Summary

Slopes where full-depth avalanches often occur have characteristic features which have not been described in detail. We define an “avalanche furrow” as a steep convergent furrow which on aerial photographs looks like a pattern scratched by a needle. We have researched morphological features of avalanche furrows and relation-ships between avalanches and avalanche furrows using photo interpretation and field survey of study areas in Hokuriku district, central Japan, and the distribution of avalanche furrows based on interpretation of 1:15 000 scale photos in the whole of Japan.

Avalanche furrows usually appear in groups of two to four. Each is composed of smoothly dissected rock.

Usually semicircular or U-shaped in section, they are 3–6 m wide and 2–3 m deep, and slightly concave in longitudinal section. Lengths range from several tens to several 100 m, and continue from near the ridge line to valley floor (Fig. 1).

Fig. 1. Terrestrial photograph of “avalanche furrow”.

Avalanche furrows appear on 76.8% of slopes where avalanches have occurred. Inversely, traces of avalanches appear in 90.2% of avalanche furrows. Distribution of inclination of avalanche furrows is centered at 35°-50°, while that of non-avalanche furrows where avalanches occurred is concentrated from 30° to 45°. Avalanche furrows are distributed in heavy snow regions of Japan, especially densely in Hokuriku district and the southern part of Tohoku district. They are good indicators of slopes where avalanches will easily occur.

“Avalanche chutes” (Reference ShimokawaShimokawa, 1980) appear in slopes where avalanches occur, formed by avalanches and distributed centrally in areas where avalanche furrows appear. We have not investigated their genesis, but we concluded from their distribution that they too are formed by avalanching snow.