https://doi.org/10.5194/nhess-2019-204 Preprint. Discussion started: 8 July 2019 c Author(s) 2019. CC BY 4.0 License. 1 Back-calculation of the 2017 Piz Cengalo-Bondo landslide cascade with 2 r.avaflow 3 Martin Mergili1,2, Michel Jaboyedoff3, José Pullarello3, Shiva P. Pudasaini4, 4 1 Institute of Applied Geology, University of Natural Resources and Life Sciences (BOKU), Peter-Jordan-Straße 82, 5 1190 Vienna, Austria 6 2 Geomorphological Systems and Risk Research, Department of Geography and Regional Research, University of Vi- 7 enna, Universitätsstraße 7, 1010 Vienna, Austria 8 3Institute of Earth Sciences, University of Lausanne, Quartier UNIL-Mouline, Bâtiment Géopolis, 1015 Lausanne, 9 Switzerland 10 4 Institute of Geosciences and Meteorology, Geophysics Section, University of Bonn, Meckenheimer Allee 176, 53115 11 Bonn, Germany 12 Correspondence to: M. Mergili (
[email protected]) 13 Abstract 14 In the morning of 23 August 2017, around 3 million m³ of granitoid rock broke off from the east face of Piz Cengalo, 15 SE Switzerland. The initial rock slide-rock fall entrained 0.6 million m³ of a glacier and continued as a rock(-ice) ava- 16 lanche, before evolving into a channelized debris flow that reached the village of Bondo at a distance of 6.5 km after a 17 couple of minutes. Subsequent debris flow surges followed in the next hours and days. The event resulted in eight 18 fatalities along its path and severely damaged Bondo. The most likely candidates for the water causing the transfor- 19 mation of the rock avalanche into a long-runout debris flow are the entrained glacier ice and water originating from 20 the debris beneath the rock avalanche.