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

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Improving American River Flood Frequency Analyses

of the sample and regional skews, as suggested in Bulletin 17-B. No historical data
were used in the analysis, reportedly because it did not appear that use of historical
data would affect the results. Finally, the expected probability adjus1ment was applied
to the estimated distributions. The three-day flow values computed by the USACE
(1998) were 215,000 cubic feet per second (cfs) for the roO-year flood and 278,000
cfs for the 200-year flood at the Fair Oaks gage. For perspective, estimates of the
three-day flow values for the 1986 and 1997 floods at Fair Oaks are 166,000 cfs and
164,250 cfs, respectively.

Reactions to USACE Analysis

The methods and results of the USACE flood frequency analysis have
prompted several questions, comments, and criticisms from representatives of local,
state, and federal govemment agencies, public interest groups, private citizens, as well
as from the Corps itself. These concerns were conveyed to the committee both orally
and in writing over the course of this study. In addition, the committee identified
other issues of concern. Below is a list of the issues that the committee recognized as
potentially critical, and that are addressed in this report:

. accuracy of the adjusted daily flows used in the flood frequency analysis;
. failure of the USACE analysis to incorporate historical data or paleoflood
information; .
. consistency of the results with probable maximum flood estimates,
envelope curves of maximum flood discharges, and rainfall nmoff modeling results;
. use of the Bulletin l7-B map skew, given that the skew map is out of date
and was developed for instantaneous flood discharges, not maximum daily flows;
. the use of the expected probability correction;
. adequacy of the log-Pearson type ill distribution for modeling flood
distributions over a wide range of exceedance probabilities;
. adequacy of the Bulletin 17-B procedure for accounting for historical data;
. the potential advantages of censoring the lower part of the distribution so
that the estimation depends only on the largest floods;
. the fact that the record from 1950 to the present has many more large
floods than the 1905-1950 record; and
. potential changes in flood probability due to global climate or regional

change.

The first two issues concern data used (or not used) in the USACE analysis,
and are discussed in Chapter 2. The next six issues concern methods of flood
frequency analysis, and are discussed in Chapter 3. The last two issues concern
climate and its bearing on the standard assumption that flood discharges are
independent and identically distributed in time; this is the focus of Chapter 4.
In evaluating the issues of data, analysis methods, and climate, the committee
strictly adhered to scientific standards. Hence, each technical recommendation,
presented in Chapter 5, is based on best judgment of what is consistent with the
scientific literature. It is important, however, to recognize the following:

. Estimations of flood quantiles and probabilities are based on a number of