LANDSLIDES IN WESTERN NORTH CAROLINA - A FIELD-BASED VIEW OF SLOPE MOVEMENTS TRIGGERED BY THE REMNANTS OF HURRICANE HELENE

On September 26-27, 2024, a Predecessor Rainfall Event, followed by the remnants of Hurricane Helene, passed over the southern Appalachians. Heavy rainfall was concentrated along the high peaks of the Blue Ridge Escarpment and other high-elevation ranges in western North Carolina (WNC). After the storm passed and communications began to be reestablished, geologists with Appalachian Landslide Consultants, PLLC (ALC) began responding to requests for landslide evaluations in WNC. Through these requests from the NC Geological Survey, county Emergency Management, the NC Department of Transportation, as well as private individuals, geologists began to collect empirical observations from the dozens of slope movements visited. The majority of these are classified as debris flows, debris blowouts, or debris or weathered rock slides. Many of the debris flow initiation zones are shallow (less than 1.5 m deep) and translational in nature. These debris flows channelize, scouring the steep portions of the drainage, and depositing their coarse load on shallower slopes. On natural slopes, debris flows often initiate at the colluvium/bedrock boundary or within colluvium, however, an abundance of them have initiated on constructed fill slopes. Many exhibit evidence of surface water sheet flow outside of the debris flow impact areas.

Debris blowouts are features that often have an arcuate head scarp, and a slide plane that transitions from planar to rotational, leaving a lip of in-place soil at the base of the circular initiation zone. Blowouts were observed to take place within colluvium, or within colluvium and residuum/weathered rock. Evidence of groundwater flow through macropores or bedrock fractures was observed in most of these features. Blowouts do not exhibit channel scour, but instead spread debris across the slope downhill. In multiple observed cases, the runoff from these blowouts eventually entered a channel downslope, and an erosion-induced debris flow was then initiated.

Both debris flows and debris blowouts impacted homes and structures below them, however, the debris flows appear to have been much more destructive. Mapping these features separately can help us understand how to refine models, and better understand the rainfall intensity and duration that lead to initiation of these slope movements.