Field-based investigation and characterization of debris-flood processes in southwestern British Columbia

Debris floods - described as rapid water flow heavily charged with sediment - present significant hazards in steep, mountain channels. These events are triggered by various factors, including heavy rainfall, rain-on-snow events, dam breaches, and oversaturated debris flows, and are often exacerbated in post-wildfire landscapes. Debris floods are common in British Columbia and typically occur in steep creeks and on their alluvial fans, posing safety and economic risks to communities and infrastructure in these areas. Despite their significance, debris floods remain poorly understood compared with other creek hazards, such as debris flows, and are challenging to model in practice. Limited post-event data further constrains both conceptual understanding and the development of reliable hazard assessments and modelling. This study addresses these limitations through detailed field and geospatial analysis of debris-flood events triggered by the November 2021 atmospheric river in southwestern British Columbia. Field investigations were conducted across thirteen catchments and their alluvial fans, documenting impact extents, erosion and deposition patterns, sediment characteristics, vegetation disturbances, and channel geometry. Airborne LiDAR data were integrated where available, with pre- and post-event differencing conducted at six sites to quantify volumetric changes; field-based estimates were applied otherwise. Additional morphometric parameters were derived including fan and catchment area, relief, watershed stream length, Melton ratio, channel gradient, channel incision, and runoff volume. A novel visualization approach, Fan-Relative Elevation Models, was developed to show alluvial fan topography relative to active channel elevation. These datasets were synthesized to produce detailed geomorphic maps of each site. Results highlighted variability in geomorphic response across creeks. Geomorphic responses and relative fan-channel incision were plotted to compare debris flood impacts with pre-existing fan-channel conditions. Highly incised channels were characterized by confined flows and dominant bank erosion, whereas less incised channels exhibited increased overbank deposition and flow spreading. Infrastructure, particularly culverts, consistently influenced flow routing and sediment distribution, often resulting in overtopping and redirection across roadways. This study provides detailed, case-study datasets and maps to further the understanding of debris-flood processes and support the advancement of hazard assessment and modelling approaches.