Geomorphic Response and Large Wood Recruitment in the Vésubie Valley (France) Following Storm Alex

Extreme rainfall events in mountain catchments can induce substantial geomorphic changes, reshaping channels, hillslopes, and surrounding environments. These changes often widen active channels, recruiting large wood from adjacent forests into sediment-laden flows, thereby increasing hazards such as altered flow patterns, sediment retention, and logjam formation. Such dynamics can exacerbate flood risks, particularly near infrastructure like bridge piers, dams and weirs. Understanding the extent of forest areas contributing to large wood recruitment and predicting mobilized large wood volumes is critical for effective hazard mitigation. This study examines the geomorphic response of the Vésubie catchment (392 km², south-east, France) to Storm Alex (October 2020), which caused intense flood and sediment transport (i.e., bedload, debris floods and debris flows) with strong large wood recruitment. Using high-resolution aerial LiDAR data from pre- and post-storm surveys, geomorphic changes in valley bottom channels and 43 tributaries (catchment sizes: 0.06–59 km²) were analysed at both catchment and 100-m reach scales via the DEM of Difference (DoD) technique. Diachronic canopy height models were used to assess forest cover loss, and the volume of recruited large wood was estimated based on data from the French national forest inventory. Results revealed massive sediment mobilization, with sediment net balances ranging from -669 m³ ± 36 m³ to -341,575 m³ ± 3,625 m³ in tributaries and -518,609 m³ ± 5,735 m³ to 326,213 m³ ± 16,912 m³ in valley bottoms. This culminated in a total sediment export of 2.14 Mm³ ± 48,985 m³ from the Vésubie catchment. Tributary erosion volumes varied by an order of magnitude, displaying spatially consistent patterns in tributaries with pronounced variability in valley bottom channels. Erosion rates showed no distinct trend with slope, with high rates observed also at low gradients. Conversely, deposition rates increased with decreasing slopes (<25%) but declined sharply in steeper channels, emphasizing the critical role of slope in sediment connectivity. Erosion rates varied widely (0.1–2.5 m³/m²) across the cascading network, reflecting diverse geomorphic responses and exceptional sediment mobility during Storm Alex. The absolute and relative reduction of forest cover extension inside the reaches well correlated with local sediment erosion, deposition, and net balance rates per unit length of reaches, indicating dependence on the intensity of geomorphic processes. The process type played a minor role. The estimated large wood volumes recruited during Storm Alex in the tributaries ranged from moderate to high when compared to literature values, while system-wide estimates exceeded the highest predictions of large wood volumes when scaled to the catchment surface. The findings provided by this extensive dataset underscore the need to integrate geomorphic and large wood dynamics into hazard assessments and protection measures in mountainous regions, particularly in case of extreme events.