Debris-flood reconstruction in a pre-alpine catchment in Switzerland based on tree-ring analysis of conifers and broadleaved trees

Past debris‐flood activity on the forested fan of Torrent de la Greffe located in the Swiss Prealps was assessed from growth disturbances in coniferous and broadleaved tree species. The study of 186 tree‐ring sequences sampled from 44 Norway spruce (Picea abies (L.) Karst.) and 55 broadleaved trees from various species affected by past activity and the sampling of an additional 49 trees for the reference chronology allowed the reconstruction of 17 debris‐flood events since AD 1900. The spatial analysis of trees affected during particular events on the geomorphic map helped the identification of five breakout locations in the torrent and affected sectors on the fan. The coupling of tree‐ring analysis of coniferous and broadleaved tree species proved to be a valuable tool for the reconstruction of past events. Debris‐flood frequency in the investigated torrent (0.16 event yr‐1) is considerably lower than the frequencies reconstructed in most other catchments in the Swiss Alps. As material for the entrainment of debris floods is not always readily available, the torrent has to be seen as supply‐limited