Page 1 Back-calculation of the 2017 Piz Cengalo-Bondo landslide cascade with 1 r.avaflow 2 Martin Mergili 1,2 , Michel Jaboyedoff 3 , José Pullarello 3 , Shiva P. Pudasaini 4 , 3 1 Institute of Applied Geology, University of Natural Resources and Life Sciences (BOKU), Peter-Jordan-Straße 82, 4 1190 Vienna, Austria 5 2 Geomorphological Systems and Risk Research, Department of Geography and Regional Research, University of Vi- 6 enna, Universitätsstraße 7, 1010 Vienna, Austria 7 3 Institute of Earth Sciences, University of Lausanne, Quartier UNIL-Mouline, Bâtiment Géopolis, 1015 Lausanne, 8 Switzerland 9 4 Institute of Geosciences and Meteorology, Geophysics Section, University of Bonn, Meckenheimer Allee 176, 53115 10 Bonn, Germany 11 Correspondence to: M. Mergili (martin.mergili@boku.ac.at) 12 Abstract 13 In the morning of 23 August 2017, around 3 million m³ of granitoid rock broke off from the east face of Piz Cengalo, 14 SE Switzerland. The initial rock slide-rock fall entrained 0.6 million m³ of a glacier and continued as a rock(-ice) ava- 15 lanche, before evolving into a channelized debris flow that reached the village of Bondo at a distance of 6.5 km after a 16 couple of minutes. Subsequent debris flow surges followed in the next hours and days. The event resulted in eight 17 fatalities along its path and severely damaged Bondo. The most likely candidates for the water causing the transfor- 18 mation of the rock avalanche into a long-runout debris flow are the entrained glacier ice and water originating from 19 the debris beneath the rock avalanche. In the present work we try to reconstruct conceptually and numerically the 20 cascade from the initial rock slide-rock fall to the first debris flow surge and thereby consider two scenarios in terms 21 of qualitative conceptual process models: (i) entrainment of most of the glacier ice by the frontal part of the initial 22 rock slide-rock fall and/or injection of water from the basal sediments due to sudden rise in pore pressure, leading to a 23 frontal debris flow, with the rear part largely remaining dry and depositing mid-valley; and (ii) most of the entrained 24 glacier ice remaining beneath/behind the frontal rock avalanche, and developing into an avalanching flow of ice and 25 water, part of which overtops and partially entrains the rock avalanche deposit, resulting in a debris flow. Both sce- 26 narios can be numerically reproduced with the two-phase mass flow model implemented with the simulation software 27 r.avaflow, based on plausible assumptions of the model parameters. However, these simulation results do not allow to 28 conclude on which of the two scenarios is the more likely one. Future work will be directed towards the application of 29 a three-phase flow model (rock, ice, fluid) including phase transitions, in order to better represent the melting of glac- 30 ier ice, and a more appropriate consideration of deposition of debris flow material along the channel. 31 Keywords: Debris flow, Entrainment, High-mountain process chain, Rock avalanche, Two-phase flow model, 32 r.avaflow 33 https://doi.org/10.5194/nhess-2019-204 Preprint. Discussion started: 8 July 2019 c  Author(s) 2019. CC BY 4.0 License.