I³MS - Gray Seminar
Prof. Dr. Nico Gray - Particle Size-Segregation and Rheology of Dense Granular Flows
, The University of Manchester, UK
Hazardous geophysical mass flows, such as snow avalanches, debris-flows and pyroclastic flows, often spontaneously develop large particle rich levees that channelize the flow and enhance their run-out. Large scale experiments with 10 cubic metres of water saturated sand and gravel flowing down the 80m USGS debris-flow flume indicate that a subtle segregation-mobility feedback effect is responsible for their formation. Within the flow large particles segregate to the faster moving near surface layers and are preferentially sheared towards the front. Here they may be over-run, re-segregated and recirculated, to create a coarse grained front that is more resistive to motion than the more mobile ?finer grained interior. As a result the large particles are shouldered to the side to create static levees that constrain the flow laterally. Simple models for particle segregation and the depth-averaged motion of granular avalanches are described and one of the first attempts is made to couple these two types of models together. This process proves to be non-trivial, yielding considerable complexity as well as pathologies that require additional physics to be included. Some of these difficulties can be overcome by incorporating a depth-averaged mu?(I)-rheology for granular flow into the model. However, the mu?(I)-rheology turns out to have regions of ill-posedness itself at high and low inertial numbers.