RECORD FLOODS, RECURRENCE, AND CLIMATE CHANGE: IRENE AND LEE

Record flood crests were produced by Hurricane Irene in parts of the Catskill Mountains of New York and by Tropical Storm Lee in the upper Susquehanna River drainage basin in 2011. Many of the same sites had experienced their previous “flood of record” in 2006 or another time in the last 20 years. Standard flood-frequency assessments at a number of gage sites indicate that the 2011 floods had apparent recurrence intervals at or greater than 500 years, but the same is true for 2006 (or, elsewhere, 2005 or 1996) floods at these same sites. Are we living through an unlucky time when major floods just happen to be occurring? Or is flood frequency changing, presumably as the result of changing precipitation patterns related to human-induced/aggravated climate change? Flood histories from long-lived (~80-100) gage sites in the region allows testing of the stationarity of flood frequency. These sites generally show that the magnitude of major low-probability floods (e.g. 1% annual exceedance) has changed over the last century, with most sites showing significantly greater magnitude estimates for low-probability floods when results using data from the last 40-50 years are compared with results using data from the previous 40-50 years. While such an analysis doesn’t prove a climate-change connection, it does imply that flood frequency has changed in recent decades, which should prompt a re-evaluation of flood hazard assessment strategies.