In areas with steep slopes, heavy rain can cause a riverbed that held very little or no water at first, to suddenly brim with fast flowing water. The rain water is collected on the slopes, then flows downhill gathering speed and all the water comes together in the river bed. The water level rises fast. The water flows over the river banks and floods the area. Speed is the keyword. It all happens fast, it rains heavily. The water flows at high speed. Because of this speed it has the strength to carry away heavy objects as you can see in the videos of Boscastle in 2004, Vaison la Romaine in 1992, and the area of Alicante in Spain in 2007. The flood stops as suddenly as it starts.
The animation shows a schematic image of a flash flood.
A flash flood is a very direct response to rainfall with a very high intensity or sudden massive melting of snow.
The area covered by water in a flash flood is relatively small compared to other types of floods. The amount of water that covers the land is usually not very large, but is so concentrated on a small area that it can rise very high.
Because of the sudden onset and the high travelling speed of the water, flash floods can be very dangerous. The water can transport large objects like rocks, trees and cars. Never drive through a flash flood, even if it doesn’t seem to be very deep: the car may be swept away by the sheer speed of the water.
When a dyke breaks along the sea or along a river, the water may flow in so suddenly and with such speed that you could compare it with a flash flood.
If you search the web you’ll find more images of flash floods.
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