Flooding occurs when water overflows or soaks land that is normally dry. It is the most common and expensive type of weather-related disaster and climate change is accelerating its intensity and severity.

Floods can affect anyone, but they tend to hit the poor hardest as their homes and livelihoods are washed away or destroyed. They also increase the risk of waterborne diseases — from hepatitis A to cholera — and can lead to an increase in mosquito-borne malaria. Floods can also be devastating for wildlife. For example, hundreds of endangered one-horned rhinos were killed when flooding swamped Kaziranga National Park in India.

The most important factors in a flood are rainfall intensity and duration. The speed at which water rises or recedes is also a critical factor. Slower systems are less dangerous than rapid ones, which may catch people unaware. Floods are often caused by localized storms and rains, but they can also occur near rivers, coastlines or dams or levees due to water being released or overflowing or because of a break or failure.

Floods can be caused by a wide range of things, including excessively heavy and prolonged rain, highly accelerated snowmelt, severe storms with winds over water or tsunamis. They can also be caused by the breaking or overflowing of dams, levees, retention ponds and other structures. Standing water left behind after a flood can be a breeding ground for bacteria and can carry toxins and chemicals that are harmful to human health. It is best to avoid touching any standing water unless it is safe to do so.