EVIDENCE FOR INCREASED FLOOD RISK IN NEW ENGLAND OVER THE PAST CENTURY USING STATISTICAL ANALYSIS OF THE PARTIAL DURATION FLOOD SERIES

Recent studies document increasing precipitation and streamflow in the Northeastern United States throughout the 20^th-21^st centuries. Low flow quantiles like the annual minimum and median flows have increased significantly over the period on many New England rivers with dominantly natural streamflow, as have annual peak discharges—especially for smaller, more frequent floods. In order to better investigate the high frequency flooding trends (events occurring at least once every five years), this study analyzes the partial duration flood series for 23 of these New England rivers. The study rivers have continuous records through 2006 and an average period of record of around 70 years, with a minimum of 59 years and a maximum of 81 years. Eighteen rivers show positive trends in flood magnitude using the Mann-Kendall non-parametric trend test. Six of these trends are significant at p<0.1. We also investigate a potential hydroclimatic shift in the region around 1970. Twenty-two rivers show increased numbers of partial duration series floods per year in the post-1970 period when comparing pre- and post-1970 records using the non-parametric Wilcoxon rank sum test. Thirteen of these increases are significant at p<0.1. On average, each river has approximately one more flood per year above the gauge threshold value. Because frequent floods are important channel-forming flows, these results have significant implications for channel and floodplain geomorphology, as well as aquatic habitat.