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."