How does a flash flood happen?

Many hydroelectric dams, such as the Hoover Dam, also serve to contain flooding.

The weather event known as a flash flood can be especially dangerous because of its sudden formation and potential for widespread destruction. Television meteorologists routinely broadcast flash flood warnings along with severe storm and tornado warnings. Cities located along rivers or under dams are especially vulnerable if the amount of water generated during a storm exceeds protective barriers.

Flash floods can occur during a severe storm.

There are a number of factors, some natural and some man-made, that can determine whether a storm system will produce a flash flood or just a significant amount of rain. One factor is the nature of the storm cell itself. A fast line of thunderstorms, for example, can move through an area so quickly that rain is absorbed by the ground or sewer systems and does not come back. On the other hand, a slow-moving, independent storm system can dump a large amount of rain on a small area and create the potential for floods to develop quickly. A short, intense burst may be preferable to a long, slow rain event.

Flash floods can create dangerous road conditions.

Assuming there is a slow moving storm system, dumping a lot of rain in the area, the next factor in a flash flood is soil absorption. If the ground is frozen or covered in ice, rainwater will not be absorbed naturally and will continue to seek lower areas. The same goes for urban areas with significant amounts of concrete and asphalt. When rain is prevented from reaching the ground, it will flow into sewer systems and natural ditches. If these drainage systems or gutters become overloaded, it can lead to flooding.

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A line of fast moving storms may be preferable to a slow moving storm system.

Rivers and streams also have a natural saturation level, and if enough rain falls in a short period of time, the water will overflow into an area known as the floodplain. If a dike or dam broke during a severe storm, a destructive flash flood would be unleashed on anything and anyone living beneath it. A flood of this magnitude is often called a 100- or 500-year flood, mainly because destruction at this level must occur only once every 100 or 500 years. This does not mean that a second flood of equal power will not occur the following year.

In flash floods, heavy rains or other water quickly submerges low-lying areas.

In short, a flash flood occurs whenever the amount of rain during a storm overwhelms natural and man-made drainage and absorption systems and continues to surge in a low-lying area. This type of flooding can occur at any time, so meteorologists often advise people to stay off the streets for several hours after a slow, intense storm. Runoff from storm systems further upstream may not reach an area for a few hours, which means that the potential for flooding still exists in some places, even after the rains have stopped.

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