A Uniform Technique for Flood Frequency Analysis

Abstract

In 1967 the U.S. Water Resources Council (WRC) published Bulletin 15 recommending that a uniform technique be used by all Federal agencies in estimating floodflow frequencies for gaged watersheds. This uniform technique consisted of fitting the logarithms of annual peak discharges to a Pearson Type III distribution using the method of moments. The objective was to adopt a consistent approach for the estimation of floodflow frequencies that could be used in computing average annual flood losses for project evaluation. In addition, a consistent approach was needed for defining equitable flood‐hazard zones as part of the National Flood Insurance Program. In 1976 WRC published Bulletin 17 which extended and updated Bulletin 15 but still recommended the use of the “log‐Pearson Type III” method. Since 1976, two updates of Bulletin 17 (17A and 17B) have been published which clarify or improve on this base method, or do both. This paper gives a brief historical review of the development of these bulletins and the motivation and justification for the adoption of this uniform technique. Special emphasis is given to Bulletin 17B, the current guidelines used by Federal agencies. Specific techniques examined are the development of regional skew, weighting of regional and station skew, the basis for the low‐ and high‐outlier tests, and the basis for the adjustment of frequency curves using historical information.