Predictability of long runout landslide motion: implications from granular flow mechanics

Quality of landslide motion prediction is directly linked to the understanding of the basic flow mechanisms. Although it is known that landslides are granular mass flows and granular flow mechanics is an established area of research, hypotheses on landslide motion are still based on simple geometrical relations and heuristic assumptions. New insights into the development of flow properties of high-speed, high-concentration granular flows are given by results of discrete particle simulations: rapid granular flows are self-organizing dynamic systems that are forced to develop a plastic body rheology. This behaviour must be described by a coefficient of internal friction μ_CM that refers to the center of mass of a flow. Coefficients of rapid granular flows of inelastic and rough particles, which are typical for common rock materials, do not vary significantly around μ _CM ≈0.45 that is definitively smaller than the friction coefficient of soil creep (≈0.6). The motion of the center of mass is superimposed by the spreading of the granular mass that is controlled by the same plastic body rheology. This combined motion is a scale-invariant self-similar process that depends only on the drop height of a landslide and its volume. This allows specification of implications that must be given special attention in the development of future models for landslide prediction.