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Flash floods

Flash floods sometimes mark the beginning of a major river flood, but usually they are separate events of only local significance, scattered randomly in space and time. They are caused by what are usually short periods of heavy and intense rain, typically in conjunction with thunderstorms. If the rainfall intensity exceeds the infiltration rate, however, the water runs off on the surface and soon gathers in the receiving waters. On sloping terrain, this results in a rapidly growing flood wave which in next to no time can forge into areas where it may not even have rained at all. On level terrain, the water does not run off quickly enough and accumulates on the surface or collects in lower-lying areas like hollows, cellars, or underground car parks.

The mechanical forces associated with the high rates of flow are a critical factor, as is the erosion potential: both can cause buildings to collapse and increase losses enormously.

It is almost impossible to forecast flash floods because they happen too suddenly. Short-term loss reduction measures are therefore more or less ruled out. Flash floods are also much shorter in duration than river floods, with most of the water having disappeared again after a few hours. Nevertheless, the average annual overall loss from the many flash flood events that happen every year is no smaller than the loss from the rare but spectacular "once-in-a-century" events on major rivers. Insurance against flash floods is no problem because the spread of risks over space and time is given.

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