flooding

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.