Quantifying the mechanical resistance of coppice trees against rockfall

Abstract

In mountainous areas, forests protect human lives and infrastructure from natural hazards such as rockfall and avalanches. Rockfall protection forests are known as cost efficient mitigation structures that substantially reduce the hazard. In recent years they have been increasingly considered in rockfall hazard assessments and for land use planning.

For a reliable assessment of the rockfall protective function of forests the use of numerical simulation models relying on detailed site specific input data is the most appropriate approach. The input data include information on the potential of trees to reduce the kinetic energy of impacting rocks. This key parameter, also known as the mechanical resistance of a tree, has been assessed for the predominant conifers of the Alps (Norway spruce and silver fir) and mature European beech trees in previous studies based on full-scale impact tests in forests.

In contrast, the mechanical resistance against rockfall of small diameter broadleaf coppice trees is hitherto unknown. Coppice forests are often situated upslope of transportation routes and settlements where they fulfill a protective function. In addition to discrete rock-tree interactions the rockfall protective function of coppice forests is enhanced by numerous tree aggregations (coppice clumps) where impacting rocks get trapped and subsequently prevented from continuing their trajectory.

The objective of this dissertation is to quantify the mechanical resistance of small diameter broadleaf trees against rockfall. The aim is to supply reliable data for rockfall simulation models and to acquire additional understanding of the rock-tree interactions that are substantially different from mature large diameter trees. The findings are also meant to provide a basis for better estimating the particular rockfall protective effect of coppice clumps.

The mechanical resistance of small diameter broadleaf trees (diameter at breast height (DBH) < 0.1 m) was assessed with full-scale dynamic impact tests. Two different test devices were used. The first was an open-air pendulum impact device where the root anchorage of the trees was replaced by a rigid restraint. The second test device was used in a coppice forest to impact trees that were part of coppice clumps.

All forest tests were done on beech (Fagus sylvatica L.). The pendulum tests included beech and for comparison also chestnut (Castanea sativa Mill.) and fir (Abies alba Mill.). The acquired data are suitable for being used as simulation model input data and they are more reliable than data of previous studies, because the impact process specific to small diameter trees (DBH < 0.1 m) was accounted for. It was found that the mechanical resistance increases exponentially with increasing tree diameter. For small diameter broadleaf trees the mechanical resistance is not determined by impact height at least for all impact heights between 0.2 and 1 m above ground. Impact tests showed that the impact angle influences the potential of trees to reduce the kinetic energy of impacting rocks. The natural root anchorage and the transition zone from the roots to the stems of coppice trees proved to sustain the stresses induced by the impacts. Hence, because these tree parts do not constitute predetermined breaking points, the tests indicate that coppice trees in their natural environment are at least as resistant to dynamic impacts as those tested with the pendulum device.

Moreover, the results show that the impact process against small diameter trees (DBH < 0.1 m) is substantially different from larger diameter trees (DBH > 0.3 m). The high flexibility of the stems leads to relatively long lasting impact durations and only negligible damage at the point of impact on the stem. As a result, the mechanical resistance of the stems is partly determined by the impactor velocity and mass. This means that the mechanical resistance of a tree to dynamic impacts cannot be regarded as a constant material property. Consequently, the comparability of the acquired data as well as its application for the quantification of the rockfall protective function of forests is limited. This fact is certainly not restricted to small diameter stems. Therefore, it has to be evaluated in future studies whether it is necessary to consider impactor mass and velocity separately or if the implicated uncertainties of combining both impactor properties by referring to its momentum or kinetic energy are tolerable in rockfall hazard assessments.

Finally, this work constitutes a decisive step towards a more objective quantification of the rockfall protective function of coppice stands. In addition to detailed data and understanding of the impact processes and breakage behaviors of the trees the dissertation highlights key issues on which future research should focus to increase the reliability of rockfall hazard assessments.