Magnitude and frequency of floods in Washington

Abstract

Relations are provided to estimate the magnitude and frequency of floods on Washington streams. Annual-peak-flow data from stream gaging stations on unregulated streams having 10 years or more of record were used to determine a log-Pearson Type III frequency curve for each station. Flood magnitudes having recurrence intervals of 2, 5, 10, 25, 50, and 100 years were then related to physical and climatic indices of the drainage basins by multiple-regression analysis using the Biomedical Computer Program BMDO2R. These regression relations are useful for estimating flood magnitudes of the specified recurrence intervals at ungaged or short-record sites.Separate sets of regression equations were defined for western and eastern parts of the State, and the State was further subdivided into 12 regions in which the annual floods exhibit similar flood characteristics. Peak flows are related most significantly in western Washington to drainage-area size and mean annual precipitation. In eastern Washington they are related most significantly to drainage-area size, mean annual precipitation, and percentage of forast cover. Standard errors of estimate of the estimating relations range from 25 to 129 percent, and the smallest errors are generally associated with the more humid regions.

A Two‐Decade Watershed Approach to Stream Restoration Log Jam Design and Stream Recovery Monitoring: Finney Creek, Washington

“…No long-term streamflow data are available; however, U.S. Geological Survey data from Finney Creek between 1943 and 1948 show a range of daily flows from 15.9 cubic feet per second (cfs) (0.45 m 3 /s) to 2,931 cfs (83 m 3 /s) (http://waterdata.usgs.gov/nwis/ nwisman/?site_no=12194500&agancy_cd=USGS, accessed April 2008). Cummans et al (1975) estimated a bankfull discharge at the mouth of Finney Creek of 5,085 cfs (144 m 3 /s).…”

“…Cummans et al . () estimated a bankfull discharge at the mouth of Finney Creek of 5,085 cfs (144 m 3 /s).…”

Fundamental Elements of Ecologically Healthy Watersheds in the Pacific Northwest Coastal Ecoregion

“…The predicted peak discharges of a 25-year recurrence interval flood in the three dominant Cascade flood regions are markedly different (Figure 6.8). Discharges calculated from the regression formulae of Cummans et al (1975) (standardized to units of discharge per unit watershed area with standard errors of 32, 39, and 50% in Regions I, II, and III, respectively) show that Region I produces the highest discharge, Region III the lowest, and Region II an intermediate amount. In addition, smaller drainage areas produce more runoff per unit area than larger ones, regardless of region.…”

“…As before, the discharge was standardized and calculated (after Cummans et al 1975) for hypothetical watersheds of three different sizes. Standard errors of the estimates are 25, 32, and 40% for the 2-year, 25-year, and lOa-year recurrence interval floods, respectively.…”