Evolution of Bulletin 17B for Flood Frequency Analysis in the United States

The current methodology recommended for flood-frequency analyses by U.S. Federal agencies is presented in Bulletin 17B. Bulletin 17 was first published in 1976; minor corrections were made in 1977 resulting in Bulletin 17A, which was later succeeded by Bulletin 17B published in 1982. The fields of hydrology and flood frequency analysis have substantially advanced since Bulletins 17-17A-17B were published. As Bulletin 17B approaches its 25^th birthday, it is time for the guidelines to be updated. New techniques are now available that correct known problems with Bulletin 17B and improvements should now become part of the Bulletin's recommended procedures. Such changes would provide: (1) statistically appropriate and efficient procedures for the computation of the regional skew and its precision that recognize that sample skewness estimators are relatively inaccurate themselves, (2) statistically effective and flexible methods for employing historical flood data in flood frequency investigations, (3) consistent and straightforward treatment of low outliers, and (4) computation of confidence intervals for quantiles correctly reflecting uncertainty in the skewness coefficient. A computational study demonstrates how Bulletin 17B confidence intervals (correctly computed for known skew) perform when the weighted skew is estimated with at-site and regional information.