豪雪と雪崩災害

Abstract

Heavy snow areas in Japan are located in the central part of the Honshyu Island along the Japan sea, where is warm compared to the Hokkaido and the north part of the Tohoku district. In this report, the disastrous avalanches during the heavy snow fall winters, lately occured in succession in Niigata prefecture, are discussed.The Japanese Islands elongate from north to south, and amount of snowfall and its properties, such as snow are crystals and density is different depending on latitude. Therefore, avalanche release, one of the snow phenomena, is different in time and location. For full-depth avalanches, for example, one peak was found in April in Hokkaido. In Niigata the maximum was in February.The types of disastrous avalanche were dry full-depth slab avalanches and/or dry surface slab avalanche. In high latitude and cold regions, the avalanche release mechanism is weakening of supporing force caused by depth hoar in the middle or bottom layer. However, in Niigata prefecture it was the increase of driving force caused by heavy snowfall.For large scale surface avalanches the weak layer was not depth hoar but a thin wet granular snow sandwiched between dry snow layers. If the wet snow freezes into a dry snow, its strength increased and the surface avalanche could not released on such a slope. Comparisons of simulations of internal frozen in field moving observations revealed that the wet granular snow can be freezed at some situations.Avalanches denude and take in the snow in the steep slope, and sedimentation starts at the gentle slope. The zone of deposit has been morphlogically defined as the zone where snow debris exists. Here, using the mass balance of denudation and sedimentation, a kinematic definition for the zone of deposit was proposed. From observation, for fulldepth avalanches, length of the zone of deposit by both definition was approximately equal. On the other hand, for surface avalanches, the length by the kinematic definition was longer than the morphological one.If the avalanche starts from the same staring zone, the avalanche track in heavy snow winter is longer than the normal year, and track itself is also different. Therefore, catastorophic avalanche disasters occur in the heavy snowfall winter.