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Dams can be formed by human agency, natural causes, or even by the intervention of wildlife such as beavers. Man-made dams are typically (15) __________ according to their size (height), intended purpose or structure.
1) International standards define large dams as higher than 15–20 meters and major dams as over 150–250 meters in height. The (16) __________ dam in the world is the 300-meter-high Nurek Dam in Tajikistan.
2) Purposes include providing water for irrigation to a town or (17) __________ water supply; improving navigation; creating a reservoir of water to supply industrial uses; generating hydroelectric (18) __________ ; creating recreation areas or habitat for fish and wildlife; retaining wet season flow to minimize downstream flood risk. Some dams can also serve as pedestrian (= a person who is walking in the street) bridges or bridges for vehicles across the river as well. Few dams serve all of these purposes but some multi-purpose dams serve more than one.
A saddle dam is an auxiliary dam constructed to confine the reservoir created by a primary dam either to permit a higher water elevation and storage or to limit the extent of a reservoir for increased efficiency. An auxiliary dam is constructed in a low spot or saddle through which the reservoir would otherwise escape.
An overflow dam is designed to be over topped.
A check dam is a small dam designed to reduce flow velocity and control soil erosion.
A wing dam is a structure that only partly restricts a waterway, creating a faster channel that resists the accumulation of sediment.
A dry dam is a dam designed to control flooding. It normally holds back no water and allows the channel to flow freely, except during periods of intense flow that would otherwise cause flooding downstream.
A diversionary dam is a structure designed to divert all or a portion of the flow of a river from its natural course.
3) Based on structure and material used, dams are classified as timber dams, arch-gravity dams, embankment dams or masonry dams, with several subtypes.
One of the best places for (19) __________ a dam is a narrow part of a deep river valley; the valley sides can then act as natural (20)__________ . The primary (21) __________ of the dam's structure is to fill the gap in the natural reservoir line left by the stream channel. The sites are usually those where the gap becomes a minimum for the required storage capacity. The most economical arrangement is often a composite structure such as a masonry dam flanked by earth embankments. The current use of the land to be flooded should be dispensable.
Read the text again and answer the following questions.
1. What is the main purpose of a dam?
2. What measurements did the earliest dam have?
3. How far from the capital of Egypt was the first ancient dam constructed?
4. Why are the Romans considered to be the pioneers in the process of dam-construction?
5. What is the connection between the word ‘dam’ and the name of some cities in Europe?
6. What is the main principle of classification of dams?
7. Where do people usually construct dams?