Debris flows in the North Pacolet River valley, Polk County, North Carolina, USA: case studies and emergency response

Abstract

The North Pacolet River valley is incised into the Blue Ridge Escarpment (BRE) near Tryon in southwestern North Carolina. The BRE is a mountain front that marks the change from the mountainous Blue Ridge physiographic province to the lower, rolling topography of the foothills zone of the Piedmont provinces. This escarpment is often comprised of steep slopes with exposed bedrock cliffs and shallow colluvial soils. The down slope sides of the escarpment have evidence of past slope movements in the form of large scale deposits, debris fans, talus slopes, and dormant debris slides. Debris flows have been documented along the BRE in multiple past storm events including those in 1916, 1940, 1996, and 2004. On May 18, 2018, debris flows again initiated near the top of the BRE slopes and travelled down to the North Pacolet River valley floor during heavy rains on soils with high antecedent moisture contents. At least 27 debris flows were initiated, travelling up to ~966 meters (~3,170 feet) down drainages below. At least 6 homes were damaged or destroyed and one fatality occurred due to these debris flows. Main highways, interstates, and multiple private roads were covered by the debris. Appalachian Landslide Consultants, PLLC (ALC) and the North Carolina Geological Survey (NCGS) responded to this emergency situation in order to provide Polk County Emergency Management information about the stability of the slopes before the arrival of Tropical Depression Alberto just 9 days after the May 18 rains. During this reconnaissance, ALC and the NCGS identified areas of potential instability in the coming rains. County Emergency Management used this information when deciding to issue a voluntary evacuation recommendation to the people of the North Pacolet River valley. This paper discusses the findings of the reconnaissance mapping, as well as a general overview of the integration of geological information into emergency response and preparation.