Stem breakage of trees and energy dissipation during rockfall impacts

Abstract

The capacity of individual trees to dissipate the energy released by rockfalls has previously only been quantified based on data obtained from static tree-pulling tests or from dynamic impact tests on wood samples. We predicted that these data are not representative of the maximum amount of energy that can be dissipated by living trees during rockfall impacts. To test this prediction, we carried out rockfall experiments on a forested slope in the French Alps. To calculate the rock's energy before and after impact, rockfalls were filmed digitally. The recordings of nine impacts causing instantaneous breakage of Abies alba Mill. trees were analyzed in detail. An exponential relationship between stem diameter at breast height (DBH) and the maximum amount of energy a tree can dissipate was highly correlated for all of our experimental data. We applied this relationship to other tree species based on published fracture energies. The relationships obtained for Cedrus spp., Fagus sylvatica L. and Picea abies (L.) Karst. were significantly correlated with data from other dynamic impact tests in the field and with maximum bending moments obtained from tree-pulling experiments. Multiple linear regressions showed that impact height influences the energy that will be dissipated by an A. alba tree, particularly for trees with a DBH less than 15 cm. For trees with a DBH greater than 15 cm, the effect of impact height was minimal up to a height of 1 m. There was a strong relationship between the amount of energy dissipated by a tree and the horizontal distance between the impact center and the vertical central axis of the tree.