Variations in debris-flow occurrence in an Alpine catchment — A reconstruction based on tree rings

Past debris-flow activity in the Birchbach torrent (Swiss Alps) was assessed using tree-ring data. Based on the analysis of 210 Larix decidua and Picea abies, 50 events were reconstructed for the period AD 1752–2006. On average, 3.2 events per decade occurred in the torrent during the last century with distinct differences between the decades and a considerable decrease in the number of events during the last decade. Comparison with debris-flow reconstructions in neighboring catchments provided more insight in the similarities and differences in debris-flow occurrence. The recurrent debris-flow activity and current geomorphic settings suggest that the occurrence of debris flows in the torrent is rather transport-limited than supply-limited. Therefore, future investigations on debris-flow occurrence in transport-limited basins should focus on triggering rainfall events.