Evaluation of Mixed-Population Flood-Frequency Analysis

A mixed population of flood flows was shown to cause quality-of-fit problems if a single-population flood-frequency distribution was used to describe the flood data. The three populations in this mix were "ordinary," tropical cyclone, and icejam–release floods. Parametric descriptions of the single and separated flood populations were evaluated using probabilityplot correlation-coefficient tests. These tests quantified how well the flood-probability distributions agreed with plottingposition descriptions of the data and quantified the differences due to the mixed-population analysis. High outliers caused the high skewness found in the single-population analyses. The tropical cyclone component was underestimated by singlepopulation analyses at gauging stations in Massachusetts that had little data.