Real-size experiments and 3-D simulation of rockfall on forested and non-forested slopes

Only a few rockfall simulation models take into account the mitigating effect of existing forest cover. The objective of our study was to improve the generic rockfall simulation model RockyFor, which does take the effect of forest stands into account, thereby developing a clear method for quantifying and modelling slope surface characteristics based on quantitative field data. To obtain these data we carried out 218 real-size rockfall experiments on forested and non-forested sites on a mountain slope in the French Alps. On the basis of a polygon map representing different diameter classes of the material covering the slope, we determine the mean obstacle height (MOH) for each homogeneous unit at the experimental sites. We proposed an algorithm for calculating the tangential coefficient of restitution using the MOH. Comparing the simulated and observed data from the real-size rockfall experiments showed that the 3-D combined deterministic-probabilistic rockfall simulation model RockyFor accurately predicted rockfall events on a non-forested (Root Mean Square Error = 17%) and a forested site (Root Mean Square Error = 12%). We conclude that for further improvement of rockfall-forest simulation on different slope types more quantitative data is required on (1) the energy dissipative capacity of shrubs and bushes (e.g. in coppice stands), (2) the effect of the slope material, (3) the rock shape as well as the rock size, and (4) the tangential and normal coefficient of restitution. Based on the presented results we can state that the RockyFor model could contribute to better taking into account the mitigating effect of the existing forest cover when planning protective measures.