Landslide dam activity in Flysch Zone of the Eastern Carpathian, Romania: distribution, stability, and failure mechanism assessment

Abstract Landslide dams are natural obstructions of river channels caused by slope failures. In Romania, the Eastern Carpathians, particularly the Flysch Zone, are the most susceptible to landslide dam formation compared to other mountainous regions in the country, due to a combination of specific lithological characteristics, rainfall conditions, and anthropogenic factors. In some cases, stable landslide dams create impoundments that can persist for decades or even centuries. This study investigates the spatial distribution of 21 landslide dams in the Eastern Carpathians, analyzing the landslide-triggering factors, the morpho-hydrological relationships between the dams and their associated impoundments assessed using six stability indices (DB, DBI, II, IS, BI, RI), and the failure mechanisms of the landslide dams that have breached. The main findings are: (1) 70% of the investigated sites are dams made by rockslides and rock avalanches, 20% are from rock and debris avalanches, and 10% were formed by shallow (earth) slides. (2) Heavy rainfall and seismic activity are the primary triggers for dam formation, although snowmelt, fluvial erosion, and anthropogenic activities, such as deforestation and infrastructure development, also play significant roles. (3) Half of the investigated sites (11 sites) have already failed, primarily due to dam overtopping, while two of the existing landslide dams are considered unstable based on stability index assessments. (4) Although landslide dam formation has not posed direct threats to human life, it has caused material losses and disrupted local and regional watercourse connectivity. Overall, this study addresses data gaps in the Eastern Carpathians, enhancing our understanding of landslide dam formation and its impacts on the region’s mountainous landscapes.