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- 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.
- 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.
-