A flood is an overflow of water onto normally dry land. It is one of the most destructive natural disasters. It usually happens when there is heavy rain, rapid snow or ice melt, the failure of manmade structures like dams and levees, or a surge of ocean water during a hurricane or tsunami. Floods can kill people and animals and cause billions of dollars in damage each year. They often cause more deaths than tornadoes, earthquakes and lightning strikes.
The most common type of flood is a riverine flood, which happens when consistent rain or a sudden release from a dam causes a river to overflow its banks. It can also happen when a coastal storm surge rushes inland during a hurricane or tsunami. The land around rivers is called a flood plain.
There are many ways to forecast and monitor flooding. One is to use a computer model. Software such as HEC-RAS,[61] TUFLOW and others combine 1D and 2D components to provide estimates of surface and subsurface water levels.
People who live in areas that are prone to flooding can take steps to reduce the risk. They can move to higher ground or build houses designed to withstand flooding. They can also install sandbags or cement barriers. They can also plant trees and shrubs to block the flow of water. Taking mitigation actions may not prevent all flooding, but buildings that are built to withstand floods experience about 80% less damage than those that are not.