Flow Frequency Mixed-Population Analysis: Examples, Tools, and Challenges

Flow frequencies are an essential part of floodplain mapping and flood mitigation projects. Fitting flow frequencies to observed annual peak flows is both an art and a science involving methods in Bulletin 17C, available information, and engineering judgment. In most studies, all the observed annual peak flows collected at a site can be used to fit a single Log-Pearson Type III distribution. However, in the case of gage sites where peak flows are driven by a combination of snowmelt, rainfall, ice, and/or rain-on-snow events the peak flow frequency might not provide a realistic fit to the data. A mixed-population flow frequency can be used in these cases to provide a better fit to the observed events. This paper provides examples from two studies using mixed-population analysis, discusses why mixed-population analysis was considered, highlights some of the methodology and tools, and describes challenges encountered. Gages considered are from the Elkhorn River Basin in Nebraska and the Yellowstone River in Montana. These sites included historical data but no paleoflood data.