Flash floods are short-term inundations of small areas such as a town or parts of a city, usually by tributaries and creeks. Heavy rain in a few hours can produce flash flooding even in places where little rain has fallen for weeks or months. If heavy rainfall occurs repeatedly over a wide area, then river or mainstem flooding becomes more likely, in which the main rivers of a region swell and inundate large areas, sometimes well after rainfall has ended. The 1993 Midwest floods were caused by 77 events over several months where rainfall of greater than one inch occurred over areas 100 to 200 miles wide and 400 to 600 miles long. Both flash flooding and river flooding threaten life and property, although the former causes more deaths and the latter more property damage.
On average, U.S. flooding kills more than 100 people a year--more than any other single weather hazard, including tornadoes and hurricanes. The average flooding toll has increased in recent decades while deaths from tornadoes and hurricanes have dropped. Almost half of all flash-flood deaths are connected to stream crossings or highway travel. Victims often underestimate the power of water when driving into flooded areas. It takes only 18 inches of water to float a typical vehicle.
A 100-year flood is one that has a one-percent chance of being exceeded in a given year. Few locations have rainfall records of more than a century, so 100-year flood values are estimates rather than certainties. Changes in watershed management, land use, and the like can affect streamflow characteristics and alter the likelihood of a given flood. Moreover, climatic patterns themselves can change. There is nothing to prevent more than one "100-year flood" from occurring at a given spot over a century.
Because flood risk is the result of both environmental and societal factors, it is difficult to single out the impact of climate variations. According to the United Nations Intergovernmental Panel on Climate Change (IPCC), the average global temperature at the surface increased just over 1.0 degree Fahrenheit in the 20th century. At the same time, according to NOAA, there has been a steady increase in the area of the United States affected by extreme precipitation events (more than two inches of rain in one day, or the equivalent in snow). However, flood reports have not increased uniformly across the country. For instance, streamflows in the Colorado River basin have decreased over the past 60 years. Any alteration of global climate can bring either an increase or decrease in precipitation or flood events at a given location. Current computer models of climate are unable to project local variations with certainty. For the most recent global and national scenarios, see the Web sites for the IPCC and the U.S. National Assessment.
According to a report by the U.S. Congress's Office of Technology Assessment, "despite recent efforts, vulnerability to flood damages is likely to continue to grow." The factors cited include
- growing populations in and near flood-prone regions
- the loss of flood-moderating wetlands
- increased runoff from paving over soil
- new development in areas insufficiently mapped for flood risk
- the deterioration of decades-old dams and levees
- policies such as subsidies that encourage development in flood plains
Other Web sites of interest include: