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

Flash floods occur very suddenly with little advance warning and can be very dangerous. The are usually caused by a sudden downpour or by an accident like a dam breaking.

Sometimes the ground is too hard or too wet to absorb water. That means that all the rain must travel downstream. Usually at least one inch of rain in an hour is needed to cause a flash flood. Sometimes an area can get hit by a flash flood even if it's sunny and hasn't rained because it might be pouring rain 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 water in the reservoir would rush down the river at the same time causing a flash flood.

The floods in Poland and central Europe this summer were caused by heavy rainfall but weren't flash floods. A heavy rain was spread out over days and the rivers weren't able to handle that much 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 try and protect themselves or evacuate. 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)



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