Avalanche dynamics – How avalanches move
Understanding the flow properties of avalanches is important for determining potential danger zones and planning protective measures. The nature of the snow and terrain are key factors.
The movement of an avalanche consisting of dry snow differs from that of an avalanche consisting of compacted, moist snow. How much destructive power an avalanche acquires as it moves and where along its path it comes to a standstill are critical factors in assessing the hazard and planning protective measures. To be able to answer these questions, we need to know how an avalanche slows down. The mechanisms that ultimately bring an avalanche to a standstill are complex and varied. Colliding blocks of snow and their fragmentation and reconsolidation, the melting of the sliding surface of the ground, airborne particles of snow from the snowpack, and the entrainment of air in the powder cloud all help to diffuse the kinetic energy of the avalanche generated by gravity. The nature of the terrain is also a key factor.
Flow behaviour experiments
To help ascertain which processes predominate in different types of snow, we conduct experiments using real avalanches. Our test site is located in the Vallée de la Sionne, at Arbaz in the canton of Valais. As an avalanche sweeps downhill, dozens of sensors measure its characteristics including velocity and impact pressure. To obtain a detailed picture of the snow flow, we combine measurements from the avalanche trajectory with modern remote-sensing techniques.
An historical perspective
In the past, avalanches were regarded as enormous snowballs that picked up everything that lay in their path. This view, which has since been shown to be false, was accepted for a long time. Not until 1863 did Swiss geologist Bernhard Studer conclude, "Nobody has ever seen avalanches, containing houses, trees and people, fly like small balls of soil through the air." The first simple rules for calculating runout distances and velocities along avalanche paths were developed in Russia in the 1930s and – following the extreme winters of 1950/51 and 1953/54 – in Switzerland.
The rules were based on the premise that the entire avalanche mass is concentrated in a single block that is slowed down by frictional processes on the ground and ultimately brought to a standstill. Although a gross simplification, this theory combined with expert knowledge gave rise to usable assessments of avalanche danger zones.
Flow behaviour and danger evaluation
Nowadays, however, we require more detailed knowledge of danger areas, for example to enable the expansion of building zones. Our RAMMS avalanche model provides more accurate assessments of the flow and deceleration behaviour of avalanches, because its interpretation incorporates the rules of fluid mechanics. In other words, it factors in the above-mentioned processes taking place inside the avalanche. RAMMS is used by engineers and planners as a tool for assessing avalanche danger and also provides important information about runout distances and flow velocities for use in the dimensioning of protective structures.
By way of RAMMS and instructions for dimensioning avalanche dams and snow sheds, we offer tools for practical avalanche protection. Our work is also relevant to natural hazards that are accompanied by similar processes, such as debris flows and landslides – RAMMS therefore offers modules for modelling these processes as well.