400 Years of Debris-Flow Activity and Triggering Weather Conditions: Ritigraben, Valais, Switzerland

Abstract Three major rainfall events have caused considerable damage in the Valais region (Swiss Alps) since 1987. Substantial debris flows originating from periglacial environments were recorded during the August 1987 and September 1993 rainfall events, whereas no debris flows occurred in October 2000. This paper aims at putting these large-area events and the apparent increase in debris-flow frequency into a wider context by reconstructing the past debris-flow activity in the Ritigraben torrent (Mattertal, Valais) with dendrogeomorphological methods. Tree-ring analysis allowed the reconstruction of 53 events, going back to the year 1605. Previously, only 10 debris flows had been known for the torrent, and these were limited to the period between 1922 and 2002. Results further show that the apparently above-average concentration of events since 1987 was mainly caused by insufficient and short archival data. In fact, debris flows occurred even more frequently in the nineteenth century than they do today. ...