The basal friction coefficient of granular flows with and without excess pore pressure: implications for pyroclastic density currents, debris flows, rock and submarine avalanches.

Abstract:

Numerous large-scale geophysical flows propagate with low apparent basal friction coefficients but the source of such phenomenology is poorly known. Inasmuch as basal friction data from natural flows are nonexistent we use numerical methods to investigate the interaction of granular flows with their substrate. Here we investigate 3D monodisperse and polydisperse fluid-particle granular flow rheology and flow-substrate interaction using discrete element modelling and coarse-graining techniques. This combination allows us to calculate the continuum fields of solid fraction, velocity, shear stress and solid pressure and compare it with force measurements on the substrate. The rheological analysis shows the occurrence of athermal creep for inertial numbers <4x10-5 leading to friction coefficients below the tangent of the angle of repose. Furthermore, we describe the interdependence of the wall friction at the flow base with slip and force fluctuations and propose a scaling law for the wall friction of flows propagating on flat and rough substrate. Additionally, solid pressure reduction, for instance by fluid drag in flows with elevated pore fluid pressure, justifies the definition of effective wall and internal friction coefficients to capture the geophysical flow rheology and the forcing on its substrate. These results are fundamental to understand the dynamics of geophysical mass flows including pyroclastic currents, debris flows, rock and submarine avalanches