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 the result of 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 contributing a large, but slow, increase in the water volume in the entire river basin.
"Coastal floods" are usually associated with tropical weather systems and will be described subsequently.
The Thursday optional Supplemental Summary Information file describes the terminology used in the official statements issued by the National Weather Service to inform the public of floods and flash floods. Also included are major meteorological factors leading to flash flooding.
To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 11 Chapter Progress Response Form, under section B. Daily Summary.