Particle Size-Segregation and Spontaneous Levee Formation in Geophysical Granular Flows

Particle size-segregation can have an important feedback on the bulk flow of geophysical granular avalanches. As a polydisperse material travels downhill the larger particles rise to the surface, where they are preferentially sheared to the flow front. This coarse-rich region experiences a greater resistance to motion and the large particles are shouldered aside to form lateral levees. Wider flows may break down into a series of these lobate, ‘finger-like’ structures. In either case, the static leveed regions channelise the finer, more mobile interior, causing the resulting run-out distances to be significantly enhanced. Modelling segregation-mobility feedback effects is therefore crucial for hazard mitigation. A new class of depth-averaged continuum models is introduced that describes the transport of large particles as well as the granular rheology. The feedback arises from a basal friction law that is composition dependent, implying greater friction where there are more large particles. Numerical simulations are used to show the spontaneous formation of leveed fingers.