Journal of the Japanese Society of Snow and Ice

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A large-scale dry slab avalanche occurred at the Makunosawa valley in Myoko on February 17,2008,which damaged many trees in the valley. We investigated the extent of damage to the artificial cedar forest. We also estimated the decrease in avalanche velocity in the forest. Our aim is to verify the role of forests in reducing the avalanche velocity. We found that the cedar trees were broken by the bending stress. The damage was most near the upper edge of the forest. The number of damaged trees decreased gradually in the forest. On the basis of these observation results, we estimated the avalanche velocity from the bending stress of the broken cedar trees. We considered two cases in the calculations: (1) only the suspension layer of avalanche flowing into the cedar forest and (2) two layers of avalanche, consisting of a suspension layer (density: kgm ) and a dense-flow layer (density:300 kgm), flowing into the forest. In result, the avalanche was estimated to flow into the cedar forest at the velocity of 26-31m s-1 with a 17m high suspension layer and a dense-flow layer less than 2m in height. The calculation result showed that the avalanche slowed down in the forest and stopped around 130m from the upper edge of the forest. This result is consistent with the observations.

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