Famously fertile floodplains such as the Mississippi Valley, the Nile River Valley in Egypt, and the Tigris-Euphrates in the Middle East have supported agriculture for millennia because annual flooding has left tons of nutrient-rich silt deposits behind. ( Learn here about flood preparation and safety tips.) Flood preventionįlooding, particularly in river floodplains, is as natural as rain and has been occurring for millions of years. Residents of flooded areas can be left without power and clean drinking water, leading to outbreaks of deadly waterborne diseases like typhoid, hepatitis A, and cholera. Potentially dangerous mold blooms can quickly overwhelm water-soaked structures.
The water and landscape can be contaminated with hazardous materials such as sharp debris, pesticides, fuel, and untreated sewage. When floodwaters recede, affected areas are often blanketed in silt and mud. In China's Yellow River Valley some of the world's worst floods have killed millions of people.
Death tolls have increased in recent decades to more than 100 people a year. In the U.S., losses average close to $8 billion a year. Photograph by Manuel Silvestri, Reuters Impacts of floodingįloods cause more than $40 billion in damage worldwide annually, according to the Organization for Economic Cooperation and Development. Mark's Square during a period of seasonal high water in Venice, Italy, on October 29, 2018. People walk on a catwalk in a flooded St. communities will face repeated flooding by the end of this century, according to a 2017 analysis it's happening in more than 90 coastal communities already. Meanwhile, melting glaciers and other factors are contributing to a rise in sea levels that has created long-term, chronic flooding risks for places ranging from Venice, Italy to the Marshall Islands. The increase in temperatures that accompanies global warming can contribute to hurricanes that move more slowly and drop more rain, funneling moisture into atmospheric rivers like the ones that led to heavy rains and flooding in California in early 2019. So-called flash floods can be extremely dangerous, instantly turning a babbling brook or even a dry wash into rushing rapids that sweep everything in their path downstream.Ĭlimate change is increasing the risk of floods worldwide, particularly in coastal and low-lying areas, because of its role in extreme weather events and rising seas. Others generate quickly and with little warning. Most floods take hours or even days to develop, giving residents time to prepare or evacuate. Coastal flooding occurs when a large storm or tsunami causes the sea to surge inland. How floods formĪ flood occurs when water inundates land that's normally dry, which can happen in a multitude of ways.Įxcessive rain, a ruptured dam or levee, rapid melting of snow or ice, or even an unfortunately placed beaver dam can overwhelm a river, spreading over the adjacent land, called a flood plain. Any area where rain falls is vulnerable to floods, though rain is not the only cause. There are few places on Earth where flooding is not a concern.
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