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Modelling of particle size segregation and its applications to geophysical problems

Thornton, A; Gray, JM; Kokelaar, P (Manchester)
Thursday 08 January 2009, 10:05-10:30

Seminar Room 1, Newton Institute


It is important to be able to predict the distance to which a hazardous natural flow (eg snow slab avalanches, debris-flows and pyroclastic flows) might travel, as this information is vital for accurate assesment of the risks posed by such events. In the high solids fraction regions of these flows the large particles commonly segregate to the surface, where they are transported to the margins to form bouldery flow fronts. In many natural flows these bouldery margins experience a much greater frictional force, leading to frontal instabilities. These instabilities create levees that channelise the flow, vastly increasing the run-out distance. A similar effect can be observed in dry granular experiments, which use a combination of small round and large rough particles. When this mixture is poured down an inclined plane, particle size segregation causes the large particles to accumulate near the margins. Being rougher, the large particles experience a greater friction force and this configuration (rougher material in front of smoother) can be unstable. This instability causes the uniform flow front to break up into a series of fingers. A recent model for particle size-segregation has been coupled, though a particle concentration dependent friction law, with existing avalance models. In this talk, numerical solutions of this coupled system are presented and compared to both large scale experiments carried out at the USGS flume and more controlled small scale laboratory experiments. These simulations and experiments show good agreement and the numerical model allows the whole of parameter space to be sampled in a systematic way. Additionally, the numerical model can be used to analyse the effect of individual assumptions, leading to a better understanding of the key mechanisms which drive the observed phenomena.


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