Simulation of sliding deadwood logs in mountain forests: towards a quantitative hazard assessment

Deadwood is an integral component of mountain forests, supporting biodiversity and contributing to protection against gravitational hazards. However, under specific conditions, deadwood may itself become a hazard when mobilised and transported downslope. Although sliding logs have been repeatedly observed in steep forests, a quantitative framework to assess their hazard potential has so far been lacking. We present a physics-based model to simulate the motion and runout of sliding deadwood logs in complex terrain. The model extends an existing rockfall simulation framework based on nonsmooth rigid-body dynamics with hard contact laws and Coulomb friction, explicitly representing deadwood log geometries and interactions with terrain, standing trees, and protective structures. Model calibration and evaluation are performed using two recent Swiss case studies in which deadwood logs up to 35 m in length travelled several hundred metres in a single rapid descent and impacted infrastructure. Simulations indicate that sliding deadwood hazard is favoured by very steep slopes > 35°, wet surface conditions, and a narrow decay-stage window characterised by the loss of bark and branches to reduce sliding friction while still retaining sufficient structural strength. Sliding trajectories are strongly controlled by micro-topography, with preferential paths along gullies, while standing trees limit downslope propagation but increase lateral spread through repeated deflections. The proposed model highlights the importance of adaptive forest management in mountain forests and provides a quantitative basis for optimising the balance between the protective and hazardous roles of deadwood.