Floods are generally caused by the combination of large amounts of precipitation and basin topography. For example, saturation of soils caused by large amounts of precipitation can lead to flooding. Urbanization of a drainage basin increases the amount of runoff reaching a channel and decreases the lag time between a precipitation event and peak flow. A variety of weather events lead to flooding, including extended wet periods, decaying tropical cyclones, intense thunderstorms, and quick snowmelt. In some cases humanly constructed structures designed to prevent flooding collapse, causing flooding or accentuating flooding. In low-lying coastal areas storm surges may cause significant flooding. Mass movement events are similar to flooding, although the proportion of sediments to water is larger than an alluvial flood with the outcome just as disastrous.
Humans respond to flooding in a variety of ways. Broadly defined these fall into two categories, structural and nonstructural measures. Structural measures include dams and dikes. Through time the efficacy of structural features has been questioned, and there has been a shift from purely structural approaches to controlling floods to a mix of structural and nonstructural flood mitigation strategies. Nonstructural measures include flood insurance, floodplain mapping, and land-use ordinances, acquisition and relocation, floodproofing, detection and response warning systems, soil bioengineering, and combined structural and nonstructural measures to reduce flood losses. Some progress is being made in addressing the hazards associated with flooding. The reduction of flood impacts continues at great expense, but vulnerability will continue to rise as long as more people build in floodplains, increasing the risk of catastrophic floods. Even the best warnings will not eliminate the risks increasingly being taken around the globe.
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