92-7 Helene's Debris Flows in the Context of the Thoroughly-Scarred Blue Ridge Landscape

Session: Landslide Inventory Mapping and Next Steps: Assessing Susceptibility, Hazard Models, Risk, and Policy

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Abstract:

The Helene weather event triggered thousands of debris flows in a rugged landscape with a well-established history of slope failure. Areas experiencing the most extreme Helene impacts bear the extensive scars of older, undocumented events, and were also heavily impacted by known storm events in 2021, 2018, 1977, 1940, 1916, and 1901. Debris flows triggered by Helene’s rainfall show a variety of relationships to these pre-Helene slope failures, with a number of Helene debris flows initiating in different portions of hollows that initiated debris flows elsewhere during previous events. Some pre-existing, intact landslides of unknown age sourced debris flows from their oversteepened zones, while many other slide masses remained largely unaffected as debris flows initiated on intact slopes 10’s of meters away. Numerous debris flows followed channels scoured by historic debris flow events after initiating on intact slopes that had experienced several extreme rainfall events during the past century without failure. Focused, statistical study of patterns of Helene debris flow initiation with respect to older failures visible in the landscape will be necessary to evaluate possible relationships. Preliminary analysis of the landscape complexity associated with Helene debris flow initiation highlights the importance of communicating debris flow hazard based on general aspects of topography, near-surface geology, precipitation, and known landscape behavior. The vast majority of Helene’s debris flows initiated in colluvial soils within hollows modeled to be capable of producing debris flows (or on road embankments), with transported material often traversing or coming to rest on known deposit areas. Debris flow density also appears to be broadly related to precipitation totals and intensity. Collectively, observed Helene debris flow behavior followed patterns identified through study of previous events, though the specifics of initiation locations appear to reflect unknown variables. The combined analysis of pre- and post-storm data suggest suggests topography observed and modeled as debris flow-susceptible should always be treated as hazardous under appropriate weather conditions, regardless of recent landslide history.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10658

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