Full scale experiments to examine the role of deadwood on rockfall dynamics in forests

Abstract. Forests are rockfall-protective ecological infrastructures, as a significant amount of kinetic energy is absorbed during consecutive rock-tree impacts. Although many recent works have considered rock impacts with standing trees, the effect of lying deadwood in forests has not yet been considered thoroughly, either experimentally or numerically. Here, we present a complete examination of induced rockfall experiments on a forested area in three different management stages. The trilogy is conducted in a spruce forest stand (i) in its original state, (ii) after a logging operation with fresh, lying deadwood and (iii) after the removal of the deadwood. The tests allow us to directly quantify the effect of fresh deadwood on overall rockfall risk for the same forest (slope, species) under three different conditions. The study yields quantitative results on the barrier efficiency of the deadwood logs as only 3.6 % of the rocks surpass the deadwood section. The mean runout distance is reduced by 42 %. Conversely, the runout distance increases by 17 % when the cleared stand is compared to the original forest. These results quantitatively confirm the benefits of nature-based mitigation measures integrated into forestry practice and we show how modern rockfall codes can be extended to incorporate such complex, but realistic forest boundary conditions.