Quantifying debris-flow – forest interactions using high-resolution LiDAR data in the Squamish – Lillooet region, British Columbia, Canada

Debris flows are one of the most common geomorphic processes in mountainous areas, and can form a great threat to local communities and infrastructure. Traditionally, mitigation efforts have focused on engineering solutions such as check dams or debris basins. Recently, focus has started to shift towards more nature-based solutions such as forest buffer zones, which require an understanding of interactions between debris flows and trees for their design. Some research into debris flow-forest interactions has been done using field data, aerial imagery or simplified physical experiments, however, quantitative knowledge of tree removal by debris flows is still lacking. This study aims to assess tree survival and removal by debris flows, and to identify controlling debris flow and vegetation properties.We use multi-temporal, high resolution airborne laser scanning (ALS) data covering multiple debris-flow events over four different forested debris-flow fans in the Squamish-Lillooet region in British Columbia, Canada, to track sediment deposition and tree removal. Tree survival patterns are linked to tree and debris-flow characteristics (tree size, location and proximity to the next tree, and deposition and erosion depth, respectively) to gain insight into the interaction between debris flows and forests.Preliminary results show that smaller trees have a higher chance of being removed by a debris flow, and that the chance of tree survival increases with distance from the fan apex and with higher tree density. Next steps include numerical simulations of debris-flow velocities to quantify the relationship between debris-flow impact forces and tree removal or survival. The results of this study will help identify optimal characteristics for resilient debris-flow forest buffer zones.