Avalanche research in the Vallée de la Sionne ¶

The Vallée de la Sionne (VdlS) in the commune of Arbaz, Valais, Switzerland, is one of the world's most important test sites for real-scale avalanche research. Here, scientists, engineers and international partners investigate how avalanches behave under natural alpine conditions.

The research focuses on understanding how avalanches form, accelerate and flow down into the valley from their starting zones, as well as on their interaction with complex terrain and engineering structures along their path. These processes are studied at full scale under natural conditions that cannot be replicated in laboratory experiments.

The site allows experts to observe natural avalanches of various sizes and with different flow characteristics, ranging from relatively minor events to extremely large avalanches, including both wet snow and powder avalanches. At the same time, measurement systems record avalanche activity, resulting in unique long-term observational data. This enables variations between seasons, snow conditions and event types to be analysed.

Together, these measurements help build a detailed understanding of avalanche processes and create a solid foundation for numerical modelling and data-driven approaches, including advanced machine learning techniques applied to avalanche behaviour and forecasting.

At the VdlS avalanche test site, researchers investigate individual particles at the smallest spatial scale. Here, they can directly observe the movement of individual snow grains and clusters within the avalanche flow.

High-speed cameras mounted on a pylon offer a unique insight into what goes on inside avalanches, capturing processes within the dense core and the suspension cloud which cannot be resolved by large-scale measurement systems. These observations reveal particle collisions, turbulent movements and interactions between flowing and suspended snow, and help to link small-scale processes to the overall dynamics of avalanches and their impact behaviour.

Various remote sensing instruments monitor avalanche motion over the entire slope. Radar systems installed in a protected bunker continuously track how an avalanche develops. A Doppler radar measures the avalanche's velocity in a similar way to police speed radar systems, while GEODAR provides information about the inside of the avalanche, including shock waves, front dynamics and changes in flow characteristics.

Additional sensor systems are spread across the site, including seismic sensors, infrasound devices and distributed acoustic sensing (DAS) systems using fibre-optic cables laid in the ground. These systems use ground vibrations, air pressure waves and continuous ground strain signals to detect passing avalanches. They serve both as monitoring instruments and as sources of information on avalanche dynamics and flow characteristics.

Drones are used to map the area before and after avalanches, supplying detailed information on the snowpack, changes to the surface and the distribution of deposits.

Together, these systems capture the avalanche's motion from release to deposition, combining velocity measurements, information on flow structure and multi-scale observations on the ground and in the air.

A key feature of the VdlS avalanche test site is a series of large obstacles positioned directly in the avalanche path,  with various instruments installed on them. These are used to analyse the interaction between avalanches and obstacles in complex terrain and to characterise the flowing snow as it passes through the measurement sections. The data collected includes velocities, pressure, flow depth, density and temperature. Taken together, these observations provide a detailed description of the internal structure of the flow and its temporal and spatial evolution.

Impact pressure is one aspect of these measurements and shows a strong dependence on the flow regime. Wet snow and dry snow avalanches, for example, produce different pressure patterns that reflect their specific flow behaviour. Overall, the data enables a comprehensive characterisation of the flow conditions of avalanches and their interaction with the terrain, linking the internal flow structure to the external effects on obstacles.

Vallée de la Sionne avalanche test site: facts and figures

• Release zones between 2,500 and 2,700 metres above sea level, valley bottom at 1,450 metres above sea level
• 3 obstacles in the avalanche path: pylon, wedge and wall
• 185 sensors spread over a distance of 2 kilometres
• Fully automated triggering of the measurement infrastructure and data acquisition system
• Volume of data per event: 2 terabytes