Flood Data, Underlying Distribution, Analysis, and Refinement

Annual flood peak data must conform to certain standards of quality control, sample size, and homogeneity for use in flood-frequency analysis. Sometimes the values for the highest observed floods of an annual flood series are much higher or much lower than expected; these values are designated as outliers and inliers, respectively. When the values for the lowest observed floods are much higher or lower than expected, these are designated inliers and outliers, respectively. Analyses of storms producing high floods perceived as outliers and droughts containing low floods perceived as outliers provide a physical basis for their being outliers. However, these outliers and inliers need to be detected and properly modified to derive unbiased design flood estimates. Test statistics have been developed for objective detection of any outliers/inliers in a given flood series after converting it to a normally distributed series with the power transformation. A design flood estimation methodology is presented.