What is a flash flood?

A flood occurs when water flows into a region faster than it can be absorbed into the soil, stored in a lake or reservoir or removed in runoff or a waterway into a drainage basin.

A flash flood is a sudden local flood characterized by a great volume of water and a short duration. It occurs within minutes or hours of heavy rainfall or because of a sudden release of water from the breakup of an ice dam or constructed dam.

Flash floods are dangerous because the waters in a flash flood often move at destructive speeds and they often occur with very little warning.

Rainfall intensity and duration are two key elements of a flash flood. Topography, soil conditions, and ground cover also play important roles. Steep terrain can cause rain water to flow toward and collect in low-lying areas, causing water levels to rise rapidly. If the soil is saturated with water, it cannot absorb more, so the excess water runs off the land quickly.

However, extremely dry soil conditions also can be favorable for flooding. Dry soil can often develop a hard crust over which water will initially flow as if the ground were concrete.

If a flash flood warning is issued, you should stay away from streambeds, drainage ditches and culverts. Move to high ground when threatened by flooding. Stay out of flooded areas. Never drive your car across a flooded road. Most flash flood-related deaths occur when people drive into floodwaters. Never underestimate the power of moving water.

Category: Phenomena
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