FLOOD07843

• County: Statewide
• Basin: Statewide
• Title: Nationwide Summary of US Geological Survey Regional Regression Equations for Estimating Magnitude and Frequency of Floods or Ungaged Ssites 1993
• Date: 1/1/1993
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

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ESTIMATION OF EXTREME FLOODS

By W.O. Thomas, Jr. and W,H. Kirby

Measures of Extreme Floods

Very large or extreme floods can be characterized
in several ways, Some examples are the Probable
Maximum Flood (PMF), envelope curve values based
on maximum observed floods (Crippen and Bue, 1977;
Crippen, 1982), and probabilistic floods, such as the
SOO-year flood, which has only a 0.2 percent chance of
being exceeded in any given year.
The PMF is defined as the most severe flood that
is considered reasonably possible at a site as a result of
hydrologic and meteorologic conditions (Cudworth,
1989; Hansen and others, 1982), The estimation of the
PMF involves three steps: (I) determination of the
Probable Maximum Precipitation (PMP) from reports
published by the National Weather Service (e.g"
Hansen and others, 1982), (2) determination of infiltra.
tion and other losses, and (3) the conversion of the
excess precipitation to runoff. In step (2), it is general
practice to assume that an antecedent storm of suffi-
cient magnitude has reduced water losses such as inter-
ception, evaporation, and surface depression storage to
negligible levels, In step (3), the conversion of precip-
itation excess to runoff is accomplished by one of a
number of techniques or models ranging from detailed
watershed models to a less detailed unit hydrograph
approach. Most Federal construction and regulatory
agencies use the less detailed unit hydrograph approach
that is based on the principle of linear superposition of
hydrographs as originally described by Sherman
(1932).
The words "probable" and "likely" in the defini-
tion of the PMF and PMP do not refer to any specific
quantitative measures of probability or likelihood of
occurrence, Moreover, a recent interagency work
group of the Hydrology Subcommittee of the IACWD
decided "It is not within the state of the art to calculate
the probability of PMF.scale floods within definable
confidence or error bounds" (Interagency Advisory
Committee on Water Data, 1986),
The definition of another type of large or extreme
flood is based on the maximum observed flood for a

given size watershed. Crippen and Bue (1977) and
Crippen (1982) developed flood-envelope curves by
plotting the maximum known flood discharges against
drainage area for 17 flood regions of the conterminous
United States. These flood-envelope curves approxi-
mate the maximum flood-peak discharge that has been
regionally experienced for a given size watershed, Like
the PMF, these flood-envelope values do not have an
associated probability of exceedance.

In general, the largest flood having a defined
probability of exceedance that is used for planning,
management, and design is the SOO-year flood. This
flood discharge has a 0.2 percent chance of being
exceeded in any given year or, stated another way, will
be exceeded at intervals oftime averaging SOO years in
length, The SOO-year flood is the most extreme flood
discharge computed in flood-frequency programs of
the U.S. Geological Survey (Kirby, 1981) and the U,S,
Army Corps of Engineers (U.S. Army Corps of Engi-
neers, 1982) that implement Federal Interagency Bulle-
tin 17B guidelines for flood frequency (Interagency
Advisory Committee on Water Data, 1982). These two
computer programs are the ones most frequently used
by the hydrologic community,

Estimates of SOO-year flood discharges are used
in defining floodplains for the flood insurance studies
of the Federal Emergency Management Agency
(FEMA) as well as by the National Park Service for
defining floodplains in National Parks. Flood-plain
boundaries based on the SOO-year flood are used mostly
for planning purposes to identify areas that would be
inundated by an extreme flood. Recent bridge failures
resulting from excessive scour have prompted the Fed-
eral Highway Administration (FHWA) to develop pro-
cedures for evaluating scour at bridges. As part of this
program, the FHWA advised the State Departments of
Transportation nationwide to evaluate the risk of their
bridges being subjected to scour damage during floods
on the order of a 100- to 500-year or greater average
return periods. Therefore, there is a defined need for
estimates of flood discharges having return periods on
the order of SOO years.

ESTIMATION OF EXTREME FLOODS 13