RESPONSES TO LANDSLIDES AND LANDSLIDE MAPPING ON THE BLUE RIDGE ESCARPMENT, POLK COUNTY, NORTH CAROLINA, USA

Abstract

Landslides occur in Polk County, North Carolina, primarily along the Columbus Promontory of Blue Ridge Escarpment (BRE), which has 400 m of topographic relief and slopes typically >20°. Bedrock is characterized as late Proterozoic to early Paleozoic metamorphic rocks within Paleozoic thrust sheets. On May 18, 2018, ∼200 mm of rainfall over a 3- to 4-hour period triggered numerous debris flows and slides along the BRE, causing one fatality and severe damage to homes and roads. The State Emergency Operations Center tasked the North Carolina Geological Survey to assess slope stability ahead of search and rescue operations and assess damage along the North Pacolet River valley. The loss of life and destruction from the 2018 storm and ongoing threats to infrastructure prompted us to map landslides throughout Polk County in 2019–2021 to fully document the 2018 landslides and place them in the context of past and ongoing landsliding. We mapped 920 varied types of landslides and attribute 241 to the 2018 storm, making it one of the largest events in North Carolina since 2004 with respect to landslide numbers and spatial frequency. The highest concentrations of landslide features in Polk County are along the slopes of the BRE, especially the Pacolet River and Green River valleys. These rivers exploit post-orogenic brittle fractures to form linear reentrants where the May 2018 and other landslides are concentrated. This article describes our landslide response and mapping efforts and relates our findings to the geomorphic and geologic framework and to past landslide events in the region.