Melting mountainsides: The debris flows and "blowouts" of Hurricane Helene

Extreme rainfall (up to ~75 cm) prior to and during the passage of Hurricane Helene’s remnants generated catastrophic landslide activity in the rough topography of the southern Appalachian Blue Ridge. Due to saturated hillslope conditions, debris flow and “blowout” (flow-slide) landslides predominated during this extreme precipitation event, with substantial loss of life and property damage resulting from slide material entering populated areas. Both debris flows and blowouts mobilized colluvium stored on slopes, with debris flows transporting substantial volumes of colluvial soil well downslope and into the drainage network. Non- or minimally-erosive “blowout” slides, though less volumetrically significant, were observed to be highly mobile and potentially hazardous despite mobilizing less colluvial soil over shorter distances. Debris flows and blowouts initiated on both natural and modified slopes, with embankment failures representing a significant source of debris flow initiation during the event. Field work by Appalachian Landslide Consultants personnel has provided additional understanding of details of debris flow and blowout initiation and downslope movement, including potential influences of bedrock lithology and structure and associated soil type on initiation and mobility of fluidized slides. Comparison of detailed post-event aerial photography to pre-existing LiDAR imagery offers further insight into the hillslope settings of debris flow and blowout initiation. These data will generate useful inputs for ongoing modeling efforts to support regional preparedness, with debris flow (or blowout) runout modeling representing a particularly important direction of study necessary to mitigating impacts of future events.

Prince (2025) Melting mountainsides: The debris flows and "blowouts" of Hurricane Helene, USGS Landslide Hazards Seminar, 14 May 2025.