Bulletin 17B Flood Frequency Revisions: Practical Software and Test Comparison Results

Bulletin 17B (B17B) provides guidelines for conducting flood frequency analyses in the United States (U.S.). Recently, the Hydrologic Frequency Analysis Work Group (HFAWG) began investigations to examine potential changes to B17B. The investigations have focused on alternative techniques for: (1) incorporating historical information; (2) addressing zero flows and low outliers; (3) use of improved plotting positions; and (4) providing correct confidence intervals. An overview of these efforts is presented, highlighting the Expected Moments Algorithm (EMA) with the log-Pearson Type III distribution. Software that implements EMA is described. The existing B17B flood frequency approach is compared with an EMA-based alternative using 82 U.S. peak-flow data sets, data set resampling, and limited Monte-Carlo simulation. In this paper, we highlight initial comparison results via a case study. The results suggest that the EMA-based methods perform well when compared to the existing B17B approach, fit easily within the current B17B LP-III/moments framework, provide much more accurate uncertainty estimates, and alleviate some technical and practical inconsistencies in B17B.