1. Chapter 33 Dam Construction

2. Objectives After reading the chapter and reviewing the materials presented the students will be able to: Give reasons why dams are constructed Describe the process of planning and designing dams Describe the features of dams Describe the process of building dams

3. Introduction A dam is a barrier that contains a body of water. The stored water can be piped to a city or used to irrigate crops. Multipurpose dam projects are dams that serve more than one purpose. Dams are built to provide a reliable water supply, provide flood control, maintain downstream flow during dry season, control erosion, generate electricity, and provide recreation.

4. Purposes of Dams Many cities rely on reservoirs for their water supplies. Another reason for building dams is to control flooding along the river or stream below the dam. Outlet works are devices used to regulate the flow of water downstream from the dam and control the level of water in the reservoir. A dam can maintain a steady flow of water downstream during dry periods. By controlling the flow of water downstream from the dam, soil erosion is reduced. Hydroelectric plant: is an electric generating facility that is powered by water pressure. Reservoirs can be used for activities such as boating, swimming, fishing, and waterskiing.

5. People Who Build Dams A farmer may have a small earth dam built across a stream to store water for livestock. Cities build dams to store water for residential and industrial uses. Several cities or counties may form a special district to construct a dam for water supply, flood control or recreation. Civil engineers design dams with the assistance from mechanical and electrical engineers and agronomists (specialists in soil and crops).

6. Planning and Designing Dams The first step in planning a large dam and reservoir project is to conduct a feasibility study. Public hearings allow citizens to participate in the planning process. Possible sites for the dam are selected. General designs and cost estimates are prepared. Environmental impact statements are required by federal law when there is a possibility that the proposed project will do serious damage to the natural environment. A cost-benefit analysis is prepared to evaluate both the costs and benefits of the project. The preferred site for a dam is where the river valley narrows. The valley upstream should be much wider to hold a large volume of water.

7. Features of Dams Major dams have an earth embankment or concrete structure to hold water in the reservoir, concrete and steel outlet works, and a spillway. Steel gates control the water level of the reservoir. The outlet works often include a hydroelectric generating plant. The spillway allows excess water to flow from the reservoir and prevents the dam from overflowing. Some dams include locks that raise and lower boats navigating the river.

8. Land Acquisition Land acquisition for a large dam can take several years to complete. Land for the dam and access roads is purchased first. Land for the reservoir and for relocation of roads, utilities, and homes is purchased as needed. The government may exercise its power of eminent domain to acquire private land when an owner is unwilling to sell.

9. Constructing Dams Building access roads and extending utilities to the site are two jobs that need to be completed early in the process. The next job to be done is the excavation of the abutments (ends) of the dam. Abutments: The ends of a dam designed to provide solid anchorage between the ends of the dam and the sides of the river valley. The next task is to construct a cofferdam (watertight enclosure) of the area where the outlet works are to be constructed. The purpose of the cofferdam is to prevent flooding of the work site. The outlet works can then be built. The spillway may be built considerable distance from the outlet works. Installation of the electrical generating equipment is typically the last major task to be completed. Once the power generating equipment is tested, the owner of the dam assumes responsibility for operation of the completed project.

10. Summary A dam is a barrier that contains a body of water. The stored water can be piped to a city or used to irrigate crops. Multipurpose dam projects are dams that serve more than one purpose. Dams are built to provide a reliable water supply, provide flood control, maintain downstream flow during dry season, control erosion, generate electricity, and provide recreation. Outlet works are devices used to regulate the flow of water downstream from the dam and control the level of water in the reservoir. Several cities or counties may form a special district to construct a dam for water supply, flood control or recreation. Civil engineers design dams with the assistance from mechanical and electrical engineers and agronomists (specialists in soil and crops). Steel gates control the water level of the reservoir. The outlet works often include a hydroelectric generating plant. The spillway allows excess water to flow from the reservoir and prevents the dam from overflowing. Some dams include locks that raise and lower boats navigating the river. The government may exercise its power of eminent domain to acquire private land when an owner is unwilling to sell. Abutments: The ends of a dam designed to provide solid anchorage between the ends of the dam and the sides of the river valley. The purpose of the cofferdam is to prevent flooding of the work site. The outlet works can then be built. Installation of the electrical generating equipment is typically the last major task to be completed. Once the power generating equipment is tested, the owner of the dam assumes responsibility for operation of the completed project.

11. Home Work 1. Why are dams built? 2. Explain the power of eminent domain.