Segregation in geophysical mass flows

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Many people remember the churning debris flow finding its path downhill after the eruption of Mt. St. Helens in 1980 or reports about another snow avalanche that destroyed half a village and killed 20 people (Flateyri, Iceland, 1995). Perhaps less well-known is the pyroclastic flow that killed a group of scientists and journalists from a ``safe'' ridge (Mount Unzen, Japan, 1991) or the historic evidence and future predictions about underwater slides that cause tsunamis to travel many hundreds of kilometers, possibly even reaching the eastern coast of the USA (Cumbre Vieja volcano, Canary Islands). These geophysical flows are so powerful and destructive because of the continuous interaction between the fluid, solid and gas phases. Segregation within these geophysical mass flows is the driving force of inhomogeneity and results in enhanced mobility of the flow. Geophysical mass flows form a significant hazard for communities and directly influence the environment, infrastructure, economy and tourism of a region. A fundamental and thorough understanding of the influence of segregation on flow dynamics is therefore necessary to mitigate damage and loss of life. The continuous interaction between the fluid, solid and gas phases in geophysical mass flows creates a complicated material that is hard to characterize. As the bulk of the material is composed of particles with different sizes and densities, the material naturally segregates. This natural sieving of the components results in inhomogeneity between and within the phases and directly influences the flow characteristics. A back-to-basic problem involving segregation in geophysical mass flows is the transport of a dry granular material down an incline under the action of gravity. The fluid component is absent and the gas phase is air, but similar physical principles apply with regards to segregation of the solid components. The proposed work aims to understand the segregation in a dry granular material, providing the core physics for segregation in geophysical mass flows. We don't have to travel far to find segregation in our own kitchen. Shaking a box of cereal at home is a good illustration of the so-called 'Brazil nut effect' --- the larger particles end up at the surface while the smaller particles sink to the bottom. Intuitively one may think that this (size) sieving, or segregation, is simply due to the smaller particles falling into the holes between the larger particles. This explanation is qualitatively correct, but is has proved extremely difficult to construct a mathematical theory that can accurately predict the speed and degree of segregation. Segregation also occurs for same size but different density particles, as the denser particles move to the bottom of the mixture. Recent work on the development of phenomenological models encounters over and over again limitations on validation because of the meager amount of experimental data. Now is an excellent time to extend the amount of experimental data on segregation and use improved modern instrumentation to quantify the speed and degree of segregation within these flows. This research proposal describes a framework to perform laboratory experiments on dry granular mixtures of beads where the density, size and initial configuration are varied so as to obtain a large experimental database of segregation outcomes. The excellent experimental facilities and the expertise in numerical modeling of granular flows at DAMTP combined with the strong research background of the applicant will ensure the successful completion of the research project. The strength of the proposal is the combination of new cutting-edge laboratory experiments with simulations of segregation mechanisms in a numerical code. The multidisciplinary nature of this proposal is complemented by a theoretical modeling effort and an experimental field component resulting in reality-based models that may be used by scientists and policymakers alike.