Uncertainty of the Assumptions Required for Estimating the Regulatory Flood: Red River of the North.

Federal guidelines specify the log-Pearson Type III (LP3) distribution as the basis for the estimation of the 100-year peak discharge that serves as the regulatory flood for floodplain management in the United States. The LP3 distribution has been shown to provide a reasonable and flexible model of flood risk. Use of the LP3 distribution involves a number of explicit assumptions about the annual maximum series that are often implicitly assumed but not shown directly to be satisfied in flood frequency analysis studies of individual stations. In this case study, the validity of these assumptions for the Red River of the North at Grand Forks gaging station is examined. None of the four examined assumptions are satisfied completely. The stationarity assumption is not met because the regional climate samples two separate climatic modes, the peak discharge time series is not a random and independent set of events, the precipitation-runoff relationships are not uniform over time, and the flood peaks result from two flood-causing mechanisms. It is recommended that the validity of the explicit assumptions of the LP3 distribution be demonstrated directly as a standard step in flood frequency analyses, both as a means of advancing estimation of the regulatory flood and as a way to better inform decisions by floodplain managers and decision makers. [ABSTRACT FROM AUTHOR]