Debris flows are common geomorphic processes in mountainous areas. When debris flows are triggered, alpine catchments containing sediment can be temporarily transformed into flowing masses and travel long distances at high velocities, making this process devastating and threatening to people, settlements and infrastructure. Currently, global surface temperature stands 1.1°C higher than during the reference period of 1850-1900. In the context of climate change, how climate change affect debris-flow activity in a long-term period is of great concern.
Dendrogeomorphology has been proved a useful tool to reconstruct and analyze the past geomorphic activity. The applicability of this approach in China remains to be demonstrated. Moreover, dendrogeomorphic reconstructions of debris flows and other geomorphic activity have also been employed to assess hydrometeorological triggers by using daily maximum precipitation due to the limitation of annual resolution. However, the accuracy of daily maximum precipitation values has never been investigated before.
This PhD thesis is motivated by the aims of investigating 1) long-term frequency and magnitude change of debris flows in the Swiss Alps, 2) accuracy of daily maximum precipitation in reconstructed debris-flow years based on dendrogeomorphology, 3) effect of climate change on debris-flow frequency and magnitude, 4) the role of sediment in the initiation of debris flows in the context of climate change, 5) the potential of dendrogeomorphology in China.
To achieve above goals different methods were applied. First, a single site dendrogeomorphic reconstruction covering the past four centuries was carried out in the Swiss Alps and the characteristic of debris-flow activity was detected based on different statistical trend tests. Then, a dense dataset of debris flows in nine catchments coming from both archival records and dendrogeomorphic reconstructions were presented to compare different triggering precipitation signals derived from different resources. Furthermore, two regional tree-ring based debris-flow reconstructions located in the eastern and western Swiss Alps were compared. Finally, the potential of dendrogeomorphology in Chian was validated by a flash flood reconstruction.
The main findings of this thesis are: 1) The absence of a direct climatic influence on debris-flow initiation at the single site Multetta indicates a dependence of debris flows on sediment discharge and recharge, 2) Annually resolved reconstructions of debris flows based on dendrogeomorphology cannot be used to analyze hydrometeorological triggers, 3) The regional average decadal frequency of debris flows has increased significantly in Zermatt, while no significant, but a slightly decreasing trend was tested in Mustair. Permafrost was widely distributed in the high-altitude catchments of the Zermatt area, so that sufficient thawed and uncovered debris was available. With barely permafrost distribution related to low elevations in Mustair, warming did not enhance the debris flow activity, 4) The potential of dendrogeomorphology in China has been proved.
This thesis represents a multidisciplinary study to improve our understanding of the effect of climate change on debris flows in a long term with consideration of sediment supply. It also has stark implications for the identification of hydrometeorological triggers in tree-ring studies.
