La dynamique des avalanches d'après les résultats des expériences de déclenchement artificiel au Lautaret

Abstract

Results of tests carried out at an experimental site on the Lautaret Pass during the 1974/75 snow season are described. Two avalanche corridors are fitted with instrumentation ; one starts at an altitude of 2,320 m and ends 90 m below, and the other drops 210 m from an altitude of 2,340 m. Snow depths vary from 0.7 m to 2.8 m. The avalanches are set off by explosives positioned by a conveyor cable and fired by a special safety unit designed by Laboratoire A.S.P. Avalanches were triggered in powder snow on the 22nd and 31st January 1975 and in powder and sheet snow on the 12th March 1975. Test measurements are as follows : 1) Particle velocities in the avalanche, using hydraulic currentmeters and cinematographic equipment. Average velocity is around 7 m/s and peak velocities up to 16 - 18 m/s. 2) Impact pressure, using strain-gauges. Peak values are recorded behind the avalanche front, ranging from 0.6 bar to 1.3 bar. 3) Specific mass of the flowing snow, by gamma attenuation and comparison with gravimetric measurement data obtained before and after the avalanche. The coefficient of increase is approximately 1.3. Measured data for various depths are shown on tables. 4) Specific mass of the aerosol, by gamma attenuation (cesium 137, activity 37 mCi, horizontal distance 2.24 m) at 5m above the bottom. However, the proportion of aerosols in the avalanche is too small for the measurements to carry significant weight. 5) Snow temperature in the avalanche, using platinum temperature probes. Temperature rises of 1.6°C, 1.4°C and 1.0°C have been recorded. Snow temperatures at various depths are listed on tables. 6) Aerosol velocity, using a standard French Meteorological Service Type W106 propeller anemometer. 7) Avalanche front velocity, depth and size of the avalanche. These are determined by photography at 4 frames/second with reference to appropriate markers along the corridor. With this equipment, an avalanche can be observed over much of its length. 8) Electrostatic charges carried by the avalanche. Attempts have been made to measure these with a Rotschild electrometer a few metres from the edge of the avalanche, but so far without significant results. Data obtained for twelve avalanches in 1973, 1974 and 1975 are listed in a general table.