Types of dams

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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?

CANALs

Read the following text. Try to understand it. Use your dictionary if necessary. Make up 8 – 10 questions to the text. Let your classmates answer your questions. Check the answers.

Canals are human-made channels for water. There are two types of canals:
- Aqueduct canals that are used for the delivery of fresh water, for human consumption, agriculture;
- Waterway canals that are navigable transportation canals used for carrying ships and boats loaded with goods and people, often connected to existing lakes, rivers, or oceans. Included here are inter-ocean canals such as the Suez Canal and the Panama Canal.

The word "canal" is also used for a city-canal in cities such as Venice, Amsterdam or Bangkok.

Smaller transportation canals can carry narrow-boats, while ship canals allow seagoing ships to travel to an inland port (e.g.: Manchester Ship Canal), or from one sea or ocean to another (e.g.: Caledonian Canal, Panama Canal).

At their simplest, canals consist of a trench (= a long narrow hole that is dug in the ground for water to flow along) filled with water. Depending on the stratum the canal passes through, it may be necessary to line the cut with some form of watertight material such as clay or concrete.

Canals need to be leveled. Different methods of solving this problem exist nowadays.

The oldest known canals were irrigation canals, built in Mesopotamia circa 4000 BC, in what is now modern day Iraq and Syria.

In ancient China, large canals for river transport were established as far back as the Warring States (481-221 BC), the longest one of that period being the Hong Gou (Canal of the Wild Geese), which according to the ancient historian Sima Qian connected the old states of Song, Zhang, Chen, Cai, Cao, and Wei. By far the longest canal was the Grand Canal of China, still the longest canal in the world today. It is 1,794 kilometres (1,115 mi) long and was built to carry the Emperor Yang Guang between Beijing and Hangzhou. The project began in 605 and was completed in 609.

In the Middle Ages, water transport was cheaper and faster than transport overland. This was because roads were unpaved and in poor condition; so greater amounts could be transported by ship. The first artificial canal in Christian Europe was the Fossa Carolina built at the end of the 8th century. More lasting and of more economic impact were canals like the Naviglio Grande built between 1127 and 1257.

Canal building progressed steadily in Germany in the 17th and 18th centuries with three great rivers, the Elbe, Oder and Weser being linked by canals. In post-Roman Britain, the first canal built appears to have been the Exeter Canal, which opened in 1563. The oldest canal built for industrial purposes in North America is Mother Brook in Dedham, MA. It was constructed in 1639 to provide water power for mills. In Russia, the Volga-Baltic Waterway, a nationwide canal system connecting the Baltic and Caspian seas via the Neva and Volga rivers, was opened in 1718.

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