Research on glacial/rock fall-landslide-debris flows in Sedongpu basin along Yarlung Zangbo River in Tibet

A large-scale glacial/rock fall occurred in the upstream of Sedongpu basin on the left bank of the Yarlung Zangbo River in Linzhi of Tibet on October 17, 2018. The runout materials were composed of glaciers and rocks, which impacted the accumulation and moraine of the lower part, then formed a debris flow and dammed the Yarlung Zangbo River impounding a barrier lake. This study aims to identify the triggering mechanisms and propose a theoretical basis for the development of landslide hazard prevention and reduction. In this paper, the authors adopted short distance observation by helicopter, statistical analysis of climate change data, interpretation of multi-temporal satellite remote images, calculation of landslide-debris flow velocity using the Scheidegger formula and the b value of seismic activity sequence calculated by Gutenberg-Richter formula as well as multi-factor assignment statistics to evaluate the site and scale of the future glacial falls. It was found that the debris flow was triggered under the condition of high and steep terrain, broken rock mass, climate warming, regional rainfall, glacier ablation, fracture activity and earthquake damage (b value being about 0.7). The volume of the deposition materials was estimated at 31×10⁶ m³ (including previous residual accumulation), which were mainly composed of gravel soil, a small amount of ice and big boulder. The whole event lasted 6.7 minutes with an average movement speed of about 20 m/s. The runout distance of the landslide was more than 8 km. Two-thirds of the Yarlung Zangbo River had been blocked before the glacial/rock fall-landslide-debris flow event that occurred on October 17, 2018. The countermeasures of disaster prevention and reduction which include adaption to the nature, comprehensive avoidance and reasonable dredging are put forward in consideration of the reality of high and steep hillslope, sparse population and traffic inconvenience in this area. The Sedongpu event that occurred on October 17, 2018 was a typical cascading landslide hazard, with a process of landslide-debris flow-dammed lake-outburst flood disaster. Such events will continue to occur frequently for a long time in the future. Based on a comprehensive analysis, the authors have reached the conclusion that a new landslide-debris flow-dammed lake event will be triggered again when the average temperature exceeds 13℃, or precipitation exceeds 5 mm/h or 10mm/d, or the earthquake PGA is greater than 0.18g.