2011 Interdisciplinary Summer School: Granular Flows

Particle Segregation in Granular Avalanches

Nico Gray, University of Manchester

Hazardous natural flows such as snow avalanches, debris-flows, lahars and pyroclastic flows are part of a much wider class of granular avalanches, that frequently occur in industrial processes and in our kitchens! Granular avalanches are very efficient at sorting particles by size, with the larger ones rising towards the free surface and the smaller ones percolating down to the base, to create inversely-graded layers. The surface layers move faster than the base, and the larger particles are therefore preferentially transported towards the flow front. Here they may be overrun, but can rise up again, by particle size segregation, into the higher faster moving layers and be recirculated. This creates bouldery flow fronts, which are common features of many geophysical mass flows. Larger less mobile particles accumulate at the flow front, and can lead to an instability in which the large grains are spontaneously shouldered aside into lateral levees, that channelize the flow and significantly enhance the total run-out. Broad fronts can also destabilize into lobes and clefts that then develop into a series of leveed fingers. We shall show the latest results of small scale laboratory experiments, large scale experiments on the 95 metre long United States Geological Survey (USGS) debris flow flume and the first attempts to develop theories for particle size segregation, large particle transport and segregation-mobility feedback effects in granular avalanches."