Experimental quantification of deadwood influence on rockfall dynamics and its incorporation into regional-scale rockfall models
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Date
2022
Publication Type
Doctoral Thesis
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Abstract
Mountain forests provide an essential protective function against natural hazards.
Various studies predict higher tree mortality and susceptibility to natural disturbances such as windthrow, bark beetle infestation, and forest fires as a result of future climate and legacies of land-use history. Therefore the fundamental research question of this thesis arises: Can disturbed mountain forests with lying deadwood still protect against rockfall or will protective deficits occur? The novel development in experimental methodology and the subsequent adaption of the results within modern numerical methods to answer this guiding question is presented in this thesis resulting in three publications. They cover different spatial scales and address different relevant questions related to rockfall processes in mountain forests. The first paper breaks ground for the experimental procedure of a 1:1 rockfall tests through an experimental rockfall trilogy in the forest. It comprises the release of approximately medicine ball sized, sensor-equipped, cubic 45 kg rocks to a forested rockfall transit area. The forest service changed the predominant forest structure between experiments, resulting in different forest stages and experimental setups: (i) the original forest, (ii) the stage after a timber cut with lying deadwood left as obstacles and (iii) the cleared stage after the removal of the deadwood. The second paper deals with similar rockfall experiments but with larger rock masses ranging up to 3.2 t, varying rock shapes on a longer and naturally, windthrow-affected forested slope. With the rock masses, the experimental complexity also increased. A multi-camera setup was elaborated and each time installed anew to achieve video-footage coverage along the entire slope. Together with the in situ rock data and high-resolution lidar point clouds of the site, a reconstruction of 63 rock trajectories was possible. The intricate experimental sequence involves removing and re-installing the logs of the deadwood in order to allow comparable living forest states, allowing for the highest possible cross-comparability of forest states across different weight classes. The effect of the lying deadwood could be quantified in unprecedented detail. Both studies show that deadwood effectively protects against rockfall but to varying extent. The deadwood retention ratio of the first experiment was above 95 %. For larger rock masses, this ratio decreased. However, the protective effect of forests is not solely dependent on deadwood: In contrast to the results of open land experiments, we demonstrated that platy-shaped rocks are less dangerous in the forest than their mass-equivalent, cubic rocks. They are more affected by tree impacts and tend to stop earlier. The results of these experiments with and without lying deadwood have been successfully reproduced in computer simulations. Therefore three-dimensional cones (cylinders) were introduced into the program RAMMS::ROCKFALL. The third paper presents an automatic deadwood generator which has been programmed to facilitate the upscaling and transferability from the two experimental sites to other rockfall and windthrow endangered areas. This approach allows programmatically placing deadwood logs into the landscape regional-wide and is serving as an input for rockfall simulation models. The thesis hence comprises high-end real-scale rockfall experiments in different forests with deadi wood, acting as a potent real-world link for new numerical modelling approaches. This combination of both, experiments and modelling, allows the upscaling of the relevant results up to a regional scale. The results are of the highest relevance and application for forest management.
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published
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Examiner: Rigling, Andreas
Examiner : Caviezel, Andrin
Examiner : Bebi, Peter
Examiner: Bugmann, Harald
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ETH Zurich
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Subject
Rockfall; Deadwood; Protective effect of forests; Rockfall experiments; Rock shape
Organisational unit
03535 - Bugmann, Harald / Bugmann, Harald
08849 - Rigling, Andreas (Tit.-Prof.) / Rigling, Andreas (Tit.-Prof.)
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Related publications and datasets
Is new version of: https://doi.org/10.5194/esurf-2022-21Has part: https://doi.org/10.5194/nhess-22-2433-2022Has part: https://doi.org/10.5194/esurf-2022-70