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At the scale of forest stands, there is a lack of quantitative, statistically valid data on the protective effect of forests against rockfall. Therefore, the first objective of this study was to quantify the velocities, rebound heights as well as the residual hazard of rockfall on a forested and a non-forested slope. The second objective was to evaluate existing rockfall protection forest management guidelines, as well as the underlying criteria. We carried out and analysed 100 real size rockfall experiments at a non-forested site (Site 1) and 102 identical experiments at a forested site (Site 2) on the same slope. We compared the obtained results with literature data on rockfall protection forests. At the non-forested site, results show that the mean maximum velocity is 15.4 m s1 compared to 11.7 m s1 at the forested site. The maximum rebound height decreases from 8 m (Site 1) to 2 m (Site 2) and the number of rocks that surpass the 223.5 m slope distance decreases from 95 out of 100 (Site 1) to 35 out of 102 (Site 2). A major effect of rockfall on a forested slope is the development of a treeless rockfall path or couloir, which had evolved after releasing 78 rocks at Site 2. The effect of such a couloir can be mitigated by cutting trees on both sides of the couloir and leaving the trunks on the slope, diagonally to the slope direction. This is a known and effective technique to reduce the effect of gaps in protection forests. During our experiments, none of the rocks attained their maximal velocity within the first 40 m. They did, however, attain destructive velocities (11–15 m s1) within that distance. Based on our observations, we propose a maximum gap size in the slope direction of 1.3 times the mean tree height, with a maximum of 40 m. Further, we present various findings that have direct implications for the management of rockfall protection forests. Finally, the results proved that forests can provide effective protection against rockfall.
Forest Ecology and Management 215 (2005) 183–195
Cemagref Grenoble, France
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