Flash Floods

Flash floods occur when water levels rise rapidly and inundate low-lying areas. The United States National Weather Service defines flash floods as floods that occur within 6 hours of their cause, which may include heavy rainfall, ice dam breakage, and dam or levee failures. Flash floods cause minor to severe damage, injury, or death, and often disrupt transportation systems. Although some regions are more likely to experience flash floods than are others, they can occur anywhere in the world.

Flash floods often occur when heavy rain falls on less permeable surfaces such as clay soils, soils already saturated with water, and concrete. Urbanization therefore contributes to flash flood hazards in two ways. Instead of infiltrating the ground, the water flows over land surfaces and through streets, increasing human exposure to flash flood hazards as well as the potential for damages and injuries.

Flash floods are a major geomorphological force that erodes soil and rock to create and [Page 1124]deepen canyons, arroyos, and steep valleys, particularly in arid or semiarid regions. Deeper channels confine water and prevent it from spreading across the surface, enhancing the risks associated with flash floods. In a canyon flash flood, the water may appear suddenly and without warning from an unseen source upstream. In such cases, the optimal method of escape is to climb to higher ground, although this is not always possible and many hikers find themselves in danger each year. The Big Thompson River flood of 1976 is one example of a canyon flash flood, in Colorado, that resulted in the deaths of 146 people.

Flash flood within a city in Iowa

Source: Lynn Betts, USDA Natural Resources Conservation Service.

Infrastructural failures may include naturally occurring structures such as ice dams as well as human-made structures such as dams or levees. Although infrastructural failures are often associated with heavy rainfall, this is not always the case. As a result, such flash floods are extremely difficult to predict and have the potential to devastate extensive areas.

Ice dams are created when water is retained behind a wall of ice. Water pressure builds up behind the dam and may cause it to break, flooding areas downstream. Ice dam failure is a typical event in spring, when rain and warm temperatures combine to melt snow and ice into the reservoir while simultaneously weakening the ice dam. Loss of life from ice dam flash floods is low despite the high frequency of these events because they are relatively predictable. A thaw prediction after one of the heaviest ice accumulations on the Yellow River in China led authorities to evacuate 13,000 people in March 2008. The ice dam burst, but no casualties were reported.

Dam and levee failures present a difficult problem for water managers and residents alike. These types of infrastructure are designed to prevent or [Page 1125]alleviate flooding and usually allow greater development within a floodplain, bringing more people and property under the threat of flash floods. When they do fail, the subsequent flash floods are often devastating to communities downstream because of their suddenness and severity. In 1975, Typhoon Nina produced more than a year's average of rain over the Banqiao Reservoir in China, causing the Banqiao and 61 other dams to fail. Entire communities were destroyed, leaving approximately 26,000 dead from the flash floods and over 100,000 more dead due to subsequent waterborne disease and famine. The storm surge from Hurricane Katrina in 2005 breached 53 flood control levees and caused another 28 to fail outright, rapidly flooding New Orleans and leaving more than 700 dead in the city from flooding alone.

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