ABSTRACTAn extreme snow avalanche occurred on Mt. Nodanishoji, Japan, in 2021. This avalanche was the second largest ever documented in Japan and destroyed many trees and structures. Despite the ...increasing interest in avalanche disaster mitigation effects of forests, there are limited opportunities to obtain actual data sets on avalanches and forests. In this study, we reconstructed the avalanche velocity based on the cedar forest damage and simulated the avalanche flow using the dynamical run-out model Titan2D to reveal the avalanche movement and ascertain the braking effect of the forest on the avalanche. We successfully simulated the whole movement of the extreme avalanche with a horizontal runout distance of 2,800 m from the starting zone at an altitude of 1,700 m to the run-out zone considering the effect of the forest by increasing the frictional parameters in the model. Whereas in the case of no forest, the avalanche spread beyond its actual reach, considering the forest, the run-out distance decreased and was consistent with the observed reach. Comparing the results with and without forests, we ascertained the distinct braking effect of the forest on the extreme avalanche and quantify it in terms of a locally increased friction angle.
Two-thirds of the 111 active volcanoes in Japan are covered with snow for several months during winter and demonstrate high hazard and risk potentials associated with snow-related lahars during and ...after eruptions. On 23 January 2018, a sudden phreatic eruption occurred at the ski field on Kusatsu-Shirane (Mt. Motoshirane) volcano, Japan. This new vent eruption from the snow-clad pyroclastic cone required forecasting of future snow-related lahars and crisis hazards zonation of downslope areas including Kusatsu town, a popular tourist site for skiing and hot springs. In order to achieve a prompt hazard assessment for snow-related lahars, a multidisciplinary approach was carried out involving characterization of proximal tephra deposits, snow surveys, and numerical lahar flow simulations using the Titan2D model. To determine the input parameters for the flow model, the consideration of snow water equivalent (SWE) immediately after the eruption (on 29 January) and in the post-eruptive period (on 12 March), was significant. In the case of Kusatsu-Shirane volcano during the winter of 2018, linear relationships between altitude and SWE, obtained at different elevations, were used to estimate the snow volume around the new vents. Several scenarios incorporating snow and snowmelt (water), with or without the occurrence of a new eruption, were simulated for the prediction of future lahars. Three lahar scenarios were simulated, including A) rain-on-snow triggered, B) ice/snow slurry, and C) full snowmelt triggered by a new eruption, and indicated the flow paths (inundation areas) and travel distances. These were useful for lahar hazard zonation and identification of potential high-risk areas. Since the input parameters required for the Titan2D flow model can be relatively easily determined, the model was suitable for the 2018 eruption at Motoshirane where historical and geological lahar records are not available for calibration. The procedure used in the study will enable rapid lahar prediction and hazard zonation at snow-clad volcanoes. Further consideration for simulating a cohesive-type flow, which was predicted by the primary deposits containing large amounts of clay minerals and could not be expressed in the Titan2D flow model, is necessary.
We constructed and analyzed a database of vehicle-stranding events due to snow to investigate the occurrence characteristics of vehicle-stranding events caused by snowfall in Japan. The database was ...constructed by collecting data from road administrator's reports, online news reports, and reports posted on Twitter from public and media organizations. As a result, 423 events that occurred between January 1986 and April 2021 were compiled in the database. The characteristics of the events were analyzed by focusing on eight items, namely the yearly number of events, area of occurrence, road classification, month of occurrence, time of occurrence, cause of occurrence, type of vehicle that triggered the event, and scale of the event. The results show that vehicle-stranding events have been occurring increasingly in recent years; the Hokuriku region has the largest number of vehicle-stranding events, but there are events in 40 prefectures including non-snowy areas; many events occur during the daytime, while there are fewer events in the early morning and late evening; and most events are caused by vehicles getting stuck on compacted-snow road, but in Hokkaido, which is a cold region, most events are due to low visibility and snow drifts. Furthermore, it was found that most large-scale events after 2010 occurred on national expressways and national highways in the Hokuriku and Sanin regions and in non-snowy areas, and were triggered by large vehicles getting stuck on compacted-snow road.
We performed core drilling at the Yamunai-sawa perennial snow patch (785 m a.s.l.) on Mt. Rishiri, Hokkaido, on September 9-10, 2018. The obtained core was 993 cm long. An ice layer was found at a ...depth of 853-993 cm, which indicates that perennial snow patches nourished by snow avalanches can have a thick ice layer, even at low altitudes. To clarify the transformation process from firn to ice, we investigated the crystal grain size, pore structure and oxygen isotope ratio of the core, and we estimated the depth to which the cold wave penetrated in early winter and the density increase in water-saturated firn in the ablation period. We found that 5-12-cm-thick transparent ice layers are superimposed ice with larger crystal grains and lower oxygen isotope ratios than in adjacent layers. The air temperature in perennial snow patches nourished by snow avalanches at low altitudes is higher than that in perennial snow patches nourished by snow drift. This higher temperature disturbs the refreezing of water-saturated firn that is due to cold wave penetration in early winter. Instead, the thick ice layer at a depth of 853-978 cm was formed by densification of water-saturated firn within two years after snow deposition. Thus, we conclude that the dominant transformation process from firn to ice in the Yamunai-sawa perennial snow patch nourished by avalanches is the same as that in temperate glaciers.
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