Cascading processes from the "Graetli" landslide - a case study of applied Integrated Risk Management in Gsteigwiler (Switzerland)

Located above the Lütschine Valley, the Grätli landslide endangers parts of the municipality of Gsteigwiler. Since 2021, in situ and remote sensing monitoring has shown frontal acceleration of an unstable rock mass of approximately 500,000 m³.

First, hazard analysis results were obtained using scenario-based process modelling, calculated with RAMMS:debrisflow and three-dimensional depth-resolved MPM simulations. The results indicate that the primary rock avalanche is expected to cause little to no damage to infrastructure. However, subsequent debris flows may impact buildings and critical infrastructure. The modelling results will be integrated into the existing hazard map, potentially affecting land-use planning decisions.

Second, a risk analysis revealed unacceptable risk levels for several properties as well as protection deficits affecting infrastructure. A safety concept involving evacuation following an initial rock avalanche could reduce the risk to an acceptable level. To address economic losses and infrastructure availability, options for structural protection measures are being evaluated in an ongoing study.

This natural hazard mitigation project, commissioned by the municipality, illustrates how the Swiss Integrated Risk Management (IRM) policy can be successfully applied as a framework to prevent major damage from cascading mass movements. Private-sector consultants and communal and cantonal authorities collaborate to address three key questions: (1) What can happen? in terms of hazard analysis; (2) What is allowed to happen? from a policy-based risk perspective; and (3) What needs to be done? by all stakeholders to mitigate unacceptable risks.