What is the difference between a high outlier and a historical flood?
High outliers and most historical floods both are exceptionally large floods. High outliers are exceptionally large floods that are contained in the systematic record, whereas historical floods were observed outside the period of systematic record. Systematic records are collected during periods of systematic stream gaging, usually continuous series of years, in which flood data are observed and recorded annually, regardless of the magnitudes of the floods. A nonsystematic record is collected and recorded sporadically, without definite criteria, usually in response to actual, perceived, or anticipated major flooding. The systematic record can be used directly in flood frequency analysis. The non-systematic record cannot be used unless additional information can be supplied to relate it to the population of all flood peaks. Bulletin 17-B requires that the non-systematic record be a complete record of all flood peaks that exceeded some threshold level during a definite historical time pe
