Flood occurs when a water level rises above an established elevation threshold and the surrounding land becomes covered with water. Floods can be caused by heavy rainfall, glacier melt or dam breakage, or the failure of levees, retention ponds, and other structures. The severity of a flood depends on the magnitude of rainfall, the volume of water involved, and the speed at which it moves.
Drowning is the most common cause of death during floods, especially flash floods. Six inches of moving water can knock a person over, and even shallower floodwater currents are dangerous because they can carry away cars and other vehicles. Hazards like lumber, debris and downed power lines can also be carried in floodwaters, and may pose a risk of injury or death to people or animals.
In addition, floodwaters are often contaminated with household and industrial chemicals, sewage, waste materials, and decomposing animal carcasses and other debris. These pollutants can make people sick if they come into contact with them, and floods can be especially harmful to children because their immune systems aren’t fully developed.
During flooding, soils can be eroded and lost, leading to crop loss, land degradation, and decreased productivity. In the long term, repeated floods can disrupt ecosystems, as they prevent soils from replenishing themselves with nutrients and water. This can lead to a decrease in biodiversity and increased vulnerability to other natural disasters, such as drought. Flooding can also lead to the spread of diseases such as cholera, typhoid and malaria, which are transmitted by mosquitoes and unclean water.