Geophysical mass flow over complex micro-topography: from grain-scale mechanics to continuum modeling

Geophysical mass flows involve granular earth materials surging down natural slopes, one of the major threats to mountainous regions worldwide. Accurate modeling of geophysical mass flows requires closure relations both within the flow (rheology) and at the flow-substrate interface (boundary conditions). However, although recent years have seen significant advances in the modeling of granular flow rheology, our understanding of how flowing granular materials interact with the substrate remains largely elusive. Here, we focus on micro-topography, i.e., geometric base roughness that is about the same size as the grain size, and investigate its effects on the granular flow dynamics as well as the associated closure relations. To systematically vary the base roughness from smooth to rough, we generate the base using immobile particles with varying particle size and spatial arrangement in laboratory experiments (with particle image velocimetry for flow kinematics extraction) and discrete element method simulations. Two granular flow scenarios are considered, including steady-state flow down inclines and granular column collapse. In the first scenario, it is found that basal slip occurs when the base roughness is below a range of intermediate values and a general slip law connecting the slip velocity, the mean flow velocity, and the base roughness is developed. In the second, transient flow scenario, basal slip inevitably occurs even for very rough bases due to inertial effects and a transient basal slip law is proposed to correlate the slip velocity with local flow properties based on kinetic theory arguments. The basal slip laws developed in this work can be readily incorporated as a dynamic boundary condition in continuum modeling of granular flows. In future work, grain-scale mechanisms relevant to more realistic geophysical flows will be investigated, including the feedback effects of pore fluid pressure on the flow mobility during basal sliding and the role of irregular particle shapes in hydro-mechanical modeling of geophysical mass flows.