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What causes flash floods? - Windows to the Universe

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What causes flash floods and the floods that took place this summer in Poland?

Flash floods are floods that happen very suddenly and with little warning. They are very dangerous. A very heavy rain or an accident like a dam break can cause one. Sometimes the ground is too hard or too wet to soak up water. The only place the rain can go is downstream. Usually rainfall of more than one inch in one hour is needed to cause a flash flood. Sometimes a place can get hit by a flash flood even if it's sunny because it might be raining on higher ground somewhere. Dams are designed to hold a river's water in reserve for when people may need it. If a dam were to break then all the trapped water would rush down the river at the same time and cause a flash flood. The floods in Poland this summer weren't flash floods. They were caused by heavy rainfall but the rain was spread out over days. So much rain fell that the rivers couldn't handle all the water. If you want to know more about the flood in Poland, check here. Sometimes these kinds of floods can be forecast so people can plan ahead. One cause of floods can be forecast months in advance--snow. Melting snow can cause dangerous floods like what happened this spring in the upper-midwest U.S.

Submitted by Leah (Washington DC, USA)
(August 22, 1997)



Last modified April 28, 2016 by Jennifer Bergman.

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