The purpose of this study is improving and evaluating the numerical snow avalanche simulation model to investigate the flow shape, run-out distance, velocity, impact force and other properties. Model has been developed by applying the Moving Particle Simulation method (MPS method), which is one of the Lagrange-like analysis methods. Previous simulation developed by authors calculated the pressure field by implicit method called Semi-Implicit MPS method. On the other hand, the Explicit-MPS method which is applicable for high-speed calculations such as parallelization for large-scale analysis was introduced. Comparison with the data observed at the artificial avalanche experiments revealed the validity of the model; our model succeeded to reproduce the flow velocity and the run-out distance reasonably well. Further, the model was applied for two avalanche disaster cases in nature. Examination also proved that the model can be a useful tool to deepen our understanding of snow avalanches.

In this study, a numerical simulation model for snow avalanches is developed by improving a single phase fluid continuum body model of debris and mud flows. The developed model considers the development and decrescence processes of snow avalanches. The model was applied to the snow avalanches occurring at Mount Pinneshiri in February 2020 and at Nasu in March 2019 to discuss the reproducibility of the flow characteristics of the snow avalanches. Numerical simulation results show that the deposition area of the snow avalanche that occurred at Mount Pinneshiri was approximately 350 and 50 m in the flow and transverse directions, respectively, which was close to the observed snow deposition area. Further, the snow avalanche reached the burial point of casualty in 50 s after its occurrence, and the average speed of the snow avalanches on the slope was approximately 15 m/s. The flow depth deepened as it flowed down, exceeding 8 m in some areas. The time from the occurrence of the snow avalanche to its cessation was approximately 200 s. The snow avalanches occurring at Nasu were recorded using an interval camera, and the reproducibility of the avalanche propagation speed was examined using the photographs. Numerical simulation results showed that the avalanche reached the lower part of the slope in 14 s after the occurrence, similar to the observed snow avalanche and the reproducibility of the propagation speed of the snow avalanche was confirmed.

In a numerical model, the dynamics of an avalanche, such as the runout distance, flow velocity, and flow thickness, are calculated under the initial conditions of the model. These initial conditions are selected according to assumed distributions, which are derived from field observations or theoretically. These distributions are called uncertainties of the model input, and the model output depends on the input. An output calculated with an input based on the distribution reflects these uncertainties. The uncertainties in inputs are propagated in outputs through the numerical model, which is called uncertainty propagation. This propagation is utilized for hazard mapping while considering the uncertainties in output. A hazard map enables us to estimate the quantitative risk of an avalanche. In this study, three methods are employed to evaluate an uncertainty: Monte Carlo (MC), Latin hypercube sampling (LHS), and polynomial chaos quadrature (PCQ). We use the models to quantify the uncertainty of initial volume of an avalanche, and then draw resultant hazard maps using each method. A comparison of maps based on MC, LHS, and PCQ revealed that (i) PCQ had the best result in terms of computational cost and map quality, and (ii) a proper condition is required for PCQ to obtain the highest map quality. Here, the condition is N_P=N_Q, where N_P represents the order of polynomial chaos expansion and N_Q denotes Gaussian quadrature points.

Powder snow avalanches and pyroclastic density currents (PDCs) are particle-gas gravity currents with stratification of particle concentrations. This paper reviews an existing two-layer PDC model and discusses the applicability of two-layer PDC models to powder snow avalanches. In two-layer PDC models, the upper region of PDCs is modeled as a dilute turbulent suspension flow and the lower region is modeled as a dense fluidized granular flow. Considering the interactions between these flows (e.g., particle transfer from one flow to the other), the two-layer PDC models can evaluate the flowing and stopping of PDCs. In particular, to evaluate the run-out distance of the dilute current of PDCs (i.e., the process in which the hot dilute current becomes lighter than the ambient air and lifts off the ground), the effect of thermal expansion of the ambient air entrained into the current is taken into account. Many physical processes of PDCs are common to powder snow avalanches. Unlike for PDCs, however, the run-out distance of the dilute current of the snow avalanches can be explained by the physical process of the fall-out of all particles in the dilute current due to decreased turbulent velocities in the flow. Incorporating this physical process into the two-layer PDC models may allow us to construct a unified model for powder snow avalanches and PDCs.

In the winter of 2020/21, the southern part of Sorachi, Hokkaido, experienced heavy snowfall. Iwamizawa recorded the snow depth of 205 cm which is the second−highest following 208 cm observed in 2011/12. We analyzed the structure of the snow layer and the factors thereof during the heavy snowfall in Iwamizawa, as well as its impact on the local economy and citizens' lives. We compared it with the results of future projections of heavy snowfall. Snow profile observations were conducted twice in the period immediately before snowmelt runoff. Compared to the winter of 2011/12, when snow was mostly lumpy, the snow in winter 2020/21 was affected by high temperatures and rainfall from mid−February onward. Furthermore, the snow texture in the latter was mainly rough with several aquifers and a high sleet content. The reason for the heavy snowfall was that a pattern of streaky clouds over the Japan Sea moved over the Iwamizawa area on many days due to westerly winds. In addition, in late February, meridionally elongated clouds to the west of northern Hokkaido merged with streaky clouds over Ishikari Bay and caused heavy snowfall when they reached the Iwamizawa area. The local economy and citizens' lives were greatly affected by the suspension of public transportation, accidents caused by falling snow, and collapsed houses. The population of Sorachi is aging rapidly, with the elderly accounting for 40 % of the total, and the risk of snow−related accidents is exceptionally high.