Flood. The account of the Deluge in the Bible and similar traditional accounts are evidence that there always have been floods large enough to affect humanity's struggle to win a living from the earth. The early civilizations that grew along the Nile, Tigris, Euphrates, Indus, and Hwang Ho (Yellow) rivers are examples of the development of civilizations in an environment where a river enforced community action for flood protection. Such an environment nurtured humans and their crops, but the river was also a source of disaster.
In China, the Hwang Ho has been the cause of repeated floods during 4,000 years of continuous settlement. The land subject to flooding has an area of about 55,000 square miles (140,000 sq km), and it has been densely populated. Because of its devastating floods, the Hwang Ho is known as "China's Sorrow." Despite the repeated setbacks caused by the floods, however, the Chinese did not abandon their lands. This generally was true of the other great early civilizations that experienced river floods. For example, in his explorations at the ancient city of Ur in Mesopotamia in the 1920s, Sir Leonard Woolley found evidence that sediments deposited by a flood became the site for rebuilding the settlement.
The accompanying table gives a chronology of notable floods that have occurred during the period since the earliest one recordedˇXa flood in China in 2297 B.C.
Most of the world's population and property are located on lands subject to the overflow of rivers or seas. For example, flood-prone lands comprise about 5% of the area of the United States, more than 10% of the Hwang Ho basin in China, almost all of the Netherlands, and nearly all of the southern part of Vietnam.
River and Coastal Lands. One major type of flood is a river flood. Many cities, including Paris, Rome, Washington, and New Orleans, occupy land subject to river floods.
The flow of a river usually is confined to a well-defined channel that meanders in the course of time from one side of its valley to the other. However, when there is a heavy rainfall or rapid snowmelt, the river overflows into a wide, flat area adjacent to the channel. This area, the natural floodway of the river, is called a floodplain, which is composed of sediments deposited by the river. Typically, a river uses some portion of its floodplain about once in two or three years. Once in a century a river may inundate its entire floodplain to a considerable depth.
A second major type of flood is a coastal flood. Coastal lands, such as offshore bars formed by sediments carried by coastal currents, occupy a position relative to the sea that floodplains do to rivers. Like floodplains, offshore bars and barrier beaches are the sites of many citiesˇXfor example, Atlantic City, N.J.; Miami, Fla.; Galveston, Tex.; and Lagos, Nigeria. All are subject to coastal floods.
Flood Damage. The floodplains, the coastal bars, and the ocean strands invite people's occupancy. Floodplain soils normally are more fertile and easier to till than uplands, and the flatlands are less costly to build on. Because rivers and oceans have been the main avenues of commerce, cities naturally were built on the contiguous lands. For these reasons, overflow of rivers or the sea destroys crops, causes extensive damage to property, cuts transportation lines, disrupts the life of cities, and takes a toll in lives. Loss of life as a consequence of floods is decreasing in industrial countries that use protective and flood-warning measures, but its toll continues unabated in less developed countries.
In the United States, the floodplains of the Ohio River and its major tributaries have been the scene of the greatest flood damages. The industrial Northeast and Middle Atlantic regions come a close second. No region is spared from flood damages, not even the deserts, where sporadic floods form fearsome walls of water that rush down normally dry channels and wash out roads and bridges.
The floods in the United States cause an average annual loss of 80 lives and a total damage of about $1 billion. Damage to rural and agricultural property accounts for about 50% of this total, to urban and industrial property about 30%, and to transport facilities about 20%. Only large annual investments in flood control keep damages in the United States from increasing.
Flood Measurements. Floods usually are measured in terms of the depth of the overflow. The depth of a river's overflow varies in relation to the rate of flow and the capacity of the channel. The depth of coastal overflow depends on the barometric pressure, the wind strength, and the configuration of the coast. However, these conditions do not provide the data needed for comparing the severity and the frequency of floods in different locations.
