Dr. Hervé Vicari

Liquefaction of saturated erodible bed sediments is often indicated as one of the main drivers for the bulking of geophysical mass flows and their extensive mobility, but this is especially difficult to capture in numerical models. In this work, we employ a two-phase depth-resolved MPM–CFD model to simulate a granular column collapsing over a water-saturated erodible bed. The solid phase is modeled using MPM, coupled with CFD to capture the behavior of the fluid phase, with a drag term accounting for the interactions between the two phases, regulated by the permeability of the medium. The model incorporates an elasto-plastic framework, utilizing a dilatant Mohr–Coulomb constitutive model based on Terzaghi's effective stress principle. The two-phase approach allows the emergence of pore pressures, liquefaction, erosion, and flow runout within the simulations. The model can naturally simulate the transition of the initially solid sediments into a liquid-like state when overridden by debris material. Through our numerical experiments, we quantify the effects of the initial dilatancy and permeability of the water-saturated bed on the runout dynamics and erosion. Contractive beds with lower permeability exhibit larger and persistent pore pressure buildup and erosion, resulting in longer runout compared to dilative beds, consistent with field and experimental observations. These findings emphasize the need to account for the hydro-mechanical properties of flow and erodible bed materials in predictive modeling of landslide hazards, moving beyond empirical formulations often used in depth-averaged models.

In mountainous regions, snow avalanches pose significant threats to both populations and infrastructure. As the flow progresses, it can accumulate additional mass by entraining bed material along its path, thereby amplifying its destructive potential. Volume increases of over tenfold the initial volume have been documented from avalanche observation and measurements. Entrainment is usually accounted for in depth-averaged models through a source term in the mass conservation equation and simplified entrainment criteria. However, quantitative assessment of erosion/entrainment rates with various flow types and bed material properties has seldomly been explored under well-controlled conditions. In this work, we investigate avalanche dynamics as well as erosion/entrainment mechanisms on the basis of depth-resolved particle-based simulations. Through such a modeling approach, processes like frontal ploughing, basal abrasion as well as erosion-deposition waves naturally emerge. Our findings highlight that, the interaction of the flow with the initially static bed material may have multiple and diverse effects on the flow behavior. First, the bed material can be eroded, but only partially entrained in the flow. Second, while the flow mobility is generally reduced by the presence of the bed, it may be enhanced in some cases due to the mechanical weakening of the bed. Finally, we propose a relationship linking the entrainment rate to a dimensionless parameter that compares the flow-induced stress to the bed shear resistance. This work contributes to a better understanding of the dynamics of snow avalanches and entrainment process and can lead to a refinement of depth-averaged formulations for practical purposes. Future research should consider factors such as rate-dependent snow rheology, air pore pressure and bed fluidization mechanisms to further refine our understanding and predictive capabilities in avalanche modeling.

For snow-avalanche hazard mapping, one needs efficient tools that nevertheless capture the essential physical processes. The code MoT-PSA (Method of Transport – Powder Snow Avalanche) described here is based on the two-layer depth-averaged formulation for mixed snow avalanches developed by Eglit and co-workers in the 1980s but is extended to 3-D terrain and uses a fast numerical scheme based on the method of transport. Compared to previous works, we introduce novel formulations for the suspension and deposition of snow from the dense core. Snow cover and air entrainment are quantified with physics-based models. A sensitivity study of the model parameters on an idealized topography shows that both the dense core and the parameters of the powder snow cloud (PSC) governing particle suspension and settling significantly affect the dynamics. As expected, we observe that snow cover entrainment favours the formation of large PSCs with long runout. The powder-snow avalanche that occurred in Lom (Norway) on 27 February 2020 is back-calculated using MoT-PSA. With plausible parameter values, the model reproduces the dense core stopping at the gully's base and the dilute PSC travelling across the frozen lake for almost 1 km.

Skiing or snowboarding down a mountain covered with fluffy new snow can provide a unique experience,  eminiscent of floating above clouds. This sensation is often intuitively attributed to the supporting effect of the interstitial air loaded by the skis or snowboard, creating a feeling of weightlessness. Interestingly, this same principle may also account for the rapid descent and extensive runout distance of much less pleasant phenomena, such as powerful mixed snow avalanches. These avalanches exhibit a complex structure, characterized by a dense basal regime overlaid and preceded by an intermittent layer of coherent snow clusters, along with a lighter suspension layer of fine particles mixed with air. Despite their significance, the  processes driving the formation of these structures and the reasons for the remarkable mobility of mixed snow avalanches remain incompletely understood. One prevailing theory proposes that this mobility could result from the fluidization of the porous snow cover, which is transiently weakened by the development of excess pore air pressures upon undrained loading by the avalanche. We test this hypothesis, by means of two-phase simulations. The solid ice grains are simulated using either the Discrete Element Method (DEM) or as a continuum through the Material Point Method (MPM). Compared to previous works, we simulate explicitly the air both within the pores and in the ambient, using CFD (specifically the Finite Volume Method) which is coupled to the conservation equations of the solid phase. Simple preliminary simulations are presented in this work. We simulate a snowboarder dropping over a fresh snow cover. The simulations show that, if the permeability of the snow cover is low enough, the pressurized pore air may lead to fluidization of the snow, favoring its mechanical weakening and suspension as a vertical jet. Instead, if the permeability of the snow is too high, pore air pressure quickly diffuses, which cannot generate significant suspension of the particles. Similar modelling approaches will be applied in future to model mixed snow avalanches interacting with an erodible snow layer. Ultimately, we expect this research to contribute to improved entrainment and fluidization models in depth-averaged numerical methods.

The mechanisms of debris flows and their interaction with mitigation structures are still not well understood. Among the research challenges, only few entrainment measurements are available in literature, as entrainment is often masked by deposition on top. In this paper, we present a simple, cheap, and effective method to measure the entrainment depths. Flume experiments have therefore been performed to assess the influence of the initial debris flow volume and of an upstream flexible barrier on entrainment. To better understand the debris flow dynamics, the flow basal stresses have been measured. A high degree of liquefaction at the base of the debris flow is observed. A mitigation measure to reduce entrainment has also been studied. A compact flexible barrier was installed in the upstream part of the channel and is observed to deflect the flow along a curvilinear path. High normal stresses are measured at the base of the overflow, which are caused by the additional centrifugal stresses from the overflow. The results from the flume tests suggest that the flow interaction with an upstream flexible barrier may significantly influence the debris flow dynamics both upstream and downstream of the barrier.