The mudflow disaster at Villa Santa Lucía in Chilean Patagonia understandings and insights derived from numerical simulation and postevent field surveys

摘要

The evaluation of potential landslides in mountain areas is a very complex process. Currently, event understanding is scarce due to information limitations. Identifying the whole chain of events is not a straightforward task, and the impacts of mass-wasting processes depend on the conditions downstream of the origin. In this paper, we present an example that illustrates the complexities in the evaluation of the chain of events that may lead to a natural disaster. On 16?December?2017, a landslide occurred in the Yelcho mountain range (southern Chile). In that event, 7?million?m3 of rocks and soil fell on the Yelcho glacier, depositing 2?million?m3 on the glacier terminal, and the rest continued downstream, triggering a mudflow that hit Villa Santa Lucía in ChileanPatagonia and killing 22?people. The complex event was anticipated in theregion by the National Geological and Mining Survey (Sernageomin in Spanish). However, the effects of the terrain characteristics along therun-out area were more significant than anticipated. In this work, weevaluate the conditions that enabled the mudflow that hit Villa Santa Lucía.We used the information generated by Sernageomin's professionals after themudflow. We carried out geotechnical tests to characterize the soil. Wesimulated the mudflow using two hydrodynamic programs (r.avaflow and Flo-2D)that can handle the rheology of the water–soil mixture. Our results indicate that the soil is classified as volcanic pumices. Thistype of soil can be susceptible to the collapse of the structure whensubjected to shearing (molding), flowing as a viscous liquid. From thenumerical modeling, we concluded that r.avaflow performs better than Flo-2D.The mudflow was satisfactorily simulated using a water content in themixture ranging from 30 % to 40 %, which would have required a source of about 3?million?m3 of water. Coupling the simulations and the soiltests that we performed, we estimated that in the area scoured by themudflow, there were probably around 2 800 000 m3 of water within the soil. Therefore, the conditions of the valley were crucial to enhancing the impacts of the landslide. This result is relevant because it highlights the importance of evaluating the complete chain of events to map hazards. We suggest that in future hazard mapping, geotechnical studies in combination with hydrodynamic simulation should be included, in particular when human lives are at risk.