276-10 Forecasting Channel Response to Extreme Floods Using River Morphology: Lessons from Hurricane Helene

Session: Natural and Urban Channel Responses to Disturbance, Management, and Restoration

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Extreme floods are powerful agents of geomorphic change, yet our ability to forecast river channel response to these rare events remains limited. In the immediate aftermath of a disaster, response efforts rightly prioritize human safety and infrastructure, leaving limited capacity for rapid assessment of geomorphic impacts. This study investigates whether channel widening in steep mountain rivers during extreme flooding can be predicted using pre-flood morphology and evaluates the feasibility of rapidly detecting and quantifying river channel changes using satellite imagery. Hurricane Helene, which struck the Blue Ridge Mountains on September 27, 2024, delivered over 20 inches of rainfall and triggered floods-of-record across many river channels, exemplifying this type of extreme event.

We quantified river response to Hurricane Helene by quantifying post-flood channel changes along the full lengths of nine rivers in western North Carolina and eastern Tennessee. Pre-event bankfull widths derived from 1 m lidar data were compared to post-event channel scour mapped from 3 m PlanetScope imagery acquired in the weeks following the storm. Results show substantial variation in channel response across rivers, with the proportion of channel length exhibiting detectable widening ranging from 19.7% (Linville River) to 93.3% (North Toe River). Averaged across catchments, channel width widened by a factor of 1.3 to 3.4. Widening was strongly concentrated in meander bends, where 75%-83% of the total detected widening occurred across rivers. Bend-scale widening occurred primarily by deposition on inner banks. Across all rivers, initially narrower and oversteepened segments (i.e., knickpoints) were more likely to undergo widening, with statistically significant correlations between pre-event channel width and slope and the magnitude of the post-event width response.

These results demonstrate that pre-flood channel geometry provides useful predictive information for identifying areas most likely to experience a measurable change in river channel geometry in response to an extreme flood. By integrating high-resolution lidar with rapid-return, moderate-resolution satellite imagery, this framework enables rapid, scalable assessment of flood impacts in mountain rivers. As extreme hydrologic events become more frequent and intense under climate change, anticipating where and how mountain rivers respond is essential for improving disaster preparedness, guiding long-term river management, and understanding the future evolution of steep mountain rivers.

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

doi: 10.1130/abs/2025AM-8213

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