FOR IMMEDIATE RELEASE:
March 31, 1997
P.O. Box 3000
Boulder, CO 80307-3000
Telephone: (303) 497-8611
Fax: (303) 497-8610
What seasonal weather patterns have placed so much of the country at risk for flooding? How does society exacerbate flooding and its dangers? What is the difference between a flash flood and a river flood? To help you answer these and other questions, below is some background information on U.S. flood risk from a newly published report by Roger Pielke, Jr., of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. Also below is a list of flood and flash-flood experts and some relevant World Wide Web sites. NCAR is operated by the University Corporation for Atmospheric Research (UCAR) under sponsorship by the National Science Foundation.
story ideas and relevant links
NWS Office of Public Affairs
Bibliography of research on water resources
U.S. Geological Survey/Colorado District
U.S. Army Corps of Engineers/Flood Plain Management Services Program
Background on flood hazards, insurance, and mitigation
Federal Emergency Management Agency
Home page and National Flood Insurance Program
What are the elements of a flash flood and a river flood? 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.
Are the death tolls from U.S. floods increasing? 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 automobile.
How can a 100-year flood occur more than once in a short period? 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.
Is global warming causing more floods? 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, the average global temperature at the surface has increased about 1.0 degree Fahrenheit this 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.
Are societal choices increasing flood risk? 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
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