A flood occurs when a body of water rises and overflows onto normally dry land. The effects of floods, both beneficial and destructive, have been recorded for at least 5,000 years, serving as the basis for myth, religious beliefs, and scientific study.
The most familiar flood story is that in the Old Testament tale. The event upon which this tale is based may have actually occurred about 3000 BC, when the Euphrates River overflowed, covering a vast area. According to the Bible, the flood resulted from 40 days of continual rain, producing high water that lasted 150 days and flood depths in excess of 25 ft.
The benefits of regular flooding were appreciated in ancient Egypt, where the floodwaters of the Nile brought fertile silt and much-needed water to the fields each year. The pharaohs, recognizing that flooding meant prosperity, levied higher taxes after floods. Accordingly, the oldest records of stream levels are from the Nile. Flooding has probably been more intensely studied and more carefully recorded than any other natural phenomenon since the birth of civilization.
Types of Floods
Floods are classified in various ways for many special purposes. In the most useful general classifications, coastal flooding of lakes and oceans is distinguished from river flooding.
Coastal flooding can be caused by high, wind-generated water waves, exceptionally high tides, giving away of coastal areas, and tsunamis. Coastal flooding is of special concern because, in many countries, population is concentrated along coastlines. For example, U.S. coastal counties - those bordering oceans or the Great Lakes - contained over 50 % of the population in the early 1990's.
Although exceptionally high tides rarely produce serious and widespread damage on their own, they may significantly increase the hazard of flooding in combination with even moderately severe storms. Hurricanes and major storms produce most coastal floods. In 1970, a major storm in the Bay of Bengal produced heavy seas that inundated the coastal regions of East Pakistan, killing some 200,000 people. Wind-generated waves well over 100 feet in height have been observed in the open ocean. Fortunately, these huge waves usually diminish in size before reaching coastlines.
River Floods
River flooding results from a variety of causes. Natural causes include rain, snow melt, and ice jams. The rate at which soil can absorb water decreases with continuous wetting. The longer a rainstorm lasts, the more likely it is that rainwater will flow across the ground as runoff and enter stream channels.
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The Mississippi River during the 1993 Flood |
Floods on major rivers, such as the Mississippi and the Missouri, result from prolonged periods of rain over large areas. This was the case in 1993, when torrential rainfall that began in April continued into May, June, and July, flooding 16 million acres in Iowa, Illinois, Minnesota, Missouri, Wisconsin, South Dakota, Nebraska, and Kansas. Dozens of deaths and damage well over $10 billion resulted as flood levels rose to record heights and many levees broke. Persistent heavy rains in California in January, 1995 caused at least 11 deaths, widespread flooding and mudslides, and severe property damage.
Flash Floods
Cloudburst floods are caused by extremely intense rainfall, 10 inches or more an hour, but they are short-lived, rarely continuing for more than a few hours at a given location. They tend to be somewhat more common in mountainous areas, where steep slopes cause water to travel at high speeds, thus eroding and carrying away natural and artificial debris. These floods often occur rapidly and with little warning.
Floods that results from snow melt and ice jams are especially dangerous because they are not necessarily preceded by heavy precipitation. Even moderate amounts of warm rain falling on a snowpack, particularly if the ground beneath the snow is frozen and unable to absorb the moisture, can cause severe flooding.
The actions of humans may also cause floods. In the most obvious case, floods result from failure of artificial structures such as dams. Dams usually fail because of poor design or siting, geologic hazards such as earthquakes and landslides, or simply old age. One of the most devastating dam failures in the U. S. occurred in February, 1972 at Buffalo Creek in Logan County, West Virginia. A dam used to impound coal-mining wastes, as well as water, completely collapsed after three days of rain, causing a 3-hour flood that took 118 lives and did $65 million in damages.
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