Debris-flow dynamics: Understanding phase separation and wave formation

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Debris flows are natural hazards that threaten people and infrastructure in mountainous terrain. They are becoming more frequent due to the increasing number of high rainfall intensity events (Petely et al., 2012, Global patterns of loss of life from landslides. Geology 40, 927-930). The destructive potential of debris flows is strongly enhanced by the development of surge waves (Fig 3 + video), which concentrate the flow into large amplitude waves that often carry large rocks and boulders. Despite nearly forty years of research there is still a lack of understanding about why these waves form, as well as a dearth of good field data sets (in-situ measurements of active flow thickness and velocity) that can be used to test models. This severely limits the accuracy of debris-flow risk analysis, because current debris-flow models are not able to capture quantitatively these waves or the particle-size segregation that concentrates the largest particles in the deepest and fastest-flowing part of the flow. This proposal aims to make a major breakthrough in our ability to predict debris-flow surge wave formation by coupling (a) new innovations (hybrid LiDAR/HD video) in environmental sensing at the Illgraben debris-flow observation station in Switzerland with (b) recent theoretical advances in modelling nonlinear wave formation and particle-size segregation made in Manchester (Gray & Edwards 2014, Gray 2018, Rocha et al. 2019). The assembled research team is uniquely suited to address this question, as it combines a diverse range of experts who have a strong theoretical background as well as expertise in laboratory and field data collection. This will enable the team to integrate state-of-the-art in-situ data currently being collected at Illgraben (and improved on as part of this proposal) into cutting-edge theoretical and numerical models in order to generate a step change in our understanding of surge waves as well as new computational tools for use by the debris-flow community in hazard risk assessment.