Effect of particle shape and initial orientation on the kinematics and runout behavior of rockfalls

Rockfalls are frequent geodisasters, characterized by gravity driven downslope movement of a solid mass of rock. The kinematics of rocks is heavily dependent on their shape, which leads to a competition between sliding, bouncing, and rolling, and thus to a large variation in runout distances. Numerical studies using simplified round particles cannot fully reproduce the complex motion resulting from the interaction with the terrain, while more realistic models so far had most of their application limited to case studies, thus systematic investigations into block shape are largely missing. In this study, we systematically investigate the effect of rock shape and initial orientation on the course and runout of rockfall events by means of a two-dimensional polygonal discrete element simulation with soft contacts. We analyze the runout behaviour and first impact on the ground for six different block geometries and 91 different initial orientations for each geometry. The obtained results quantify the effect of particle angularity and elongation and show that the kinematic and dynamic observables of rockfall events can not be expressed as an explicit function of the initial orientation but have to be treated in a statistical way. We find that the expectation values and standard deviations for the maximum runout distance and the normal restitution coefficient show a clear shape dependence and saturate with increasing particle roundness, while for the impact angle and tangential restitution coefficient, we notice stronger differences with respect to particle elongation.