LYCOS RETRIEVER 
Floods: Rivers
built 641 days ago
At the junction of three rivers—the Allegheny, the Monongahela, and the Ohio—Pittsburgh, Pennsylvania, has suffered several devastating floods. On March 16, 1907, heavy rains and melting snow brought the river stage to 36.6 feet, causing the greatest flood in the city to date. Residents were caught completely unaware due to the rapid rise of the water, but surprisingly few lives were lost—estimates range from 6 to 12. However, damage to property and business was great, with total losses estimated at $5,000,000. Electricity was cut off and hundreds of workers found themselves out of work due to mill and industrial plant closures.
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Flash floods occur when excessive rainfall sends a river out of its bank in a period of several hours or less (Flood and Flash Flood Occurrence), usually during the night. (Prediction and Mitigation of Flash Floods) The usually are the result of cloudbursts or torrential rains. (Confronting Natural Disasters: An International Decade for Natural Hazard Reduction) Flash floods can reach heights of twenty feet and higher. (Flash Flood Safety Rules) This type of natural disaster is common mainly in desert and mountainous regions. However, they can still occur and cause damage where streams flow in narrow canyons, where there are steep terrain and high surface run off rates. They ... can occur whenever thunderstorms are present.
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The worst floods usually occur when a river overflows its banks. An example of this is the January 1999 Queensland floods, which swamped south-eastern Queensland. Floods happen when soil and vegetation cannot absorb all the water. The water then runs off the land in quantities that cannot be carried in stream channels or kept in natural ponds or man-made reservoirs.
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Levees are artificial river banks built to control the spread of flood waters and to limit the amount of land covered by floods. Levees provide protection from some floods but can be over-topped or eroded away by large floods. For example, levees failed to protect vast areas in the Mississippi and Missouri River valleys during the record-setting floods that occurred in 1993.
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Most large rivers in densely populated areas have been modified to help prevent floods, and to obtain new land for urban development as well as assuring agricultural production. In many cases, hydrological engineering structures have changed river courses and channels considerably, with meanders and branches straightened and redirected.
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In western countries, rivers prone to floods are often carefully managed. Defences such as levees, bunds, reservoirs, and weirs are used to prevent rivers from bursting their banks. Coastal flooding has been addressed in Europe with coastal defences, such as sea walls and beach nourishment.
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