River floods typically occur with the melting of winter snow coupled with heavy spring rainfall, resulting in overfilled river basins. Heavy rainfalls from tropical storm systems may also produce river flooding.
Coastal floods are the result of ocean waters being driven inland, most commonly by tropical storms, or less commonly as a result of tidal waves driven by earthquakes.
Urban floods, an increasingly common flood, are the result of urban/suburban sprawl, where developed land areas lose their ability to absorb rainfall. Development may increase runoff up to six times over what would occur naturally in its absence.
Flash floods commonly result from rainfall of exceptional intensity and duration, and may occur within minutes or a few hours of such rainfall. Other causes may be the failure of dams or levees, or the releases of ice jams.
