Particle densities, velocities and size distributions in large avalanches from impact-sensor measurements

Abstract

In winter 1998/99, high-frequency pressure measurements with 10 cm sensors mounted 1–19 m above ground were carried out in the upper run-out zone of the avalanche test site at Vallée de la Sionne, Switzerland. Two large dry-snow avalanches clearly revealed a three-layered structure, with surprisingly low pressures in the suspension (or powder-snow) layer. The height of the saltation layer varied between 1 and > 3 m. From the duration, impulse and frequency of single-particle impacts (observed in the saltation layer and intermittently in the dense flow),particle-size and velocity distribution functions as well as strongly varying saltation-layer densities were found. With improved methods for peak detection and correction for grazing impacts, pressure measurements will become premier tool for testing granular flow models.

Physical vulnerability of reinforced concrete buildings impacted by snow avalanches

“…Inside this layer, the pressure ranges from the pressure at the top of the dense part to the pressure at the base of the powder part. The retained profile is in accordance with the existing data (Schaer and Issler, 2001;Naaim-Bouvet, 2003;Gauer et al, 2007).…”

“…The approximation by a linear increase and decrease leads to suppose that the high frequencies of a real avalanche signal do not contain significant energy. Nowadays, experimental works are still in progress to measure and more accurately describe the avalanche signal in terms of energy transfer (Schaer and Issler, 2001;Thibert and Baroudi, 2010, etc.). The next step will be to load the structure with a real snow avalanche signal in order to increase the accuracy of the derived vulnerability curves.…”

“…In the case of the powder avalanches, strong fluctuations of pressure according to time are observed for in situ measurements at real scale (Norem, 1995;Schaer andIssler, 2001 Fig. 3 andRammer, 2001).…”

“…3. Time evolution of two pressure sensors lying at 0.9 m (plot 1) and 2.1 m (plot 2) above the snow cover level (Schaer and Issler, 2001).…”

Observations and modelling of snow avalanche entrainment

“…Typical release masses are of the order of several thousand tons. Since impact pressures can be as high as 500 kPa (Schaer and Issler, 2001), any direct avalanche measurements must be made at highly reinforced obstacles. Direct measurements are therefore possible at only a few selected points along the avalanche path.…”