No abstract

AbstarctAccurate measurements of snow avalanche flows in forests are rare. To understand how forests can stop avalanches, we study a mixed flowing avalanche that stopped in a cedar (Cryptomeria japonica) forest in Japan. The large-scale dry slab avalanche occurred in the Makunosawa valley in Myoko on 17 February 2008 and damaged many trees. The site contains a geophone and other devices to detect avalanche activity. A nearby meteorological station provides weather information. Post-event observations were made of the scale of the avalanche and tree damage. These showed that the avalanche released from the east-southeast-facing slope at 1700 ma.s.l. Snow debris reached ~770ma.s.l; the horizontal runout distance was ~3000 m. The area of the runout zone was ~10 ha, and the snow water equivalent of debris was 400–1500 mm. The mass of the avalanche was roughly estimated at 5–10×107 kg (1–2×105m3). We used snowpack simulation models to estimate the height of the fracture zone. The simulations revealed that a faceted snow layer formed at 2 m and the stability index was 0.5 (poor) at the time of the avalanche release, due to heavy snowfall. We estimated the avalanche velocity from the bending stress of the broken cedar trees, in order to verify the effect of forests in reducing avalanche velocity. The avalanche was estimated to flow into the forest at a velocity of >26–31ms–1, with a 17 m high powder snow layer and a dense-flow layer <2m in height. It appears to have slowed down and stopped at ~130m after penetrating the upper edge of the forest.

No abstract

Irie et al.

Analysis of Windthrow Factors in Windbreaks: In the Case of Bibai, Hokkaido by Typhoon No.18 in 2004. J. Jpn. For. Soc. 91: 307 312. Windthrow factors in windbreaks in Bibai, Hokkaido, which was damaged by typhoon No.18 max wind speed 21 m/s in 2004, were analyzed. The windbreaks investigated were plantations of Picea abies, Larix leptolepis, Betula platyphylla, or Fraxinus mandshurica. Diameter at breast height DBH and the type of damage e.g., uprooted , snapped , or undamaged were determined for each tree in 21 quadrats. The vertical profile of soil hardness was also measured using a simplified dynamic cone penetrometer in each quadrat. The maximum moment of resistance against uprooting was measured by pulling the trunk of a standing tree laterally for each species to determine why the sensitivity to windthrow differed. The sensitivity to windthrow decreased in the order L. leptolepis P. abies B. platyphylla F. mandshurica. The sensitivity to windthrow had a bell-shaped distribution with a peak at DBH 30 cm. L. leptolepis and B. platyphylla were more likely to be uprooted , while P. abies was more likely to be snapped. Soil hardness was not closely related to the sensitivity to windthrow. The moment of resistance against uprooting was higher for F. mandshurica than for the other three species. The sensitivity to windthrow might be related to tree height and leaf mass, in addition to the moment of resistance against uprooting.