Initiation mechanisms and dynamics of a debris flow originated from debris-ice mixture slope failure in southeast Tibet, China

Glacial debris flows are a common catastrophic hazard in alpine regions due to glacier recession and permafrost thawing. They are frequently triggered by heavy rainfall and outburst floods. To date, studies on debris flows originated from debris-ice mixture slope failures triggered by glacier melting in low-intensity rainfall conditions are still limited. In this study, a typical glacial debris flow that occurred in September 2020 on the west side of Namcha Barwa in southeast Tibet, China, is taken as a benchmark to reveal the initiation mechanisms and dynamics of debris-ice landslide-debris flow hazard chains. The flow discharge, erosion, entrainment, deposition, and dynamic process of the debris flow are evidenced by both field investigations and numerical simulations with a single-phase cell-based simulation program, Erosion–Deposition Debris Flow Analysis (EDDA) (Chen and Zhang, 2015) and a multi-phase flow model r.avaflow (Pudasaini and Mergili, 2019). This hazard chain was originated from a debris-ice mixture slope failure with a volume of 1.14× 106 m3 in a temperate area under little rainfall. The mobile mixture entrained saturated moraines and water in the ravine channel and rapidly transformed into a viscous debris flow, traveling about 9.78 km and entraining 50% more materials along the path. The peak discharge and frontal flow velocity at the outlet of the ravine reached 4700 m3/s and 11.4 m/s, respectively. About 3.75× 105 m3 of debris deposited on the riverbed and partially blocked the Yarlung Tsangpo, causing severe damage to local infrastructures. This study provides a scientific basis for understanding the evolution process of a debris-ice landslide-debris flow hazard chain and for minimizing its adverse impact on key infrastructures in the region.