Flood-Frequency Analysis Updates for Bulletin 17B and The U.S. Geological Survey PeakFQ Program

A large portion of the U.S. population, infrastructure, and industry is at risk of flooding. Floods have caused more damage annually in the United States than any other natural disaster. Flood-frequency analysis provides information about the magnitude and frequency of flood discharges based upon records of annual maximum instantaneous peak discharges collected at streamgages. The information is essential for defining flood-hazard areas, for managing floodplains, and for designing bridges, culverts, dams, levees, and other flood-control structures. Bulletin 17B (B17B) of the Interagency Advisory Committee on Water Data (IACWD) codifies the standard methodology for conducting flood-frequency analyses in the United States. B17B specifies that annual peak-flow data are to be fit to a log Pearson Type III distribution. Specific methods are also prescribed for improving skew estimates using regional skew information, tests for high and low outliers, adjustments for low outliers and zero flows, and procedures for incorporating historical flood information. The authors of B17B identified various needs for methodological improvement and recommended additional study. In response to these needs, the Advisory Committee on Water Information (successor to IACWD), Subcommittee on Hydrology (SOH), Hydrologic Frequency Analysis Work Group (HFAWG) has recommended modest changes to B17B on the basis of a series of studies completed in June 2013. These recommendations include the adoption of the expected moments algorithm (EMA) which retains the essential structure and moments-based approach of the existing B17B procedures for determining flood frequency. EMA can accommodate interval data, which simplifies analysis of datasets containing censored observations, historic and (or) paleo data, low outliers, and uncertain data points, while also providing enhanced confidence intervals on the estimated discharges. In addition, the HFAWG has also recommended the adoption of the multiple Grubbs-Beck (MGB) test, a generalization of the Grubbs-Beck method that allows for a standard procedure for identifying multiple Potentially Influential Low Flows (PILFs). The SOH requested that the U.S. Geological Survey implement these changes in a user-friendly, publically accessible program (PeakFQ).The updated USGS program PeakFQ implements both the standard Bulletin 17B and EMA procedures for flood-frequency analysis of peak-flow records. Single and MGB test outlier screening is available for both the standard Bulletin 17B and EMA procedures