Abstract
In 1971-72 the topography and stand conditions were investigated on the slope at Mitsumata Village, Niigata Prefecture, where a great surface avalanche occurred on January 9th, 1918, and 158 human lives were lost.
1) The slope in question faces WSW and is on the leeward in the winter season. The average inclination near the head of the slope is 26 degrees on the windward side and about 40 degrees on the leeward side. Hence, snow cornices readily form at the slope head every winter.
2) The growth of trees is poor on the head and upper half of the slope, and trees are buried under the snow in the winter season. On the lower half of the slope tree growth is relatively good, the density attaining to 1000-4000/ha. The average tree diameter at breast height is 10-15 cm.
3) Over a period of 53 years from 1918 to 1971, 10 heavy snowfalls were recorded in which the amount of a continuous new snowfall (snow depth over 150 cm, water equivalent over 200 mm) was nearly the same as that at the time of the Mitsumata avalanche, but there are no records of extensive avalanche release since 1918.
4) There still seems to exist the danger of an avalanche at the present time since the stand conditions on the top and upper slope are almost identical with those in 1918 and the snow cornices form every winter. Therefore, a series of snow fences should be constructed at the head of the slope. to prevent cornice formation.
