Leveraging hillslope connectivity for improved large-scale assessments of landslide risk

Abstract. Landslides hinder sustainable development in mountain regions, threatening livelihoods and impacting linear and water infrastructure. Susceptibility maps are a common tool for estimating and managing landslide hazards, exposure, and risks. Yet, susceptibility maps omit hillslope connectivity, a critical shortcoming for mapping the magnitude of landslide hazards, including cascading hazards from slope failure and downslope runout. Herein we propose the COHESION (COnnected HillslopE SusceptibIlity for slOpefailure and ruNout) approach to couple susceptibility mapping with an assessment of hillslope connectivity to identify downslope-connected landslide objects (LSOs) and associated runout pathways. As we demonstrate for the Kaligandaki basin in Nepal, analyzing LSOs enables estimating the magnitude of slope failures in terms of mobilized sediment volume and to quantify additional impacts from landslide runout. After calibration using a remotely sensed landslide inventory, we find that 16 % of the basin’s slopes are susceptible to failure, while an additional 9 % of the basin area is impacted by runout. Around 33 % of buildings and 65 % of roads in the basin are on susceptible slopes, while more than 27 % of buildings and 69 % of roads are in landslide runout pathways. Omitting runout from landslide assessments would thus result in a major underestimation of risk. Our results emphasize the importance of connectivity for slope stability modeling on landscape scales, leading to improved assessments of slope hazards and management of river basin sediments.