The Bulletin 17B guidance has guided the development of peak flow frequency analyses within the United States since the early 1980's. This guidance recommended the use of the Log Pearson Type III probability distribution for annual peak flows on unregulated streams fit by the Method of Moments (Interagency Advisory Committee on Water Data, 1982). Using the Bulletin 17B methodology, distribution parameters are estimated from the moments of the sample data (i.e. mean, standard deviation and skew). Bulletin 17B also includes adjustments to those parameters for non-standard data (missing years, flows below gage base, or estimated flows), low outliers, and historical events. Adjustments can be made with a series of specified procedures including the conditional probability adjustment and the historical adjustment (weighted moments) procedure. The Bulletin 17B methodology characterizes the peak flow in each year as either (1) part of the systematic (observed) record, for which a point estimate of flow exists or (2) an historical event from a time before a gage was present but for which an estimate of flow can be made.
The Bulletin 17C guidance brings a change to the computation of peak flow frequency within the United States. This guidance incorporates changes motivated by four of the items listed as future work within Bulletin 17B and more than 30 years of post-Bulletin 17B research on flood processes and statistical methods (England, et al., 2019). As part of the Bulletin 17C methodology, the moments/parameters of the Log Pearson Type III distribution are estimated using the Expected Moments Algorithm (EMA). Like Bulletin 17B, the Bulletin 17C/EMA (17C EMA) methodology also estimates distribution parameters based on sample moments, but does so in a more integrated manner that incorporates non-standard, censored, or historical data at once, rather than as a series of adjustment procedures (Cohn, Lane, & Baier, 1997). The use of Bulletin 17C procedures can also provide improved confidence intervals for the resulting frequency curve that incorporate diverse information appropriately, as historical data and censored values impact the uncertainty in the estimated frequency curve (Cohn, Lane, & Stedinger, 2001). Within the 17C EMA methodology, every annual peak flow in the analysis period, whether observed or not, is represented by a flow range. That range might simply be limited to the gaged value when one exists. However it could also reflect an uncertain flow estimate. The analysis period also requires a specified perception threshold in each year.