What are Aqueducts?
Water often has to be transported from its source, such as a lake or river, to where it is needed for drinking, or irrigation, or for other purposes.
The term "aqueduct" is commonly applied to large channels that carry the main water supplies of cities. The water may be carried above or below the ground, either as a free-flowing stream or under pressure in a pipeline.
The most familiar kind of aqueduct is an open channel carried by a bridge across a valley. But pipelines and tunnels are also classed as aqueducts.
Aqueducts were well known to the Greeks, but were perfected by the Romans, and many are still standing. One at Nimes in France stands more than 45 meters high and has three tiers of massive stone arches.
Why Aqueducts Are Needed
Water may be led from one place to another in several ways. Under the ground it is usually carried in pipes or in big tunnels. Over the ground it may be carried in small open channels or in larger one called canals. If the channel or canal has to cross a valley or hollow it may be carried on a bridge and it is this arrangement which is usually called an aqueduct.
From ancient times, cities have been built on the banks of rivers because it was eaiser to travel and to bring goods by water than by land. Before water was laid on in houses, the people of the city washed themselves and their clothes in the riverand took their animals down to drink there.
They also got rid of much of their refuse and sewage matter by throwing it into the river. As a result the river water became dirty and unpleasant to drink, besides being so full of of germs that it often made people ill. So it became necessary to find a special supply of unpolluated water.
One way of dealing with this problem was to build an aqueduct to connect the city with a source of pure water. Because the water in a channel always flows downhill. This source of pure water has to be higher than the city unless the water can be pumped.
Aqueducts across valleys used to be built by erecting layers of arches one above the other auntil the top of the topmost layer reached the level of the channel at that point of its jounrey to the city.
One of the earliest known aqueducts was built of masonry about 691 B.C. for the city of Nineveh in Assyria. It was more than 50 miles (80 km) long and as wide as a main road. At one point, it was carried over a valley by a bridge about 920 feet (280 meters) long. No comparable structure was built until Roman times.
The Greeks built one of the first tunnel aqueducts in the late 6th century B.C. on the island of Samos in the Aegean Sea. It was about 1,100 feet (340 meters) long and brought water through the base of a hill 900 feet (275 meters) high. As in most early Greek aqueducts, the water was carried by a terra-cotta pipe. Elsewhere, stone pipes or pipelines made of wood and iron were used.
The Roman aqueducts are the most famous and were the most extensive of those in the ancient world. More than 200 of them were built, and many may still be seen in Spain, France, northern Africa, and Asia Minor as well as in Italy.
The first Roman aqueduct was built about 312 B.C. Appius Claudius Caecus built a great road called the Via Appia (Appian Way) and a great conduit, the Aqua Appia (Aqueduct of Appius).
The Aqua Appia was more than 16 kilometers long and ran underground all the way. Forty years later another underground conduit, the Anio Venus, which led from River Anio, was dug. It was more than 50 kilometers long.
In the 2nd century B.C the first above-ground channel with real aqueducts leading to Rome was built. This was the Aqua Marcia, which was nearly 90 kilometers long. "The pride of ancient Rome," was built about 144 B.C. and probably the first in Italy to be supported for some distance by arches.
The 11 aqueducts that eventually supplied water to the city of Rome carried about 80 million gallons (303 million liters) daily, a generous supply of about 38 gallons (144 liters) for each resident. Four of these aqueducts remained in use until the 20th century. Among the best-known of the Roman provincial aqueducts is the Pont du Card near Nimes, France. This was built in about 19 B.C. by the Roman general Agrippa to bring water from the Gard River to Nimes. The bridge is 270 meters long and it carries a channel, covered by stone slabs, 120 broad and 130 centimeteers deep.
Ancient Greek and Roman aqueducts were usually free-flowing channels built of masonry, or brick and concrete. Their course was almost horizontal, descending slightly toward the point of delivery, and often underground. When an aqueduct had to cross a valley, its gently sloping course was maintained by carrying the channel on a series of arches or on a wall.
The few aqueducts built during the Middle Ages and the Renaissance were usually similar to the Roman bridge aqueducts, which were still in use. A notable exception was the New River, a long open-canal aqueduct built for the city of London in 1613.
In modern times, water is usually conducted under pressure through pipelines called inverted siphons. These are laid to follow the general contours of the land and must be airtight. As long as the points along the pipeline route lie on a lower level than its point of origin and there is no air in the pipes, enough pressure will be built up on the downward course to carry the water over hilltops. This type of construction was little used in anotent times because, with a limited supply of metal, it was difficult to make the pipes both airtight and strong enough to withstand pressure. The development of cast iron, steel, and concrete pipes has made possible the widespread use of more efficient pressure pipelines.
The Old Croton water supply for New York City, completed in 1842, was the first large modern aqueduct in the United States. Most of its channel lies in a covered trench on the surface of the ground. The trench, about 8 feet (2.4 meters) in diameter and 38 miles (61 km) long, is built of concrete and masonry lined with brick. One of the last aqueduct bridges of the Roman type was constructed to carry the trench across the Harlem River. The Old Croton was designed to carry about 22 gallons (83 liters) of water per day for each resident, but by 1883 this supply had proved inadequate. The New Croton, built in that year, was one of the earliest to consist almost entirely of deep underground tunnels. One of these passed under the Harlem River.
Wide and deep underground tunnels have been made possible by modern machinery and engineering methods. Though costly, they are the safest and most reliable aqueducts, supplying many large cities in the 20th century. Modern tunnels and pipelines are generally made of steel or reinforced concrete.
Of the many aqueducts built during the early 1900s, the Los Angeles aqueduct, finished in 1913, is among the most remarkable. Its course of more than 200 miles (320 km) from the Owens River to Los Angeles consists of canals, covered conduits, tunnels, and pressure pipes. The 95 mile (153 km) Catskill aqueduct in New York was also built at this time.
An even more ambitious project, the Colorado River-Los Angeles aqueduct, was begun in 1939. One of its tunnels runs through solid rock for 13 miles (21 km). Many of the tunnels are equipped with pumps to help carry water along its 242 mile (389 km) course. Branches of the aqueduct supply several other cities in addition to Los Angeles. This aqueduct is expected to provide an adequate water supply until the year 2000.
Among modern aqueducts may be mentioned that of Glasgow which brings water to that city from Loch Katrinel that of Machester, which taps Thirlmere; and that of Liverpool, with Lake Vrnwy in North Wales as its source.
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