275-5 Sediment Storage Dynamics in Headwater Catchments following the Hurricane Helene Extreme Precipitation Event

Abstract:

Hurricane Helene caused unprecedented flooding, landslides, and debris flows throughout headwater catchments in the Southern Appalachian Mountains in September of 2024, resulting in loss of life and extensive damage to downstream property. Despite the regional scale of Helene’s disturbance, downstream flooding and sediment deposition varied substantially among neighboring catchments, suggesting differences in river corridor capacity to store and attenuate sediment. Improved understandings of watershed response and downstream hazard cascades are increasingly important as extreme storms, like Helene, become more frequent and severe in this region. To identify factors which increase catchment capacity to absorb disturbance, we investigated the relationship between sediment storage, geomorphic complexity, and human use in hurricane impacted headwater catchments in the Pisgah National forest. Sediment storage is used both as a proxy for flood water attenuation and as a direct measure of catchment disturbance resilience. We predict that high-storage reaches will be characterized by greater biogeomorphic complexity, including multithread planforms, islands, logjams, and/or evidence of beaver activity, whereas low-storage reaches will exhibit simpler channel morphology. We further predict that high storage reaches will display greater variability in channel geometry and sediment size, reflecting an increased capacity to retain and store sediment during Helene. We use a combination of longitudinal profiles and cross sections to characterize reach scale sediment storage volume and geomorphic complexity, and pebble count surveys to characterize sediment storage type. Forest composition surveys were conducted near each reach, as a proxy for logging legacy and human use. Catchment scale characterization was conducted using pre- and post- event LiDAR and aerial imagery. Although catchments vary in total storage, preliminary results indicate that multithread planforms, islands, and log-jam presence correspond with zones of greater sediment storage in every catchment. Presence and scale of these geomorphic features appear to correspond with logging legacy and human-use, with more homogeneous forest compositions and greater human disturbance corresponding with less extensive storage reaches and fewer instances of wood in channels. These findings suggest that logging legacies and ongoing human-use are associated with differences in modern channel complexity, and influence sediment storage following disturbance events. By continuing to investigate the complex interactions between disturbance, ecology, and human-use, we will improve our ability to identify headwater catchments that buffer or amplify downstream flooding and sediment delivery.

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