Flash flooding is most commonly caused by heavy rainfall over a short period of time, from a tropical system or an unusually heavy thunderstorm event. Less common causes include dam and levee breeches or the release of ice jams. Flash floods come on quickly and with little warning, developing in less than six hours from the initial rain or water event. They can move with great speed and strength, uprooting trees, picking up large boulders, destroying bridges, roads, and homes in a matter of moments. Flash flooding is responsible for at least 80 percent of all weather-related deaths in the United States each year, mostly due to people becoming trapped in automobiles. As little as 2 ft. (0.6 m.) of water can lift and move a full-sized commercial vehicle.
Although flash flooding is commonly associated with canyons or narrow valleys, where geography dictates the flow of excess water, or arid regions where the ground is not able to rapidly absorb large amounts of rainfall; urban areas are often affected by the phenomena. Buildings and impervious surfaces such as roadways and parking lots collect tremendous amounts of rainfall and divert it into storm drains, which can quickly be overwhelmed, sending the overflow into communities that are often unprepared for the threat.
Globally, flash floods are responsible for an average of 5,000 deaths and millions of dollars of property damage each year. Many regions do not have the forecasting or notification technology to alert vulnerable populations to oncoming flood events. Since 2006, the World Meteorological Organization (WMO) has been working to implement a system known as the Flash Flood Guidance Center with Global Coverage, which would give developing countries greater ability to mitigate loss of life in flash-flooding events.
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