PUBLICATIONS--Transmittal of Report on Feasibility of Assigning a Probability to the Probable Maximum Flood (PMF)

In Reply Refer To:                                         December 2, 1986
WGS-Mail Stop 415


Subject:  PUBLICATIONS--Transmittal of Report on Feasibility of
                        Assigning a Probability to the Probable
                        Maximum Flood (PMF)

The concept of PMF has been in use for almost half a century for
designing major dam spillways and other installations, such as
nuclear power plants, which are intended to withstand the most
severe flood that is reasonably possible.  The definition and the
computational procedures of the PMF both disclaim the possibility
of associating a probability with this event.  In recent years,
however, there has developed a growing desire to quantify the
probability of occurrence of floods of PMF magnitude.  This desire
arises on the one hand out of a concern for the safety of PMF-
based designs, and, on the other hand, out of a concern for proper
integration of cost and risk factors in engineering design work.

The Hydrology Subcommittee of the Interagency Advisory Committee
on Water Data in 1984 established an interagency work group to
investigate the feasibility of assigning a probability to the PMF.
The work group searched the engineering and hydrologic literature
on frequency analysis of extreme floods and concluded that the
state of the art does not at this time provide a basis for
assigning probabilities in a reliable, consistent, and credible
manner to the PMF or to extreme floods of magnitude approaching
the PMF.  Support for this conclusion and additional conclusions
are contained in the work group's report, which is transmitted

Although computation of the PMF is outside the scope of the Water
Resources Division's (WRD) mission (Surface Water Branch Technical
Memorandums 83.05 and 77.03), the work group's report should be of
interest to WRD hydrologists.  Most of the considerations and
conclusions of the report apply equally well to other kinds of
hypothetical extreme floods, such as envelope-curve floods, and to
other aspects of flood-frequency analysis that are within WRD's
mission.  Therefore, please bring this report to the attention of
hydrologists involved in flood-frequency analysis in your

                                       Verne R. Schneider
                                       Chief, Office of Surface Water


WRD Distribution:  A, FO-LS