PROGRAMS AND PLANS--New WRC Guidelines on Flood-Frequency Analysis

In Reply Refer To:                                         December 17, 1975
EGS-Mail Stop 415


          Flood-Frequency Analysis

The purpose of this memorandum is to inform field offices of the
current status of recommended techniques for flood-frequency
analysis.  Sometime after the first of the year, the Water
Resources Council will issue a new report entitled, "Guidelines
for Determining Flood-Flow Frequencies." That report will
supersede WRC Bulletin 15 (December 1967) as the standards by
which Federal agencies are to define flood-frequency curves from a
gaging station record.

The new guidelines provide a hierarchy of procedures rather than a
direct "cookbook" technique.  Within this hierarchy are various
alternatives that may be employed in the several analytical steps
necessary to define a frequency curve.  The guidelines suggest
that a frequency curve defined quickly by simple techniques may be
satisfactory for some uses, but that a very detailed analysis may
be justified for some complex or costly projects.  Inherent in
this concept is an assumption that the more complete or detailed
the analysis the more reliably defined will be the frequency

The Surface Water Branch has been considering how Water Resources
Division analysts should react to the available alternatives, has
prepared some written guidelines that are being tested, and is
having the computer programs modified to provide analytical
efficiency.  A basic tenet of the Branch is that the analyst will
be allowed as much freedom as possible in selecting and following
the analytical options.  A minimum level of analytical effort will
be specified but the analyst is encouraged to utilize as many of
the optional routines as he judges may be necessary, useful, and
practical for his region, his data, and his project.  It should be
recognized that a more detailed and complete analysis might be
justified for a HUD Flood Insurance Study than for a regional
regression analysis.

More details will be provided subsequently, but minimum analytical
standards will include: (a) use of a log-Pearson Type III
probability distribution with skew coefficient determined by
weighting of a regional (map) and computed value, (b) test for the
existence of low outliers and frequency curve adjustments for
outliers as required, (c) adjustment for any known historic peak
data, (d) examination of computer output and of the computed
station frequency curve errors, and (e) weighting of computed
station frequency curves with regional regression estimates to
obtain an improved estimate.

When the new guidelines are used to compute frequency curves, the
limitations on curve extension described in Surface Water Branch
Technical Memorandum No. 73.16 can be liberalized.  It will then
be permissible to present in a regional flood-frequency report the
lOO-year flood estimates for all sites having ten or more years of
flood record.  Coordinates of two frequency curves should be
provided for each site.  One set of coordinates defines for the
station the log-Pearson Type III frequency curve based upon
weighted skew and is provided to document input for regional
analysis.  The second set of coordinates defines the curve
computed by combining the station frequency curve with regional
estimates to form a "weighted estimate" for the site.

The WRC requirement to coordinate lOO-year flood estimates with
other concerned agencies still exists.  Project planning must
provide two to three weeks for other agency review and should
consider that the more detailed the frequency analysis, the more
acceptable will be the defined station frequency curves.

For ongoing projects, each district will have to judge the
desirability of switching to the new analytical techniques.  If
this switch would unduly hinder the completion of a project, there
is no need to redo the analysis, but the general acceptability of
curves defined by the new guidelines may justify some extra
analytical effort.  Guidance should be sought from the Regional
staff.  All future starts in frequency analysis should use the new

Although the printed copies of the WRC report may not be available
for some time, a xerox copy of the report will be available for
short-term loan.  Interim guidelines on the minimum level of
effort are being tested and will be provided as soon as possible.

Each District should acquire and maintain expertise in applying
the new analytical guidelines, and should assure the validity and
completeness of the flood-peak data file.  Public announcement of
the new guidelines quite possible will spur increased interest in
frequency determinations.  Because of the complexity of the
methodology, little analytical capability will exist outside of a
few Federal agencies.

                                       Harry H. Barnes, Jr.
                                       Chief, Surface Water Branch

WRD Distribution:  A, FO-L