OVERVIEW OF THE 1999 MEASUREMENTS AT THE SLF TEST-SITE VALLEE DE LA SIONNE

ABSTRACT: Three large avalanches were artificially triggered on January 30th, February 10th and February 25th 1999, at the Vallee de la Sionne test site of the Swiss Federal Institut for Snow and Avalanche Research (SLF). This paper presents an overview of the recorded measurements and discusses the quality of each measurement method. Two automatic stations measured the weather and· snow conditions continuously at the site during the whole winter. Avalanche velocities were measured using Doppler radar that were positioned in a bunker on the foot of the opposing slope. In addition, video recordings were taken from different observation points. Along the avalanche track, three pairs of FMCW radar were buried in the ground to measure flow depths and entrainment/deposition rates. Pressure measurements were taken on both, a 21 m high, oval-shaped pylon and a wedge in the lower part of the avalanche track. Snow volume were measured with photogrammetric methods. The size of the avalanches increased chronologically. The last avalanche of February 25th had a width of more than one kilometre and a mean fracture depth of about 2.1 m. This avalanche burried the shelter on the foot at the opposite side of the avalanche with more than 4m of snow. The overall deposition volume of all three avalanches was more than 1'300'OOOm3. Some of the obstacl~s were destroyed, since the avalanches were much larger than assumed design loads