Flood magnitudes in different locations are compared on the basis of the recurrence interval, which is the average time between floods that exceed a given size. For example, if in a 50-year period five floods had a rate of discharge exceeding 1,000 cubic feet (28 cubic meters) per second, a flood of this magnitude would be said to have a recurrence interval of 10 years. Another way to compare floods is on the basis of their probability of occurrence in any single year. Thus a flood having a recurrence interval of 10 years has a 10% probability of occurring in any single year.
River Floods. River floods may occur in any season, but they are more likely in spring and in winter. In the spring, the two chief causes of flooding are heavy rains and a marked rise in temperature; in the winter, the chief factors are frozen soil, deep snow, and a thaw. Floods during winter and spring occur on major rivers and affect large areas. Floods during the warm seasons are due to torrential rainstorms and therefore occur on the smaller streams.
Coastal Floods. Floods along coasts are of two kinds. Some are caused by meteorological disturbances, such as hurricanes and other storms at sea; others by seismic disturbances, such as submarine earthquakes, landslides, and other disturbances of the sea bed. Both kinds of disturbances have preferred locations. Hurricanes and typhoons commonly occur in the West Indies and the East Indies, respectively, and extratropical cyclones occur in the Atlantic. Sea waves of seismic origin, called tsunamis, are most common in the Pacific, although one of the most destructive tsunamis struck Lisbon after an earthquake in 1755.
Flood hazard information influences attitudes toward floods, home building, and the prudent use of land. Partly for this purpose, a comprehensive national program for managing flood losses was started by the United States government in 1966. As one part of this program, the U.S. Geological Survey prepares and publishes maps showing areas subject to relatively large floods. These maps and other information on floods are provided by the local office of the U.S. Geological Survey.
Individuals also can play a significant role in reducing losses due to flooding. Homes and other buildings can be designed to include provisions for movable bulkheads that can be placed in windows and doors in times of flood warning, placement of electrical and plumbing systems to avoid damage by floodwater; and arrangement of storage space above flood levels.
Coastal Flood Protection. In the case of coastal floods, there is no practical way to provide full protection on most exposed shorelines. The buildup of beaches and sand dunes by means of artificial barriers may provide a partial solution, but such barriers may seriously interfere with the natural beauty and recreational value of beach areas. For these reasons, the principal ways to reduce damage from floods due to hurricanes and other sea storms are adequate warning services, evacuation plans, proper building design, and zoning of the more hazardous areas.
River Flood Protection. In the case of river floods, various measures can be taken to reduce flood damages, including confining the river water, storing the water, or increasing the channel capacity.
Many stores in the United States sell NOAA (National Oceanic and Atmospheric Administration) weather radios, which allow listeners over a range of about 40 miles (60 km), depending on topography, to hear updated local weather warnings continuously broadcast by the agency's National Weather Service. Such warnings may include announcements of a flash flood or flood watch (indicating flash flooding or flooding is possible within a designated watch area), a flash flood or flood warning (indicating flooding is imminent), or an urban and small stream advisory (indicating the occurrence of flooding of small streams, streets, and such low-lying areas as railroad underpasses and urban storm drains).
Flood Forecast Conditions. For small headwater streams there is not sufficient time for forecasts because a flood peak may occur within minutes after a heavy rainfall. Flash warnings issued by a local official on the basis of rainfall reports are the best course of action that can be taken under these conditions. For medium-sized rivers draining areas larger than 100 square miles (260 sq km), accurate forecasts based on rainfall reports are possible because a flood peak occurs hours after a heavy rainfall. For larger rivers draining areas greater than 20,000 square miles (50,000 sq km), even more accurate and longer forecasts are possible. For these rivers the forecasts can be based on reports of actual flood heights observed upriver. For instance, forecasts of flood heights on the lower Mississippi can be made from observations of flood heights in the upper river and its major tributariesˇXthe Ohio, the Missouri, and the Arkansas riversˇXas much as a week in advance.
Walter B. Langbein, U.S. Geological Survey
Source: . "Flood." Encyclopedia Americana. Grolier Online http://ea.grolier.com/cgi-bin/article?assetid=0159710-00
(accessed August 13, 2007).