A Brief History of Debris Flow Occurrence in the French Broad River Watershed,Western North Carolina

Abstract

The Appalachian mountains of North Carolina have a long history of producing destructive debris flows. Steep slopes, a thin soil mantle, and extreme precipitation events all exacerbate the probability of slope instability in the region. For this study, modern accounts of debris flows have been reviewed to construct a history and estimate the frequency of debris flows in the French Broad watershed. Major debris flow forming events occurred in 1876, 1901, 1916, 1940, 1977, and 2004. In western North Carolina, debris flows are activated primarily by either a series of two storms or hurricanes tracking through the area within a 6-20 day period or a prolonged moderate rainfall event lasting several days. In general, precipitation greater than 125 mm (~5 inches) in a 24-hour period can generate debris flows. Although the recurrence interval of individual debris flows may be on the order of thousands of years, when assessed at the level of the French Broad watershed, the average frequency of mass wasting from 1876-2004 is 16 years. Individuals living in the mountainous regions of western North Carolina must be vigilant in monitoring weather conditions and steep hillslopes, especially during intense rainfall events.