FLOOD FREQUENCY EXPECTED AND UNEXPECTED PROBABILITIES by D.M. Thomas Prepared by the U.S. Geological Survey 1976 Open-File Report 76-775 TABLE OF CONTENTS Abstract Introduction Frequency concepts Discharge variations Probability variations     Expected probability Discussion Summary and conclusions References FIGURES 1. Time-sampling variations in 10-year records 2. Discharge histograms 3. Frequency curves on rectangular coordinate graph 4. Probability histograms TABLES 1. Theoretical number of years in a 100-year period that floods exceed a 3 percent chance design discharge estimated from a 10-year record at 20 homogeneous but independent sites ABSTRACT Flood-frequency curves may be defined either with or without an "expected probability" adjustment; and the two curves differ in the way that they attempt to average the time-sampling uncertainties. A curve with no adjustment is shown to estimate a median value of both discharge and frequency of occurrence, while an expected probability curve is shown to estimate a mean frequency of flood years. The attributes and constraints of the two types of curves for various uses are discussed. LizardTech's Djvu plug-in is needed to view these reports. CLICK HERE TO VIEW THE ENTIRE REPORT For questions or comments, contact K. Van Wilson.