“IT RAINETH A RAINY RAIN:” A BRIEF HISTORY OF DEBRIS FLOWS IN WESTERN NORTH CAROLINA FROM 1847 TO 2004

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. Through the Pleistocene, temperature and moisture fluctuations associated with the transition between glacial to interglacial cycles destabilized exposed soil and rock. These prehistoric debris flows have formed prominent relict debris fans and a rolling, hummocky topography that can be discerned from aerial photography and topographic maps.

Modern accounts of flooding in western North Carolina associated with hurricanes and other strong storms exist back into the early 1700's. The earliest recorded instance of a debris flow occurred on July 7, 1847 when debris flow or “waterspout” scars were noted in the mountains north of what is now Hayesville, N.C. Other major events occurred in 1876, 1901, 1916, 1940, 1977, and 2004.

The locations of prehistoric and modern debris flows, and their associated geomorphic features, are good indicators of areas that may be prone to future slope instability. Noted North Carolina politician and scientist Thomas L. Clingman describes in detail 40-60 “waterspouts” that formed in Macon and Jackson Counties, NC on June 15, 1876. Of these, one of the largest fell from Fishhawk Mountain, southeast of Franklin, N.C. More than 100 years later, another destructive debris flow mobilized on this mountain and inundated the community of Peeks Creek killing 5 people and destroying 15 homes on the night of September 16, 2004.

Antecedent moisture and particularly rainfall intensity, seems to play a crucial role in triggering debris flows. In western North Carolina, debris flows are activated primarily by either a series of two storms or hurricanes tracking through the area within a 10-20 day period or a prolonged moderate rainfall event lasting several days, particularly if followed by high intensity rainfall. In general, precipitation greater than 125 mm (~5 inches) in a 24-hour period can generate debris flows. Continued study of the history of debris flows will help identify recurrence intervals of these events, areas susceptible to slope movements, and triggering mechanisms that are particular to North Carolina.