Geomorphic activity and related large wood recruitment during debris flows and debris Floods: Storm Alex in the Vésubie valley (France)

• Storm Alex (October 2020) triggered extreme geomorphic activity in Vésubie (FR). • Debris flows and debris floods caused channel widening, recruiting large wood. • Geomorphic changes and forest loss were estimated using LiDAR-derived data. • Large wood production is deduced from eroded forest areas and forest stand density. • An equation predicting forest loss from total sediment mobilized was proposed. Extreme floods in forested catchments cause geomorphic changes and large wood recruitment, worsening flood hazards. The study examines the geomorphic response and large wood recruitment in the Vésubie River catchment (392 km 2 , southeast France) during Storm Alex in October 2020. Using high-resolution LiDAR-derived DEM, the research quantifies geomorphic changes in the main branches and 43 active tributaries, estimating sediment mobilization and channel dynamics at (sub-)catchment and 100-meter reach scales. Forest cover changes were analyzed by comparing pre- and post-event canopy height models (CHM), allowing for the estimation of large wood recruitment. The results show significant geomorphic changes, with extreme erosion and deposition rates observed at both the (sub-)catchment and reach scales. A total forest cover loss of 121 ha was estimated, with recruited large wood volumes per unit of catchment surface ranging from 4 – 445 m 3 /km 2 in the tributaries and 45 – 95 m 3 /km 2 in the main branches. A positive correlation was identified between forest cover loss and both erosion and deposition rates, suggesting that forest loss resulted from trees being scoured by erosion or buried by deposition. An empirical approach was developed to predict forest cover loss based on total sediment mobilization by implementing different equations for optimistic, intermediate, and catastrophic scenarios. The research highlights the interconnected nature of geomorphic processes and their role in large wood dynamics. The findings provide valuable insights into large wood recruitment and the impact of intense geomorphic activity on channel dynamics, demonstrating complex and tight interactions between sediment transport, forest cover, and flood hazards in mountain catchments.