Post-Wildfire Debris Flow Susceptibility and Preconditioning Factors in the Central Okanagan

Debris flows are a common hazard in British Columbia, but studies on their occurrence in the temperate continental regions of the province are primarily limited to anthropogenic and specific large events. We document 197 debris flows in and around the Regional District of Central Okanagan and the physical properties governing their magnitudes and spatial distributions. These physical properties were used to create a Random Forest model to determine predictive variables associated with debris flow initiation. Key predictive variables include cross-sectional curvature, basin Melton ratio, slope, burn severity, valley depth, and distance to stream, with a final precision-recall area-under-curve (PR-AUC) score of 0.953. Results indicate that debris flow susceptibility is highest in cross-sectionally concave channels on slopes surpassing 10° within narrow basins, particularly those that have been burned within the last three years. These environments are found primarily along low-order channels along canyon walls within the study area. Two-thirds of all debris flows examined occurred in areas burned within three years before their occurrence. Spatial trends point to the proclivity of debris flows to be clustered in space due to fire and local hydrological input effects; fire increases the potential and magnitude for both runoff and infiltration-generated debris flows and creates a debris flow regime akin to those in more arid montane environments through sediment loading, though with lower magnitude flows. Based on debris flow occurrence dates, runoff-triggered debris flow events are minor and rare outside of fire areas, whereas infiltration-triggered debris flow events tend to be more major and commonly occur in isolation as well as in clusters. We encourage further study utilizing inventories in similar regions throughout the province, as well as improved documentation of surficial materials and properties of soils to better delineate susceptible terrain.