Characteristics of rock-ice avalanches and geohazard-chains in the Parlung Zangbo Basin, Tibet, China

Over the past 50 years, catastrophic rock-ice avalanches have frequently occurred in the Parlung Zangbo Basin, such as those involving the Guxiang and Tianmo gullies, causing serious casualties. The initial slope failures generally occurred in high mountains at an elevation of >4000 m above sea level (asl), which subsequently triggered long run-out geohazard chains, such as debris avalanches, debris flows and outburst floods. In this paper, based on satellite imagery, aerial photography, interferometric synthetic aperture radar (InSAR) technology, and field surveys, the characteristics of these rock-ice avalanches are analyzed. The results indicate that high-risk zones are located in the upstream and downstream sections of the Parlung River, where intense geohazards occurred. Based on the movement characteristics of geohazards, four disaster chain modes were summarized, namely rockslide-river blocking-flood, rock and ice avalanche-glacier lake outburst-flood, rockslide/rock-ice avalanche-debris avalanche/debris flow-river blocking-flood, and debris flow-river blocking-flood. Several factors jointly determined the propagation mode and high mobility observed during rock-ice avalanche events: (i) the pipe-like terrain favoured long-runout propagation; (ii) detached ice and snow quantities can be greatly increased via entrainment, thus reducing friction between the moving mass and basal layer. (iii) The debris-moraine may provide abundant materials easily scraped by debris flows. (iv) Meltwater provided by ice and snow at the base of the flow could lubricate debris to improve mobility after mixing with avalanches' debris. Choosing Tianmo gully as an example, dynamics analysis was conducted in by DAN-3D. In the future, possible increases of failure events in formerly glaciated and permafrost areas are likely because of ongoing changes in climatic conditions. This study provides insight into multi-stage avalanche motion in the glacier regions. The results constitute a reference for hazard zonation in similar mountainous areas.