Study of the extent of damage to a subalpine forest by large-scale snow avalanches and estimation of the avalanche velocity on Mt. Iwate, Japan during winter 2010-11

Abstract

Two large-scale avalanches occurred on the west-facing slope of Mt. Iwate during the winter of 2010-11, which damaged 7 ha of subalpine forest. Both were likely to be dry-slab avalanches, with starting zones higher than the tree line at around 1730m a.s.l., because they seem to have penetrated the forest at high speed. According to photographs and a snowpack model simulation, the avalanches were seemingly triggered by record snowfall of December 31 and occurred until January 6. We investigated the extent of damage to the forest in July 2012 with the aim of knowing the behavior of avalanche flowing in the subalpine forests. The result of the investigation on 85 Mariesʼ firs and nine Ermanʼ s birches in the avalanche path showed that the greater the altitude, the higher the rate of thin trunks, while the greater the altitude, the higher the rate of fractured trees and the lower the rate of uprooted trees for Mariesʼ firs. In the case of Ermanʼs birches, 8 of 9 were uprooted, probably due to their larger modulus of rupture and a higher rate of thick trunks. We estimated the avalanche velocity based on the bending stress of broken trees. Assuming two layers: a suspension layer (density: 3 kgm⁻³) and a dense-flow layer (300 kgm⁻³), the avalanche velocities in four investigation areas were estimated at minimum of 21〜42ms⁻¹，33〜53ms⁻¹，24〜39ms⁻¹and 22〜33ms⁻¹ in order of altitude with a 3msnow depth and a dense-flow depth h2 of 2m or 1m.