Evaluation of mixed-population flood-frequency analysis

A mixed population of flood flows was shown to cause quality-of-fit problem s 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 ice-jam-release floods. Parametric descriptions of the single and separated flood populations were evaluated using probability-plot corr elation-coefficient tests. These tests quantified how well the flood-probab ility distributions agreed with plotting-position descriptions of the data and quantified the differences due to the mixed-population analysis. High o utliers caused the high skewness found in the single-population analyses. T he tropical cyclone component was underestimated by single-population analy ses at gauging stations in Massachusetts that had little data.