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Causes of floods

Hydrologic cycle

Human populations

Human influence on flooding

Weight of water and force of floods

Flood intervention


Flooding, although it usually carries a negative connotation, is a natural process, the response of a river system to the presence of too much water during an interval of time. Rivers and streams are governed by a simple equation, Q = A x V, where Q is discharge (amount of water), A is the area of the river channel, and V is velocity. When excess discharge is present in a river or stream, at first the water moves more quickly (V increases) and perhaps some channel erosion takes place (i.e., A increases). If discharge Q increases too rapidly, however, water must move out of the channel (the confining area, A) and out onto the surrounding area, known as the floodplain. The floodplain is the area that floods first.

Floods are caused by a variety of factors, both natural and man-made. Some obvious causes of floods are heavy rains, melting snow and ice, and frequent storms within a short time. The practice of building homes and towns near rivers and other bodies of water (i.e., within natural floodplains) has contributed to the disastrous consequences of floods. In fact, floods have historically killed more people than any other form of natural disaster, whether from catastrophic events such as the 2004 Indian Ocean tsunami that, or seasonal flash floods that claim lives mainly when people attempt to drive through them. Because of this, people

have tried to manage floods using a variety of methods with varying degrees of success.

Causes of floods

Many floods are directly related to changes in weather. The most common cause of flooding is rain falling at extremely high rates or for an unusually long period of time. Additionally, areas that experience a great deal of snow in winter are prone to springtime flooding when the snow and ice melt, especially if the thaw is relatively sudden. Furthermore, rainfall and snowmelt can sometimes combine to cause floods.

Sometimes, floods occur as a result of a unique combination of factors that only indirectly involve weather conditions. For instance, a low-lying coastal area may be prone to flooding whenever the ocean is at high tide. Exceptionally high tides may be attributed to a storm caused by a combination of factors, like low barometric pressure and high winds. Finally, floods sometimes can occur regardless of the climate. Examples are volcanic heating and rapid melting of a snow pack atop a volcanic mountain or under a glacier, failures of natural or man-made dams, or tsunamis (seismic ocean or large lake waves caused by earthquakes). The 2004 Indian Ocean tsunami was caused by a 9.1 magnitude earthquake beneath the Indian Ocean west of Sumatra, Indonesia, that sent its vibrations out into the ocean in all directions. The resulting tsunami created waves of up to 100 feet (39 m) that battered the coasts of India, Sri Lanka, Thailand, Indonesia, and the west coast of Africa. In one of the deadliest disasters in modern history, flooding from the tsunami killed 230,000 people.

Hydrologic cycle

An underlying influence on many floods is the hydrologic cycle. This is the evaporation of water from the oceans into the atmosphere that falls as rain or snow on land. The water then runs off the land (or is absorbed by it) and, after some period of time, makes its way back to the oceans. Scientists have found that the total amount of water on Earth has not changed in three billion years; therefore, the hydrologic cycle is said to be constant. The same water has been filtered by soil and plant use and purified by temperature changes over many millennia. Rivers and streams may feed water into the ground, or where springs persist, groundwater may supply water to streams (allowing them to flow even when there is a drought).

Although the hydrologic cycle is a constant phenomenon, it is not always evident in the same place year after year. If it occurred consistently in all locations, floods and droughts would not exist. Thus, some places on Earth experience more than average rainfall, while other places endure droughts. It is not surprising, then, that people living near rivers eventually endure floods.

Human populations

For millennia, human populations have chosen to live near bodies of water. There are three main reasons for this: (1) the soil near the waters is very fertile and can be used for growing crops; (2) the bodies of water themselves are sources of drinking and irrigation water; and (3) water courses support transportation and facilitate commerce and trade.

While floods can have disastrous effects, they leave behind silt and other sediments that make the land surrounding rivers and other bodies of water rich and fertile. The soil deposited by moving water is known as alluvial soil. At first, populations avoided settling directly on the low-lying land, called floodplains, surrounding the rivers and instead built their villages on terraces or bluffs close to but higher than the rivers. Examples of cities developing on such terraces are Washington, Paris, and Budapest. The advantages of building there is that towns are relatively safe from floods because they are situated higher in elevation than the natural floodplain, but they are also close to fertile land so food is accessible. As populations grew, however, they needed the extra land near the rivers and, therefore, moved closer to the water.

