Risk Assessment of the Lower Reaches of the Yarlung Tsangpo-Brahmaputra River Exposed to Glacier Landslide Risk Chains

The lower reaches of the Yarlung Tsangpo-Brahmaputra River are important hydropower development bases in the future. However, frequent glacier landslide hazard occurred in this region, posing serious threats to the safety of local communities and infrastructures. A glacier landslide hazard chain can form a long-run mass flow and generate a large flood, travelling more than hundreds of kilometers away from the initiating hazard site. This study takes remote sensing, field investigations and numerical simulations to make risk assessment on the river system. A comprehensive framework is developed, considering the impacts of near-field and far-field hazards. The findings suggest that the presence of extensive, nearly saturated sediments on the glacier valley floor significantly increases the mobility and intensifies the scale of the mass flow when these sediments are incorporated. Topography plays a key role in influencing the behavior of mass flow. When the valley outlet intersects the river course at a perpendicular angle, topographic barriers lead to an abrupt stop, resulting in the formation of high barrier dams. In contrast, if the glacier valley runs nearly parallel to the river channel, the mass flow is able to travel further upon entering the river, thereby impacting a larger area within the river channel. The formed mass flow can traverse river channels in mountainous regions, during which geo-material gradually accumulates, leading to the formation of barrier dams. Barrier dams can break suddenly, leading to breaching floods that significantly extend the downstream impact, ranging from several kilometers to potentially hundreds of kilometers. Among the regions at risk, the Sedongpu-Ganglang reach faces the greatest vulnerability to river damming and subsequent breaching floods. Downstream areas along the Yarlung Tsangpo-Brahmaputra River are comparatively less likely to experience greater threats from these events than those posed by local monsoon floods. These findings serve as a valuable basis for developing strategies to manage glacier hazard chains, contributing to better disaster preparedness and risk mitigation in affected regions.