flooding

Repeated flooding at Chiswell, Dorset, England, led to the construction of large-scale beach defences. This consisted of a sea wall, as shown here, and huge cages of pebbles (known as gabions) laid down on top of the beach. Some of the floods were caused by water soaking through the pebble beach, and flooding the low-lying village behind the beach. To prevent this, engineers placed steel piles down through the beach, diverting water away from the village of Chiswell.

Repeated flooding at Chiswell, Dorset, led to the construction of large-scale beach defences. This consisted of a sea wall, shown here, and huge cages of pebbles (known as gabions) laid down on top of the beach. The gabions are partly covered by shingle thrown up during a storm.

The inundation of land that is not normally covered with water. Flooding from rivers commonly takes place after heavy rainfall or in the spring after winter snows have melted. The river's discharge (volume of water carried in a given period) becomes too great, and water spills over the banks onto the surrounding flood plain. Small floods may happen once a year – these are called annual floods and are said to have a one-year return period. Much larger floods may occur on average only once every 50 years, or once every 100 years.

Flooding is least likely to occur in an efficient channel that is semicircular in shape (see channel efficiency). Flooding can also occur at the coast in stormy conditions (see storm surge) or when there is an exceptionally high tide. The Thames Flood Barrier was constructed in 1982 to prevent the flooding of London from the sea.