EFFECTS OF HURRICANE HELENE ON RIVER CHANNELS IN WESTERN NORTH CAROLINA: PRELIMINARY OBSERVATIONS FROM REMOTE SENSING

Hurricanes can rapidly reshape landscapes as intense precipitation leads to flash flooding of mountain channels. On September 26, 2024, western North Carolina and northern Georgia received up to 30 inches of rainfall in a 24 hour period, leading to flash floods across the affected areas. Here we present preliminary observations of some of the most affected river gorges in the upper Broad River and Green River drainages between Zirconia and Chimney Rock, NC. Observations are drawn from high-resolution (<3m/pixel) aerial and satellite orthoimagery, dense-return (>20 returns/m²) airborne LiDAR and super-high resolution (<3 cm/pixel) drone-based structure-from-motion photogrammetry. Orthoimagery and LiDAR illustrate extensive channel bank widening, localized erosion and aggradation and dramatic reorganization of in-channel islands. Structure-from-motion photogrammetry further illustrates an increase in channel roughness through large boulder excavation and transport. Future work will include LiDAR point-cloud differencing of channel reaches and repeat drone flights to continue monitoring changes in channel roughness. Our results are broadly consistent with similar observations made in central Puerto Rico following Hurricane Fiona and support the general conclusion that flash floods leave a lasting legacy on mountain streams by dramatically changing channel cross-sectional area and increasing channel roughness. These effects have the potential to increase future flood risk as channels respond to geometric changes and increase the frictional resistance to flow, and have implications for greater hazards as the global climate warms and storms increase in intensity. Our results also demonstrate the value of rapid response remote sensing for documenting and preserving the effects of flash flooding and the synergistic value of integrating multiple types of remote sensing across multiple scales of observation.