Debris Flow event on Osorno volcano, Chile, during summer 2017: New interpretations for chain processes in the Southern Andes

Abstract. Debris flow generation on volcanic zones at the Southern Andes has not widely studied, despite the enormous economic and infrastructure damage that these events can generate. The present work contributes to the understanding of these dynamics based on a study of the 2017 Petrohué debris flow event from two complementary points of view. First, a comprehensive field survey allowed to delimitate that a rockfall initiated the debris-flow due to intense rainfall event. The rockfall lithology corresponds to lava blocks and autobrecciated lavas, predominantly over 1500 m.a.s.l. Second, the process was numerically modelled and constrained by in situ data collection and geomorphological mapping. The event was studied by back analysis using the height of flow measured in road CH-255 with errors of 5%. Debris flow volume has a high sensitivity with the initial water content in the block fall zone, ranging between 4.7x10⁵ up to 5.5x10⁵ m³, depending on the digital elevation model (DEM) used. Therefore, debris flow showed that the zone is controlled by the initial water content available previous to the block fall. Moreover, our field data suggest that future debris flows events can take place removing material from the volcanic edifice. We conclude that similar events could occur in the future and that it is necessary to increase the mapping of zones with autobrecciated lava close to the volcano summit. Finally, the study contributes to understanding debris flows in the Southern Andes since the Osorno volcano shares similar features with other stratovolcanoes in the region.