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News Release

U.S. Department of the Interior
U.S. Geological Survey

Release: March 30, 1998
Luis A. Fusté, Information Officer
(253) 428-3600, ext 2653

Washington's big floods provoke a new USGS flood study

Washington's streams and rivers raging over their banks, pushing muck and stumps and debris across roads and into basements. Muddy neighbors throwing up sandbags along frail levees. Folks grabbing the kids and the dog and the cat and hightailing it for drier, safer ground.

Washington's historic flood peaks of February 1996 and other floods in recent years have prompted a new study of the state's flood frequencies that will help update the flood hazards facing people, homes and other buildings, bridges, dams, and similar constructions in flood-prone areas. The findings of the study have just been released in a new report by the U.S. Geological Survey (USGS), Department of the Interior.

The USGS report is based on annual peak discharge data through September 1996 from streamflow gaging stations on naturally flowing streams throughout the state. It presents updated calculations of flood probabilities on these gaged streams. For sites on ungaged naturally flowing streams, the report presents equations developed to estimate the magnitude and frequency of floods for any of those sites. The USGS conducted the study in cooperation with the Washington State Department of Transportation and the Washington State Department of Ecology.

From the maximum annual instantaneous discharge data gathered from 527 stream-gaging stations around the state, USGS hydrologists determined the state's 2-year, 10-year, 25-year, 50-year, and 100-year flood frequencies. These are the magnitudes of peak stream discharges that will probably occur at any of the 527 stations on average once during any period of 2, 10, 25, 50, or 100 years.

"Technically, we express these flood frequencies as `exceedance probabilities,'" said USGS hydrologist Steve Sumioka, the principal author of the report. "That simply means the probability that a flood of a given magnitude will happen in any one year. So, for instance, a flood peak with an exceedance probability calculated at 0.01, or 1 percent, has a one-in-one-hundred chance of being exceeded in any given year. Flood magnitudes with these one-in-a-hundred odds are those big floods sometimes called `hundred-year floods,'" Sumioka explained.

The equations to estimate flood frequencies on ungaged naturally flowing streams were developed from the frequencies calculated for gaged streams together with data about the size of the drainage basins upstream of the stream gages and the average amounts of rainfall over the drainage basins.

The new USGS report, Magnitude and Frequency of Floods in Washington, updates a previous USGS study of data collected only through 1979.

Magnitude and Frequency of Floods in Washington, by S.S. Sumioka, D.L. Kresch, and K.D. Kasnick, is published as U.S. Geological Survey Water-Resources Investigations Report 97-4277. Limited copies are available for reading at the office of the U.S. Geological Survey, Water Resources Division, 1201 Pacific Avenue, Suite 600, Tacoma, Washington 98402-4384. The report can be purchased from the U.S. Geological Survey, Branch of Information Services, Box, 25286, Denver, Colorado 80225, telephone (303) 202-4610. Electronic copies are also available from the home page of the USGS Washington Water Science Center at web site

For additional technical information, send email to Steve Sumioka at or call him at (253) 428-3600, extension 2645.

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