Magnitude and frequency of floods in western Oregon

Abstract

A method for estimating the magnitude and frequency of floods is presented for unregulated streams in western Oregon. Equations relating flood magnitude to basin characteristics were developed for probabilities of 0.5 to 0.01 (2to 100-year intervals). Separate equations are presented for four regions: Coast, Willamette, Rogue-Umpqua, and High Cascades. Also presented are values of flood discharges for selected probabilities and of basin characteristics for all gaging stations used in the analysis. Included are data for 230 stations in Oregon, 6 stations in southwestern Washington, and 3 stations in northwestern California. Drainage areas used in the analysis range from 0.21 to 7,280 square miles. Also included are maximum discharges for all western Oregon stations used in the analysis. INTRODUCTION A special need for updated flood-frequency information for Oregon results from recent emphasis on flood-plain zoning, flood insurance, and design adequacy of hydraulic structures. Studies of flood magnitude and frequency are based on analyses of available streamflow records. Very few long-term records for streams with less than 10 mi2 of drainage area were available when the last flood-frequency reports were prepared for Oregon by the U.S. Geological Survey. Many data have been collected since a small-stream flood-data program was started in 1952 in cooperation with the Oregon State Highway Commission (now Oregon Department of Transportation, Highway Division). This program was expanded in 1965, through funds provided by the U.S. Forest Service, to include many previously unsampled areas in national forests. Inclusion of these data has enlarged the flooddata base, both areally and in range of basin size. This analysis was limited to western Oregon, because of the large number of gagingstation records available for the western part of the State and the deficiency of peak-flow information for many areas of the eastern part. Limiting the analysis to western Oregon allows timely use of urgently needed flood information in a rapidly developing area. An eastern Oregon flood-frequency analysis will be presented in a later report. In describing flood frequency in this report, the term exceedance is used in preference to the term recurrence interval. However, both terms are used in most of the tables, graphs, and illustrative problems. For example, a flood with a 0.01 probability is a flood that has one chance in a hundred of being exceeded in any one year. This is a 100-year flood under the recurrence interval terminology.