Combined effects of grain size, flow volume and channel width on geophysical flow mobility: 3-D discrete element modeling of dry and dense flows of angular rock fragments

We have carried out 3-D numerical simulations by using a discrete element method (DEM) to study the mobility of dry granular flows of angular rock fragments. These simulations are relevant for geophysical flows such as rock avalanches. The model is validated by previous laboratory experiments. We show that: 1) the finer the grain size, the larger the mobility of the center of mass of granular flows, 2) the smaller the flow volume, the larger the mobility of the center of mass of granular flows and 3) the wider the channel, the larger the mobility of the center of mass of granular flows. The grain size effect is due to the fact that finer grain size flows dissipate intrinsically less energy. This volume effect is the opposite of that experienced by the flow fronts. Six different channel cross sections are tested. We introduce here a new scaling parameter χ that has the product of grain size and the cubic root of flow volume at the numerator and the product of channel width and flow length at the denominator. The linear correlation between the reciprocal of mobility and parameter χ is statistically highly significant. Parameter χ implies that the mobility of the center of mass of granular flows is an increasing function of the ratio of the number of fragments per unit of flow mass to the total number of fragments in the flow. These are two characteristic numbers of particles whose effect on mobility is scale invariant.