Thursday, 23 November 2017


Note: This document is a repeat of that appearing as a Supplementary Information File for last Thursday, 16 November 2017.

A flood occurs when water overflows the confines of a stream or other body of water and accumulates over low-lying areas. Floods are classified as flash floods, river floods and coastal floods.

A "flash flood" is a dangerous rise in water level of a stream in a few hours or less caused by heavy rain, ice jams, earthquake and earthslide, or dam failure. A flash flood usually occurs within 6 hours of the rain event, typically a thunderstorm. In some cases, the heavy precipitation can produce a wall of water, moving at incredible speeds and with sufficient force to roll boulders, tear out trees, destroy buildings and bridges and scour out new channels. In many cases, flash floods can cause automobiles to be swept away in just 2 feet of moving water. Since the 1970's, flash flooding has caused an average of 200 fatalities per year in the United States.

These flash flood events often differ in rapidity and violence from "river floods" that are seasonal, resulting from spring rains and melting snow. River floods may crest slowly and persist for as much as a week. The record spring 1997 floods on rivers in the Dakotas and Minnesota, notably the Red River of the North, were caused by rapid spring thaw of the heavy snow cover from a record number of blizzard-producing snowstorms. The great Midwest floods of 1993 were essentially a combination of river flooding produced by a persistent weather pattern, but also by the many local flash flood events on tributaries to the Mississippi and Missouri Rivers. All these local floods contributed a large, but slow, increase in the water volume in the entire river basin.

The following list includes terminology used by the National Weather Service for those public statements pertaining specifically to floods, especially those flash floods produced by thunderstorms. The terms are given in alphabetical order, not severity or immediacy:

Important meteorological factors contributing to flash floods include rainfall intensity and rainfall duration, or specifically, a combination of the rate at which the rain falls and the length of time over which it persists. The sudden and potentially deadly flash floods are typically associated with excessive rainfall events from local convective thunderstorms. Meteorological causes of flash floods then include slow moving thunderstorms, multiple thunderstorms that repeatedly track over the same area (also called the "training effect") and heavy rain from tropical weather systems, such as hurricanes. Situations conducive to flash floods would occur with those thunderstorms that develop when the atmospheric moisture content through a deep tropospheric column is high (including high surface dewpoints); little vertical wind shear through the cloud; the repeated development of convective cells over the same area; and the absence of strong wind gusts, hail or tornadoes.

Hydrological factors that contribute to flash flooding include topography, soil conditions and ground cover. Flash floods often occur in mountainous terrain with steep slopes that permit rapid runoff rather than slow percolation into the soil. This sudden stream burden can produce large flood crests that move rapidly downstream tearing out everything in their path. Flash floods can occur when additional precipitation falls upon soil and vegetation that have become saturated and cannot absorb additional rain or snowmelt. The soil may be saturated because of previous rains or because the soil is frozen and the additional water cannot percolate into the soil. Vegetation usually reduces the runoff rate. However, urban areas are especially prone to flash floods because these areas have more non-porous surfaces, such as roads and buildings, and little vegetation, thereby increasing runoff. Existing storm sewers may not be able to handle this increased runoff from heavy rains falling in a short time interval. In urban areas heavy rain events can produce street "ponding" from an accumulation of water on roads. Many fatalities result when people drive into flooded roadways or streams.

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Prepared by Edward J. Hopkins, Ph.D., email
© Copyright, 2017, The American Meteorological Society.