Flooding | Research & Encyclopedia Articles

Technically, flooding occurs when the water level in any stream, river, bay, or lake rises above bank full. Bays may flood as the result of a tsunami or tidal wave induced by an earthquake or volcanic eruption; or as a result of a tidal storm surge caused by a hurricane or tropical storm moving inland. Streams, rivers and lakes may be flooded by high amounts of surface runoff resulting from widespread precipitation or rapid snow melt. On a smaller scale, flash floods due to extremely heavy precipitation occurring over a short period of time can flood streams, creeks, and low lying areas in a matter of a few hours. Thus, there are various temporal and spatial scales of flooding. Historical evidence suggests that flooding causes greater loss of life and property than any other natural disaster. The magnitude, seasonality, frequency, velocity, and load are all properties of flooding which are studied by meteorologists, climatologists, and hydrologists.

Spring and winter floods occur with some frequency primarily in the mid-latitude regions of the earth, and particularly where continental climate is the norm. Five climatic features contribute to the spring and winter flooding potential of any individual year or region: 1) heavy winter snow cover; 2) saturated soils or soils at least near their field capacity for storing water; 3) rapid melting of the winter's snow pack; 4) frozen soil conditions which limit infiltration; and 5) somewhat heavy rains, usually from large scale cyclonic storms. Any combination of three of these five climatic features usually leads to some type of flooding. This type of flooding can cause hundreds of millions of dollars in property damage, but it can usually be predicted well in advance, allowing for evacuation and other protective action to be taken (sandbagging, for instance). In some situations flood control measures such as stream or channel diversions, dams, and levees can greatly reduce the risk of flooding. This is more often done in floodplain areas with histories of very damaging floods. In addition, land use regulations, encroachment statutes and building codes are often intended to protect the public from the risk of flooding.

Flash flooding is generally caused by violent weather, such as severe thunderstorms and hurricanes. This type of flooding more frequently occurs during the warm season when convective thunderstorms develop more frequently. Rainfall intensity is so great that the carrying capacity of streams and channels is rapidly exceeded, usually within hours, resulting in sometimes life-threatening flooding. It is estimated that the average death toll in the United States exceeds 200 per year as a result of flash flooding. Many government weather services provide the public with flash flood watches and warnings to prevent loss of life. Many flash floods occur as the result of afternoon and evening thundershowers which produce rainfall intensities ranging from a few tenths of an inch per hour to several inches per hour. In some highly developed urban areas, the risk of flash flooding has increased over time as the native vegetation and soils have been replaced by buildings and pavement which produce much higher amounts of surface runoff. In addition, the increased usage of parks and recreational facilities which lie along stream and river channels has exposed the public to greater risk.

Urban Runoff

Battan, L. J. Weather In Your Life. San Francisco: W. H. Freeman, 1983.

Critchfield, H. J. General Climatology. 4th ed. Englewood Cliffs, NJ: Prentice-Hall, 1983.

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