• underlying assumptions, the validity of which cannot be absolutely established (e.g., the assumption that flood discharges are independent and identically distributed in time).
  • Even if the underlying assumptions are reasonably correct, there are large standard errors in flood frequency analysis; even flood records 100 years in length have insufficient information to allow accurate estimates of quantiles such as the flood flow exceeded with a probability of 1% in any year (100-year flood discharge).
  • The differences between our best estimate of flood quantiles (such as the 100-year flood discharge) and those of the USACE are small compared to the likely uncertainties in the estimates.
  • Critical policy decisions in the American River basin, such as certification of the levees, Sacramento's floodplain status, and the adoption of flood mitigation strategies, are extremely sensitive to the official estimates of flood probabilities and quantiles. Hence, even though our best estimates are not significantly different from those of the USACE in a statistical sense, the differences may have significant policy implications.

The last issue is particularly important, and is discussed in Chapter 5.

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