Flood Frequency Analysis in the Context of Climate Change

A fundamental assumption in classic flood frequency analysis is that geomorphologic and climatic characteristics of a watershed are stable over time. This allows hydrologists to assume available data represent a single time-invariant population of extreme events. Increasingly, people are concerned about the impacts of climate change on the hydrologic cycle, and they doubt that the assumption of stationarity is valid. So the question is, what should be done? This paper evaluates different methods that can be used for flood frequency estimation that allow for climate change, including time-varying parameters that capture trends, using a limited short-term flood record or window, and adopting a safety factor (e.g. 30% increase) applied to the design flood estimators. A Monte Carlo re-sampling study is employed to estimate the 100-year-flood 50 years beyond the end of a 100-year flood record. Though small trends (e.g. +0.25%) in annual maximum series are not statistically detectable in most U.S. watersheds with a 100-year record, the 100-year flood will be significantly underestimated if such positive trends are neglected. When the trend magnitudes are known and employed to update log-Pearson type III (LP3) parameters, the 100-year flood estimators have the smallest log-space mean squared error (LMSE). Employing trends estimated from the record is an alternative for high trend magnitudes; such estimators have small biases but large variance. Use of a short-term flood record and safety factors only worked well for the low trend magnitudes such as 0 or 0.25%. Clearly, flood risk management in an uncertain world will be a challenge.

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