PROGRAMS AND PLANS - Guidelines on Publishing Flood-Frequency Estimates

August 29, 1977



SURFACE WATER BRANCH TECHNICAL MEMORANDUM NO. 77.17

Subject:  PROGRAMS AND PLANS - Guidelines on Publishing
                               Flood-Frequency Estimates


This memorandum revises limitations on the publication and
distribution of estimated flood-frequency information.
Limitations are specified for both the frequency of a known
discharge, and for the discharges of rare frequencies.  Previous
limitations or descriptions of limitations that hereby are
superseded were provided in Surface Water Branch Technical
Memorandums 69.11, 70.05, 73.16, and 76.07.

The purpose of the limitations is to minimize the distribution of
estimated variables that have an inherently large degree of
uncertainty and that may be improperly and/or detrimentally
interpreted by the public.  Our judgements about the desirability
of publishing and distributing flood-frequency estimates are based
upon assessments of the reliability, the need, and the likelihood
for proper interpretation of the data.  For example, the
estimation of the recurrence interval of a very large flood, say
one with a 500- or l,OOO-year recurrence interval, provides
information of low reliability and of only passing interest.  Such
information also is frequently misinterpreted as indicating that
an area is unlikely to be seriously flooded for some time in the
future.  We wish to discourage the presentation of such
information in either formal reports or press releases.

We have no intention of limiting the availability of truly needed
information even though it may have limited reliability.  Indeed,
we encourage the estimation and presentation for such purposes as
planning and designing projects, regulating land-use, and setting
insurance rates.

Based upon these considerations we provide the following
guidelines for estimating and publishing of flood-frequency
information:

Frequencies of observed flood discharges will be estimated only
for recurrence intervals of 100 years or less.  For larger
discharges the recurrence intervals preferably should be indicated
as "greater than 100 years."  Presentation of the ratios of
observed flood discharges to a 50- or 100-year discharge in lieu
of a large recurrence interval is permissible, but in general is
discouraged because of the poor quality and limited utility of the
information.

Discharges may be estimated and published for any recurrence
interval up to 100 years.  For recurrence intervals greater than
100 years the discharge may be presented only in response to a
specific request.  For example, only those discharges for
recurrence intervals up to and including 100 years would be
estimated and published in a regional flood-frequency report but
discharges having recurrence intervals of up to 500 years would be
presented in a Flood Insurance Administration study.

Flood-frequency information should be estimated as outlined in
Water Resources Council Bulletin 17 and in most instances will be
established through use of computer program J407.  Please note
that WRC Guidelines and Surface Water Branch Technical Memorandum
76.07 state that a minimum of ten years of record may be used to
estimate floods with recurrence intervals up to 500 years.
Program J407 provides estimates of discharge for recurrence
intervals of up to 500 years and this output may be provided to
requestors.




D. M. Thomas
Acting Chief, Surface Water Branch

WRD Distribution:  A, B, S, FO, PO