Floods are a major natural disaster. They cause death and injury, damage buildings, disrupt livelihoods and destroy crops. They also increase the risk of waterborne diseases like cholera and malaria. Floods are often a result of prolonged drought. In many places, they occur as a sudden and rapid rise of water, usually after heavy rain. The intensity, duration and location of rainfall are key factors determining whether floods will happen or not.
Several things can trigger flooding, including dams breaking or the water level reaching its maximum height inside a river channel. Some rivers have a history of chronically flooding. Floods can also be caused by clogged drainage systems, and the presence of materials that block or slow water flow.
Water levels in rivers may also rise when dams overflow or when snow and ice melts during warmer seasons. The resulting streamflow or runoff can be higher than usual and overtake a river bank, or it can pool in areas behind or along the banks.
Climate change is making floods more likely worldwide. A 2021 study found that the frequency of “100-year” floods (meaning there is a 1 percent chance they will occur in any given year) has increased in Western Europe due to warming.
Floods affect all communities, but they have a greater impact on those with lower incomes and less access to information and services. These include people living in poverty, refugees and internally displaced persons. During floods, they can be forced to abandon their homes and are at high risk of illness from waterborne diseases like cholera and malaria.