In 2006, in the United States alone, there were almost 3,800 settlements containing 2,500 or more people located in an area likely to flood. Furthermore, according to another estimate, nearly 1.5 billion people worldwide still farm their crops in alluvial soil; this is almost one-third of the worlds population. Likewise, ever since the Mesopotamians established the cradle of civilization between the Tigris and Euphrates Rivers in the Middle East in about 3000 BC, populations have been attracted to rivers for transportation and trade. Narrow stretches of rivers have always been especially attractive locations to take advantage of the natural commerce along a trade route.

Human influence on flooding

Although human populations have been victims of natural flooding, their presence and subsequent activities near rivers has also contributed to the problem. In naturally occurring conditions, vegetation captures significant amounts of precipitation and returns it to the atmosphere before it has a chance to hit the ground and be absorbed by the earth; however, certain farming practices, like clear-cutting land and animal grazing, hamper this process. Without the natural growth of vegetation to trap the rain, the ground must absorb more moisture than it would otherwise. When the absorption limit is reached, the likelihood of flooding increases. Similarly, construction of concrete and stone buildings contributes to the problem of flooding. While rain is easily absorbed into sand and other porous materials, it is not easily absorbed by man-made building materials, such as pavement and concrete. These substances cause additional runoff that must be absorbed by the surrounding landscape.

Weight of water and force of floods

Floods are among the deadliest and most expensive forms of natural disaster on Earth. In the United States alone, flooding resulting from hurricane Katrina in 2005, due to both the storm surge and the partial failure of floodwalls in New Orleans, resulted in more than 100 billion dollars in damage (making it the costliest natural disaster in U.S. history) and over 1,000 deaths. Eighty percent of the city of New Orleans was flooded in the aftermath hurricane Katrina.

Water, when unleashed, is virtually impossible to stop. The reason behind this is twofold: water is heavy and can move with significant speed. For instance, while a single gallon of water weighs 8.5 pounds (3.5 kg), the weight of high volumes of impounded water really adds up. Hoover Dam alone holds back the waters of Lake Mead, which is about 15 miles (24 km) long and contains around 10.5 trillion gallons (40 trillion l) of water; multiplication shows that this water weighs almost 90 trillion pounds (41 trillion kg). Added to its weight is the fact that water can travel up to 20 miles (32 km) per hour. As it picks up speed, it also picks up incredible strength. In fact, moving under certain conditions, 1 inch (2.54 cm) of rain can have the same energy potential as 60,000 tons (54,400 metric tons) of TNT.

Flood intervention

Because of the potential of a flood to destroy life and property, men and women have, for centuries, developed ways to prepare for and fight this natural disaster. One common way to manage floodwaters is to construct dams to stop excess water from inundating dry areas. Another way is to divert floodwaters


Alluvial soils Soils containing sand, silt, and clay, which are brought by flooding onto lands along rivers; these young soils are high in mineral content, and are the most productive soils for agriculture.

Floodplain A clearly defined border of flat land along a river that is composed of sediment, which was deposited by the river during periodic floods or instances of high water.

Hazard zoning Examining historical records, geological maps, and aerial photographs to predict likely areas where flooding could occur. Used for planning the location of new settlements.

Minimizing encroachment Carefully planning where buildings are located so that they do not restrict the flow of water.

away from populated areas to planned areas of flood storage. To this end, flood control reservoirs are kept partially empty so that they can catch floodwaters when the need arises. These reservoirs then release the water at a slower rate than would occur under flood conditions; hence, reservoirs give the soil time to absorb the excess water. About one-third of reservoirs in the United States are used for this purpose.

Two other ways to safeguard life and property are known as hazard zoning floodplains and minimizing encroachment. When hazard zoning a floodplain, planners look at such things as historical records of 40-year floods, geological maps, and aerial photographs to predict likely areas where flooding could occur. Rather than relocating populations, hazard zoning is used for planning the location of new settlements. Minimizing encroachment means carefully planning where buildings are located so that they do not restrict the flow of water or cause water to pond excessively; however, as long as people choose to live in low-lying, flood-prone areas, scientists and engineers can only do so much to protect them from the risks of floods caused by both natural conditions and human activities.

See also Alluvial systems; Hydrology; Soil conservation; Watershed.



Collier, Michael and Robert H. Webb. Floods, Droughts, and Climate Change. Tucson, AZ: University of Arizona Press, 2002.

Dingman, S. Lawrence. Physical Hydrology, 2nd. ed. Upper Saddle River, NJ: Prentiss Hall, 2002.

World Commission on Dams. Dams and Development: A New Framework for Decision-Making. Earthscan Publications, 2001.


Water Encyclopedia. Irrigation Systems, Ancient <> (accessed on November 24, 2006).

United States Geological Survey. Floods (November 1, 2006) <> (accessed November 28, 2006).

Kathryn Snavely

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