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Flash Floods: What are they, and who is at Risk?

A flash flood is a localized flood characterized by a rapid onset. Most often the result of heavy precipitation, flash flooding occurs when the amount of water accumulating in an area surpasses the ground’s ability to absorb it. As the ground becomes oversaturated, surface water runs toward low ground, collecting both speed and debris as it makes its way. It is their sudden onset and swift currents that make flash floods especially dangerous and a risk that everyone should consider.

Flash flooding can occur anywhere, even in populated cities where the possibility is often overlooked. In fact, storms over urban areas can generate more severe flooding faster and than in suburban or rural areas. This is because buildings, roads and other infrastructure in densely populated areas reduce the amount of rain that can be absorbed into the ground. The excess water becomes runoff that accumulates and can turn into a flash flood threat.

Areas located near mountains, hills, canyons and other steep land features are more susceptible to flash flooding. As water rushes downhill it quickly fills rivers, lakes and creeks beyond their capacity. It doesn’t take long for small trickles of water to transform into dangerous currents. In fact, according to the National Severe Storms Laboratory, a creek only six inches deep in a mountainous area can become a 10-foot-deep raging river in less than an hour if a storm lingers over an area too long.

Although less common, flash flooding can be triggered in other ways. Warm temperatures can turn snowpack into snowmelt, which can quickly turn into flash flood hazard. Floating debris or ice jams can cause flooding upstream, but they can also case flash flooding downstream if they are suddenly removed. Finally, the failure of manmade structures such as dams and levees, can leave entire cities underwater.

A recent dam collapse in Michigan triggered catastrophic flash flooding that forced thousands to evacuate and caused hundreds of millions of dollars in damage. The flooding ripped buildings off their foundations, twisted roads and bridges and destroyed practically everything in its path.

And most of us won’t soon forget the devastation Hurricane Katrina left behind. As the storm made its assault on the city of New Orleans, levee failures generated flash flooding so severe that, when all was said and done, nearly 80 percent of the city was engulfed in water.

While we don’t like thinking about them, disasters like these should serve as a reminder that flash flooding can present a threat any time of year. To be prepared, property owners should understand their flood risk. Flood preparedness resources available at DisasterSafety.org and IBHS.org provide a good place to start. Additionally, property owners should consider a flood insurance policy to ensure they have the coverage they need when disaster strikes.

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