Washed Away: Incorporating Bank Erosion into Highway Risk Assessments

Rivers are dynamic systems that shift over time across floodplains and alluvial fans. While these natural geomorphic changes often bring ecological benefits, they also pose a significant threat to transportation infrastructure. Highways, often located in valleys alongside rivers, are particularly vulnerable to these hazards. This research focuses on bank erosion-related components of a landslide and flood risk assessment conducted for Highway 8, a 60 km-long road that runs parallel to the Nicola River in British Columbia, Canada. In November 2021, an extreme flood caused severe bank erosion that washed out Highway 8 in 25 locations, cutting off access to communities along the corridor for nearly a year. Highway 8 is also subject to geotechnical hazards (e.g., rock fall and debris flows) and intersects catchments burned by wildfires earlier in 2021, which further complicated response and recovery efforts. During the subsequent highway re-construction, a risk assessment was initiated to systematically assess the risk of highway closure due to landslide and flood-related geohazards. The highway was divided into smaller segments ranging from tens to hundreds of meters, and each geohazard was characterized in terms of its probability of impacting a particular segment and the associated road closure duration. The study compared the overall risk posed by different geohazards, considering the duration of closures on an annualized basis. To simulate bank erosion, a probabilistic, physically-based model was used. During the design flood scenario, approximately 18 km of the highway had a 25% or higher probability of being impacted by bank erosion, with a total closure duration of about 2 years. Among the highest-risk sites identified, the majority were related to bank erosion hazards. This study emphasizes the critical importance of considering geomorphic changes when planning and designing highways to ensure their long-term resilience and functionality.