Full-scale avalanche experiments at Ryggfonn, Norway and studies on the dynamics of powder-snow avalanches

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Among two types of snow avalanches the release and dynamic characteristics of wet snow avalanches are rather simple, but those of dry snow avalanches, especially of powder-snow avalanches, are complex and more detailed information is required. The present study was initiated in aiming to make various observations about internal structures and dynamic properties of powder-snow avalanches.Natural snow avalanches were observed at Shiai-dani in Kurobe Canyon by taking video-films and installing various sensors including impact pressure sensors and ultrasonic anemometer. One of the findings was the record of avalanche winds detected by a ultrasonic anemometer. The avalanche wind was observed to precede the avalanche body.Snow avalanches were artificially released at Ryggfon site in Norway. So far three avalanches were observed ; in one avalanche the mean running speed was 22 m/s, the volume and maximum depth of the debris were 20, 000 m^3 and 5 m respectively. The observation of size distribution of the constituent snow blocks showed that the larger ones tended to be accumulated at the front part of avalanches.Laboratory experiments of snow avalanches were also carried out by flowing numerous ice spheres (diameter 2.9 mm) in a 5-m long chute at temperatures from 0 to -30ﾟC. Measurements of velocities and densities of ice spheres showed significant inhomogeneity in the internal structure vertically as well as horizontally.