Mass movements such as landslides and rockfalls are driving forces in mountainous regions and pose a major threat to its people and infrastructure. The alpine village of Brienz in Switzerland is threatened by both an ongoing landslide and the danger of a major rockslide. Regular rockfalls also threaten the village. The entire area is therefore strictly monitored. The additional creation of a drainage system is intended to slow down or stop the landslide by reducing the water pressure in the mountain. While these approaches enable authorities to gain knowledge about the present state of the landscape and to plan on the future course of action, they do not give any information about the past development of the landslide. Dendrogeomorphology offers the possibility to reconstruct the time before the start of monitoring, by analysing the annual formed tree rings.
In this Master’s thesis different methods are applied to gather information about past movement in a subarea of the Brienzer landslide. In addition, the approaches used, and the obtained results are compared to each other to assess their reliability. Furthermore, location and visual inclination of individual trees are discussed as indicator of the extent of disturbance to be expected. First, a visual inspection of all chronologies was used to get a basic impression of possible activity phases. Second, the chronologies of each tree were examined individually, and years of disturbance were noted manually. Third, using the eccentricity index with different thresholds (based on the average, median, and artificially set), the years of disturbance of each tree were also calculated. Fourth, an attempt was made to confirm identified event years by making thin sections and search for reaction wood. However, this was not successful, probably due to a too small sample size or errors in setting the cores.
The results of tree-ring analyses showed that as far back as the tree rings went (~1820), the area was in motion and still is today. The sites experienced periods of activity, that were separated by resting phases in 1850-1875, 1945-1955, and 1975-1985. The comparison of the different methods has also shown that it is important to interpret the results with caution, as manual assessment and eccentricity index calculations sometimes identified different activity phases. One advantage of the eccentricity approach is that it is sensitive enough to notice the landslide increase, while the manual approach failed to do so. But this sensitivity is also a disadvantage. If the arbitrary threshold is set low, "false" events (noise) are detected, if it is set too high, events are ignored. The manual approach on the other hand is not dependent on a general threshold but on the estimation of each individual event by itself. Lastly, high numbers of disturbances in individual trees could often be associated with cracks in the soil in close proximity. On the other hand, both inclined trees and vertical trees show equally often high numbers of disturbances. Consequently, based on the results of this work, location appears to be a better disturbance indicator than visible damage on the tree.
Tree ring analyses not only allow to record the history of a landslide dating back for several centuries, which is a great advantage compared to other monitoring methods, but also to monitor dynamics within a sliding area based on analysis of individual trees and their locations. Some predictions for the future might also be possible for example an acceleration of the slide if the tree ring analysis indicates an increase in event number per year. For all these reasons, the inclusion of dendrogeomorphology in the comprehensive monitoring of landslides should be considered
