Landslides commonly occur on earth's surface, in a wide range from subaerial landscapes to submarine oceanic regions. These events can trigger the generation of hazardous tsunamis that can have major consequences such as overtopping of dams and reservoirs, and coastal flooding and erosion. Globally, tsunamis have caused more than 250,000 fatalities over the past 30 years, triggered by landslides, earthquakes, and other sources combined (Gusiakov et al., 2019), and for this reason they are one of the primary natural hazards to coastal communities and infrastructure. The triggering conditions of tsunamis are often associated with large surface deformations caused by seismic events (

A granular front emerges whenever the free-surface flow of a concentrated suspension spontaneously alters its internal structure, exhibiting a higher concentration of particles close to its front. This is a common and yet unexplained phenomenon, which is usually believed to be the result of fluid convection in combination with particle size segregation. However, suspensions composed of uniformly sized particles also develop a granular front. Within a large rotating drum, a stationary recirculating avalanche is generated. The flowing material is a mixture of a viscoplastic fluid obtained from a kaolin-water dispersion with spherical ceramic particles denser than the fluid. The goal is to mimic the composition of many common granular-fluid materials, such as fresh concrete or debris flow. In these materials, granular and fluid phases have the natural tendency to separate due to particle settling. However, through the shearing caused by the rotation of the drum, a reorganization of the phases is induced, leading to the formation of a granular front. By tuning the particle concentration and the drum velocity, it is possible to control this phenomenon. The setting is reproduced in a numerical environment, where the fluid is solved by a lattice-Boltzmann method, and the particles are explicitly represented using the discrete element method. The simulations confirm the findings of the experiments, and provide insight into the internal mechanisms. Comparing the time scale of particle settling with the one of particle recirculation, a nondimensional number is defined, and is found to be effective in predicting the formation of a granular front.

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The granular column collapse is a simplified version of granular flows such as landslides, avalanches, and other industrial processes mobilized in air or within a fluid. In this configuration, the particles collapse in an accelerating phase, reaching a state of constant spreading velocity until they decelerate and stop. Granular flows commonly involve particles of different sizes, a property termed polydispersity. Understanding the role of polydispersity remains a challenging task that is often analyzed with nearly monodisperse systems and demanding a series of simplifications when coupled with a fluid in a numerical model. Here, we study the effect of particle-size polydispersity in dry and immersed granular columns, using a finite element method-discrete element method model for fluid-particle interactions. We show that the velocity of the column collapse and runout distance decrease with an increase in the level of polydispersity in immersed conditions, and remain nearly independent of the level of polydispersity in dry conditions. Moreover, we find that the runout scales with the spreading front kinetic energy, weighted by the ratio between the particles' density and the density difference between particles and fluid. This scaling helps in identifying the governing processes in polydisperse granular columns, unifying the runout description of both dry and immersed collapses, and indicating that the column initial packing fraction is the governing parameter.

The use of the geotechnical centrifuge to obtain scaled physical models is a useful tool in geomechanics. When dealing with granular flows, however, the traditional scaling principles are challenged by the complex rheology of the material and by the non-trivial effects of the Coriolis apparent acceleration. In a laboratory centrifuge, obtaining a clear understanding of these effects is further complicated by the technical difficulties in obtaining flows in steady conditions. In this work, the scaling principles for granular flows are studied using a numerical model based on the discrete-element method. In this way it is possible to obtain a steady flow in a rotating reference frame, and to explore the variation of macroscopic properties by changing the scaling factor and the distance from the rotation centre. The outcome is compared with the prediction obtained with a continuum theory for frictional flows. Results show that granular flows scale consistently only when the Coriolis acceleration is negligible, and are severely altered otherwise. The augmented acceleration field is also responsible for an alteration of the flow state, driving the system towards the inertia-driven collisional regime.