Disaster Mitigation of Debris Flows, Slope Failures and Landslides 597 Assessing the Spatio-Temporal Debris-Flow Activity on a Forested Cone using Tree-Ring Data

Although debris-flow are a common process in Swiss Alps and repeatedly cause damage, research in Switzerland started only after the severe events of 1987 and data on past events is very limited. It was therefore the purpose of this study (i) to reconstruct former debris-flow events and (ii) to date debris-flow deposits on a forested cone in the Swiss Alps (Grosse Grabe, Valais). The analysis was based on a detailed geomorphic map (1:1′000) of all forms related to debris-flow activity such as lobes, levees and abandoned flow paths. In addition, 152 tree-ring sequences from 71 heavily disturbed trees (Larix decidua Mill. and Picea abies (L.) Karst.) growing in the deposits were analyzed and growth disturbances related to debris-flow activity assessed, such as scars and callus tissue, rows of traumatic resin ducts, the onset of reaction wood as well as abrupt growth decrease or release. Furthermore, 71 undisturbed trees growing in the abandoned flow paths were sampled so as to approximate the time elapsed since the last event passed through. So far, 40 event years between 1782 and 2005 could be identified. The coupling of tree-ring data with the geomorphic map will allow spatial representation of reconstructed event years