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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FIGURE 2.7 Selected peak discharges and regional envelope curve. SOURCE: Meyer, 1994.

periods of regulated flow. Note that most of the larger Sierra Nevada streams have
been regulated during the period since 1950, when floods have been most severe on
the American River and neighboring rivers.
Other problems result from spatial correlation and heterogeneity. Floods on
large rivers in California are highly correlated in space, making it difficult to estimate
probabilities of exceeding envelope discharges. For example, the annual flood
discharges of the seven Sierra Nevada rivers used in Chapter 3 to compute a regional
skew (Table 3.2) have an average pairwise cross correlation of 0.87. Furthermore
California streams and rivers are highly heterogeneous with respect to flood
magnitudes. Basins with the same drainage area are likely to vary in the magnitude
of the floods they produce. For example, four floods on the American River (Table
2.2) have peak discharges within 10% of the envelope discharge of 267,000 cfs; of
these, the peak discharge of the 1997 flood was estimated to be about 10% larger.
Given the precision of flood peak estimates, these four observations essentially lie on
the Meyers envelope curve, indicating that the curve does not provide an upper
bound for American River floods. This combination of strong heterogeneity and
spatial correlation makes it difficult to estimate probabilities of exceeding envelope
discharges.

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36

Improving American River Flood Frequency Analyses