摘要：In the morning of 23 August 2017, around 3×106 m3 ofgranitoid rock broke off from the eastern face of Piz Cengalo, southeastern Switzerland.The initial rockslide–rockfall entrained 6×105m3of a glacier and continued as a rock (or rock–ice) avalanche before evolving into achannelized debris flow that reached the village of Bondo at a distance of6.5 km after a couple of minutes. Subsequent debris flow surges followed inthe next hours and days. The event resulted in eight fatalities along itspath and severely damaged Bondo. The most likely candidates for the watercausing the transformation of the rock avalanche into a long-runout debrisflow are the entrained glacier ice and water originating from the debrisbeneath the rock avalanche. In the present work we try to reconstructconceptually and numerically the cascade from the initial rockslide–rockfall to the first debris flow surge and thereby consider two scenarios interms of qualitative conceptual process models: (i) entrainment of most ofthe glacier ice by the frontal part of the initial rockslide–rockfalland/or injection of water from the basal sediments due to sudden rise inpore pressure, leading to a frontal debris flow, with the rear part largelyremaining dry and depositing mid-valley, and (ii) most of the entrainedglacier ice remaining beneath or behind the frontal rock avalanche anddeveloping into an avalanching flow of ice and water, part of which overtopsand partially entrains the rock avalanche deposit, resulting in a debrisflow. Both scenarios can – with some limitations – be numericallyreproduced with an enhanced version of the two-phase mass flow model(Pudasaini, 2012) implemented with the simulation software r.avaflow, basedon plausible assumptions of the model parameters. However, these simulationresults do not allow us to conclude on which of the two scenarios is the morelikely one. Future work will be directed towards the application of athree-phase flow model (rock, ice, and fluid) including phase transitions inorder to better represent the melting of glacier ice and a more appropriateconsideration of deposition of debris flow material along the channel.
