Text 6. Viaduct. Aqueduct.

Viaductis like a bridge, except that it crosses over dry land instead of water. Some viaducts do cross water, but they also cross dry land instead of merely extending from one bank to the other as a bridge does.

Most viaducts consist of a series of supports under beam-and-slab or arch construction. Viaducts carry railroad tracks over valleys and gorges. Some viaducts are.jilt higher than the general level of the land to carry railroads over highways or to make a safe crossing for highways over railroads. The ancient Romans built the first viaducts. The aqueducts they built to carry water to cities often also served as roadways. One of the longest viaducts ever built was a portion of the 110-mile (177-kilometer) Key West extension of the Florida East Coast Railway. Viaducts that extended over the open sea formed 30 miles (48 kilometers) of this extension. Parts were destroyed by a hurricane in 1935, but these were rebuilt as a highway a few years later. The main part of the pier viaduct over the mouth of the River Tay in Scotland has 84 steel spans and is over 2 miles (3.2 kilometers) long. The Tunkhannock viaduct on the Lackawanna Railway is one of the largest steel and concrete viaducts in the world. The Tunkhannock viaduct is 2,375 feet (724 meters) long and includes 10 spans of 180 feet (55 meters) each. Another famous viaduct is the 3-mile (5.6-kilometer) Pulaski Skyway between Newark and jersey City. Other '.veil-known viaducts are the Pecos River viaduct in Texas and the Landwasser viaduct across Albula Pass, in the canton of Craubunden, Switzerland.

Historically, many agricultural societies have constructed aqueducts to irrigate crops. Archimedes invented the water screw to raise water for use in irrigation of croplands, Another widespread use for aqueducts is to supply large cities with clean drinking water Some of the famed Roman aqueducts still supply water to Rome today. In California, USA, three large aqueducts supply water over hundreds of miles to the Los Angeles area. Two are from the Owens River area and a third is from the Colorado River. In more recent times, aqueducts were used for transportation purposes to allow canal barges to cross ravines or valleys. During the Industrial Revolution of the 18th century, many aqueducts were constructed as pan of the general boom in canal-building. In modern civil engineering projects, detailed study and analysis of open channel How is commonly required to support flood control, irrigation systems, and large water supply systems when an aqueduct rather than a pipeline is the preferred solution. The aqueduct is a simple way to get water to other ends of a field. In the past, aqueducts often had channels made of dirt or other porous materials. Significant amounts of water arc lost through such unlined aqueducts. As water gets increasingly scarce, these canals are being lined with concrete, polymers or impermeable soil. In some cases, a new aqueduct is built alongside the old one because it cannot be shut down during construction. The Central Arizona Project Aqueduct, the largest and most expensive aqueduct system ever constructed in the United States.

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