Comparison of Powder snow avalanche simulation Models (RAMMS and SamosAT) based on reference events in Switzerland

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17574BU

200204855

ABSTRACT: In this paper, we compare two advanced avalanche dynamics simulation tools: SamosAT and RAMMS. Both tools apply similar depth-averaged approaches to model the avalanche core, including cohesive extensions to the classical Voellmy flow rheology. However, they apply vastly different approaches to model the formation and motion of the powder cloud. The extended RAMMS model considers streamwise variations of avalanche flow density that are coupled to the statistical configuration of the snow particles. Expansions and contractions of the particle configurations define the mass and energy fluxes driving the formation of the powder cloud. SamosAT assumes mass transfer into the powder cloud proportional to the shear stress acting on the dense flow surface and models the powder cloud as a three-dimensional, twophase flow of ice-particles and air. RAMMS adopts a two-phase depth-average approach for the powder cloud leading to dramatic reduction in calculation times. However, information over the height of the cloud is lost. In RAMMS snow entrainment is considered as a collisional-thermomechanical process that amplifies random mechanical energy fluxes that enhance the configurational energy of the core and therefore the formation and strength of the powder cloud. Also in SamosAT entrainment can be included in various ways through the dense flow layer, or directly into the powder cloud, taking into account threshold values of particle-Froude numbers. However, in the standard operational setting entrainment is currently not activated. We apply both simulation tools to three well-documented reference events from Switzerland. We compare not only the predicted process area, but also peak velocities and impact pressures. Both simularities and differences in the simulation results exist.Keywords: powder snow avalanche, SamosAT, RAMMS, snowcover, entrainment, case studies.