A flood is an unusual high-water stage in which water overflows its natural or artificial banks onto normally dry land. Floods can have catastrophic consequences, wreaking uncontrollable havoc. This often happens as a result of excessive rainfall over short periods of time, but can also be caused by ice jams during the spring rise, storm tides and tsunamis. Floods damage property and endanger the lives of humans and animals. They cause soil erosion and the deposition of sediment which can result in long-term environmental problems such as the destruction of natural habitats and spawning grounds. Floods cause problems with drainage and economic land use, as well as traffic delays and the impairing of navigation and hydroelectric power. Floods incur millions of dollars in financial losses each year.
Floods are not always disastrous, however, as in the case of the Nile River before the Aswan Dam was built. The Nile flooded every spring bringing much needed enrichment and moisture to the fertile soil of its floodplains.
Floods can cause great damage to land and water-related constructions which can have disastrous consequences for people and economies, both short and long-term. Thus, it is important to take floods into consideration when planning the construction of bridges and dams, for example, as well as good use of land. For these reasons floods are measured in terms of height, volume of flow and the area submerged so that good flood control measures, such as storage reservoirs and protective levees, can be implemented. Sometimes, however, the discharge volume of a stream varies greatly from month to month and year to year. A flash flood, for example, comes suddenly and unexpectedly, does not last very long and can be extremely dangerous. Another factor to be considered in the design and engineering of structures such as dams and reservoirs that may be affected, are those floods that are expected to occur only once in a hundred or perhaps a thousand years.