Dangerous mud: debris flows

We research debris flows in various ways: we observe their behaviour via automatic monitoring stations, trigger experimental debris flows in the laboratory, and simulate their courses and spread using a computer.

Understanding processes through field observation

Eine Simulation mit RAMMS::DEBRISFLOW zeigt auf einer Landkarte die möglichen Fliess- und Ausbruchswege für ein grösseres Volumen. — In 2013, the Bielzug near Herbriggen, Gde. St. Niklaus (VS) was very active. The simulations with RAMMS::DEBRISFLOW showed potential flow and break out paths for a larger volumne. (GIF: WSL)

We have set up multiple automatic debris flow monitoring stations in the Swiss Alps which provide us with important data sets for describing flow processes and for developing and calibrating numerical simulation programmes. Various measuring systems are used and tested at the stations.

An automatic debris flow monitoring station is currently in operation at the Illgraben. Other stations were installed in previous years and have also provided key data sets: Dorfbach, Schipfenbach, Preonzo, Riascio.

Debris flows occur in the Illgraben several times a year. This makes the site interesting for research. WSL has instrumented the channel with technical installations and observes the events in detail. It has also implemented a prototype system for a debris flow alarm system there.

Ein Murgang kommt den Illgraben hinunter. — Thanks to the geological structure of Illgraben, several of these events occur each year, which makes it an ideal place to study debris flows. Photo: Christoph Graf/WSL

Blick von oben auf den Illgraben bis hinunter ins Rhonetal — When it rains, the yellowish loose rock material mixes with water and becomes a slushy mass that slides into the valley as a debris flow. It also carries larger pieces of rock. Photo: Christoph Graf/WSL

Laboratory studies

At WSL's large-scale laboratory we can study the trigger, transit and deposition behaviour of debris flows on a smaller scale. In a debris flow channel, we investigate the flow behaviour of different debris flow mixtures and test measuring instruments before they are used in the field. The debris flow channel (see video "Das Ding", Diagonal 2/2017) can also be used to demonstrate the characteristic features of debris flows to visiting groups.

The debris flow laboratory at the WSL offers the possibility to investigate debris flows at a small scale. This allows scientists to study, for example, flow behavior for various natural materials, investigate debris flow impact, explore the erosion process, and test instruments before deployment in the field. The channel is also used to illustrate the debris flow process for visitors.

Application and development of simulation programmes

Numerical simulation programmes enable us to make reliable calculations of the flow behaviour of debris flows in the torrent on a debris-flow fan, which but also to identify critical conditions for overloading the torrent and indicate possible flow paths and at-risk areas outside of the torrent channel. We explore the limitations and possibilities of simulation programmes based on exemplary ample applications and devise recommendations and guidelines for their use in hazard analysis. This work is carried out within the RAMMS Rapid Mass Movements project.

Contribution of research to debris flow protection

The findings from new field data and observations, laboratory experiments and calculations with numerical simulation programmes expand the knowledge of the complex and still insufficiently known phenomenon of debris flows. In addition, they contribute to the improvement of methods and working tools for practical use in the assessment of natural hazards and form the basis for the development and assessment of protective measures.