PUBLICATIONS--"Guidelines for Determining Flood Flow Frequency," Revised Bulletin 17B of the Hydrology Committee, U.S. Water Resources Council In Reply Refer To: October 16, 1981 EGS-Mail Stop 415 SURFACE WATER BRANCH TECHNICAL MEMORANDUM NO. 82.04 Subject: PUBLICATIONS--"Guidelines for Determining Flood Flow Frequency," Revised Bulletin 17B of the Hydrology Committee, U.S. Water Resources Council Attached are the new Water Resources Council (WRC) guidelines that supersede WRC Bulletin 17A as the standards by which Federal Agencies are to compute flood-peak-frequency curves from gaging- station records. This is the second revision of the original Bulletin 17 that was published in March 1976. The WRC work group that produced the present guidelines (Bulletin 17B) has been dissolved and there are no plans for further revision of the guidelines. Changes of Bulletin 17A are indicated by asterisk (*) marks in the text of Bulletin 17B. The pages that were revised are summarized on page v of the Foreword. These changes were discussed in Surface Water Branch Technical Memorandum No. 81.06 that documented the changes in flood-frequency analysis program J407. The revised guidelines in Bulletin 17B were incorporated into computer program J407 on August 7, 1981. The only revision in Bulletin 17B that will be discussed here is the new procedure for weighting station and generalized skew. Please refer to Surface Water Branch Technical Memorandum 81.06 for information on the other revisions. Station and generalized skew are now weighted inversely proportional to their mean square errors. The standard error (square root of the mean square error) of the generalized skew must be input to program J407 unless the generalized skew map in Bulletin 17B is used. If no value for the standard error of the generalized skew is input to the program, the value from the Bulletin 17B map (0.55) is automatically used. Please refer to the WATSTORE User's Guide Volume 4, Chapter I, Section C (dated June 1981) for the changes in data input format for computer program J407. The change in the skew weighting is the one revision which will cause the greatest difference in flood-frequency estimates computed using Bulletin 17A versus Bulletin 17B. This change in skew weighting reduces the influence of generalized skew and will produce a frequency curve which, on the average, agrees more closely with the station data than the same frequency curve computed from Bulletin 17A. Note that an errata sheet has been included with Bulletin 17B. Please make the necessary pen and ink changes to your copy of Bulletin 17B. Any questions or problems in interpreting the new guidelines should be directed to Will Thomas (FTS 928-6837) in the Surface Water Branch. Marshall E. Moss Acting Chief, Surface Water Branch Attachment WRD Distribution: A, S, FO, PO "This memorandum does not supersede any existing SWB memorandum."