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Debris flow activity in the region of the Swiss National Park

A view from the debris flow cone of Multetta

Jiazhi Qie (Université de Genève) mit Adrien Favillier, Markus Stoffel, Christophe Corona

Debris flows are mass movements of granular solids mixed with water that can reach high velocities. They are a major hazard in mountainous environments, causing damage to infrastructures, and loss of life. A thorough understanding of the temporal and spatial characteristics of past debris flow activity is therefore essential to predict their frequency, magnitude, or extent. This knowledge of past debris flow behavior is even more important as changing environmental conditions may affect the occurrence and physical characteristics of the process. To date, our knowledge is limited by the lack of complete and continuous historical records for most debris flows. In recent decades, dendrogeomorphic reconstructions of past debris flow activity have been developed across the Alps. However, tree-ring based reconstructions rarely exceed 150 years and are limited to a few regions. The old-growth forests on the numerous debris flow cones and the very low human pressure make the area of the Swiss National Park an ideal study environment to decipher the long-term natural activity of the debris flows.

We reconstructed 58 debris flow events between 1626 and 2009 —one of the longest reconstructions worldwide— from 478 Swiss mountain pines in the Multetta torrent. We reconstructed tree-ring series. Our reconstruction shows no clear temporal trend in debris flow frequency. However, the spatial distribution of growth disturbances across the cone, allows us to identify patterns of debris flow events with episodic shifts in the position of the main debris-flow channels. Finally, the comparison between our chronology and precipitation records —available since 1763— suggests a summer (JJA) daily rainfall intensity control with a sharp increase in the probability of debris flow release above 60 These results contribute to a better understanding of the evolution of debris flow activity in a high-alpine environment since the Little Ice Age (LIA) in both spatial and temporal terms.