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Floods

A FLOOD IS the intrusion of water into normally dry land. While floods are a natural part of the ecological cycle and have some benefits for the health of the biosphere as a whole, flooding has always been one of the most devastating types of natural disasters for humans, responsible for thousands of deaths and billions of dollars in property damage each year. Climate change is expected to increase the risk of flooding for people around the world, both by raising global sea levels and increasing severe inland flooding events.

Flash Flooding

Flash flooding is most commonly caused by heavy rainfall over a short period of time, from a tropical system or an unusually heavy thunderstorm event. Less common causes include dam and levee breeches or the release of ice jams. Flash floods come on quickly and with little warning, developing in less than six hours from the initial rain or water event. They can move with great speed and strength, uprooting trees, picking up large boulders, destroying bridges, roads, and homes in a matter of moments. Flash flooding is responsible for at least 80 percent of all weather-related deaths in the United States each year, mostly due to people becoming trapped in automobiles. As little as 2 ft. (0.6 m.) of water can lift and move a full-sized commercial vehicle.

Although flash flooding is commonly associated with canyons or narrow valleys, where geography dictates the flow of excess water, or arid regions where the ground is not able to rapidly absorb large amounts of rainfall; urban areas are often affected by the phenomena. Buildings and impervious surfaces such as roadways and parking lots collect tremendous amounts of rainfall and divert it into storm drains, which can quickly be overwhelmed, sending the overflow into communities that are often unprepared for the threat.

Globally, flash floods are responsible for an average of 5,000 deaths and millions of dollars of property damage each year. Many regions do not have the forecasting or notification technology to alert vulnerable populations to oncoming flood events. Since 2006, the World Meteorological Organization (WMO) has been working to implement a system known as the Flash Flood Guidance Center with Global Coverage, which would give developing countries greater ability to mitigate loss of life in flash-flooding events.

Inland and Riverine Flooding

Flooding within a watershed, or riverine flooding, is another common form of inland flooding. Like flash flooding, riverine flooding is generally caused by rainfall or runoff that is too heavy to be absorbed into the watershed, sending the water over the bounds of the river or stream's banks and inundating nearby flood plains. Unlike flash floods, they build slowly, over a period of many hours or days, and last for a longer period of time, often more than a week, and sometimes over a month. Flooding along the Mississippi River Valley in 1993 lasted for 45 days, with some areas still partially flooded for 183 days; a flood event in Bangladesh in 1998 lasted for 68 days before finally receding.

Riverine flooding tends not to be as deadly as flash flooding, but causes great damage to property and agricultural lands, as was seen in the Mississippi Valley floods of 1993. High water displaced 70,000 people in nine states, damaged 50,000 homes, destroyed 12,000 acres of farmland, and caused an estimated $15 billion in losses.

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