Journal of the Japanese Society of Snow and Ice

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The geometries of avalanches, such as the inclination of the release or deposit zone, horizontal run-out distance (HL), and vertical height of avalanche drop (h), were investigated using aerial photographs taken during three snowmelt seasons in 1997, 2001 and 2002 around Mt.Naeba and Mt.Torikabuto located near the border between Niigata and Nagano Prefectures. From the aerial photographs, 1139 avalanches were identified, including 30 surface-layer avalanches and 1109 full-depth avalanches. The inclination of the release zone of surface-layer and full-depth avalanches ranged mostly from 40-45°. The frequency histograms of the direct angle of elevation from the outer end of the deposit zone to the highest point of the release zone of full-depth avalanches had a normal distribution with the highest frequency in the range of 38-40°. The horizontal run-out distance of surface-layer avalanches and full-depth avalanches averaged approximately 1.7 and 1.3 times larger than the vertical avalanche drop in regression analysis, respectively. The horizontal run-out distance of some avalanches reached beyond 1 km; however, no avalanche, except some surface-layer avalanches, showed the mobility of large-scale avalanches because the excessive travel distance (Le=HL−h/tan32°) was approximately 0m or less than 0m. The relation between avalanche frequency and magnitude (area of the release zone or horizontal run-out distance of the avalanche) exhibited a power-law relationship, and the power-law exponents (α) of avalanches that occurred in each of three winters were similar. However, the power-law exponents of the avalanches differed in each of three areas or by avalanche type.

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