Floods have a variety of causes including heavy rainfall, tidal surges and the failure of manmade structures such as dams. During a flood an area of land that is normally dry is covered by water.
Heavy rainfall over a short time period caused the 2004 Boscastle flood in north Cornwall, UK. The village of Boscastle, which is located in a steep-sided valley, was overwhelmed when rivers running through the area broke their banks.
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More info on: http://www.bbc.co.uk/science/earth/natural_disasters/flood