structural steel, 21 million pounds of gates and valves, and 840 miles of pipes to complete. Here is how Hoover Dam was constructed:

1. Tunnels, 37 to 50 feet in diameter, are drilled in the steep canyon walls of the Colorado River. Metal pipes are then fitted into fitted into the tunnels.

2. Two small earth and rock dams, or cofferdams, are constructed, one just below the tunnel openings and another just below the dam site but above the tunnel outlets. The cofferdams keep water out of the dam site.

3. As the river backs up, water is diverted through the pipes and around the dam site.

4. Once the river has been diverted, construction of the dam site begins.

5. Loose and protruding rocks on the canyon wall are removed for stability.

6. To allow for quicker cooling of the concrete, construction of the dam is done in block sections. To further speed up the cooling process, cold water is pumped through pipes embedded in the concrete blocks.

7. Diverted water empties out of diversion tunnels and continues downstream.

* Note: Hoover Dam stores 31 million acre feet of water, enough to cover the entire state of Pennsylvania one foot deep.

Finding a Site

Dams can be build on almost any river. Where a dam is built depends on an area’s need for water. The size of the dam, however, dictates where on the river the dam is constructed. A stable bedrock and consistent river flow are essential.

Primary Uses for Dams

* Irrigation. One of the main function of dams. The water that dams are able to store in reserviors can easily be distributed to farmland, making food production possible year-round.

* Flood Control. Dams can be used to control the flow of the river to prevent flooding downstream.

* Hydroelectric Power. Dams the size of Hoover Dam can produce billions of kilowatt hours of electricity a year.

* Sport and Recreation. Fishing, boating and camping are a few of the many recreational use of reservoirs.

A Changed Landscape

Harnessing the flow of a river, particularly by large dams, transforms the landscape. Since a large area will be submerged by thereservoir that the dam creates, the environmental balance of the dam site must be seriously considered.

Types of Dams

* Masonry dams. These are dams made of solid materials, the most popular of which is concrete.

A. Gravity dams depend on the wieght of the materials used in ther construction for stability.

B. Hollow dams are concrete dams with a hollow portion inside the main body.

C. Buttress dams are dams that are held up by supporting buttresses.

D. Flat-slab dams are designed to have the wieght of the water hold the slabs down.

* Embankment dams are constructed with layers of earth materials. The most popular of this type of dam is the earthfill dam.

* Timber dams are relatively small and are usually built where timber is abundant. The timber is weighted down with rock.

How a Huydroelectric Dam Works

1. Intake towers or gates are opened, letting water in.

2. The incoming water is guided through pipes in the canyon wall toward the power plant.

3. The water’s velocity turns the turbines, generating electricity.

4. Water then flows out to continue downstream. *

Turning the Turbines

1. Water is guided toward the turbine.

2. The water spirals horizontally around the turbine where guide vanes direct it to hit the turbine blades with maximum efficiency.

3. The turbine drives the generator shaft, producing electricity.

4. The water flows out through the turbine’s center and continues downstream. *

Generating Energy

Hoover Dam uses AC generators. Here is how the other type of generator, DC, works.

1. The generator shaft spins a coil between poles of an electromagnet.

2. As the cuts through the lines of force, an electric current flows through the coil.

3. Since the flow of electrons through the generator’s coil reverses every half turn, the commutator also rotates, contacting each half with alternate carbon brushes. This ensures that direct current is produced because one brush is always negative and the other positive.

Sources: Bureay of Reclamation Lower Colorado Region; The World Book Encyclopedia; The Way Things Work; High Steel, Hard Rock and Deep Water.

Researched by DAVID F. MONTESINO / Los Angeles Times