FLOOD04413

• County: Statewide
• Basin: Statewide
• Title: Improving American River Flood Frequency Analysis
• Date: 1/1/1999
• Prepared By: National Research Council
• Floodplain - Doc Type: Educational/Technical/Reference Information

Data Sources

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Ely et al. (1993) used paleoflood data from 19 rivers in Arizona and southern
Utah to conclude that "the largest floods in the region cluster into distinct time
intervals that coincide with periods of cool, moist climate and frequent El Nino
events. The floods were most numerous from 4,800 to 3,600 years before the present
(B.P.), around 1,000 RP., and 500 years RP., but decreased markedly from 3,600 to
2,200 and 800 to 600 RP." Figure 2 in Ely et al. (1993) indicates that about 70% of
the extreme floods represented in the documented paleoflood record of the last 5,000
years occurred during the last 600 years. Unless the paleoflood record is grossly
incomplete, this suggests that extreme floods in Arizona and southern Utah do not
behave as independent and identically distributed random variables.
The committee is interested in the use of flood frequency analysis to predict
and mitigate future flood risk. If floods are not well modeled as independent and
identically distributed over the time period for which paleoflood information is
available, use of this information in conventional flood frequency analysis may lead
to biased estimates offuture flood risk. For example, based on the data ofEly et al.
(1993), conventional use of a 5,000-year paleoflood record from Arizona and
southern Utah would result in underprediction of the future risk of extreme floods
there. Estimation of the bias in this case or in any other case requires specification of
a model of floods that accounts for climatic variability. As discussed in Chapter 4,
we do not at this time have sufficient understanding of climate dynamics to
confidently specifY such a model.

Regional Analyses of Hydro meteorologic Extremes

Regional analyses of hydrologic extremes are based on the concept of
substituting space for time. The idea is that a rare event that occurs in one part of a
large homogeneous region could occur at other locations. Regional methods allow
the analyst to make use of such rare events. Two kinds of regional analyses are at
issue in the American River-the envelope curve of maximum observed flood
discharges and the probable maximum flood (pMF).
A flood envelope curve is a mathematical expression that provides an upper
bound of observed maximum instantaneous peak discharges for some region as a
function of drainage area. Envelope curves have been long used in flood hydrology
(Crippen and Bue, 1977; Costa, 1987), and were particularly useful before alternative
methods of regional flood analysis were developed. A recent innovation is the
incorporation of peak discharges estimated from paleoflood data (Enzel et a!., 1993).
A flood envelope curve is useful for "displaying and summarizing data on actual
occurrences of extreme floods" (IACWD, 1986, p. 71). However, the envelope curve
itself offers no means of estimating flood exceedance probabilities. Estimation of
such probabilities requires the use a statistical framework, which in turn requires
careful evaluation of regional homogeneity and spatial correlation.
For more than 50 years, the PMF has been used in the design of hydraulic
features of high-hazard dams. The PMF is defmed as "the maximum runoff
condition resulting from the most severe combination of hydrologic and
meteorologic conditions that are considered reasonably possible for the drainage
basin under study" (Cudworth, 1987, p. 114). PMF estimates are derived from