Frequency and spread of debris floods on fans: A dendrogeomorphic case study from a dolomite catchment in the Austrian Alps

Growth disturbances in tree-ring series have been regularly used to date debris-flow events in mountain environments. In contrast, no studies are available to date that have reconstructed debris floods by means of dendrogeomorphology. Therefore, the aim of this study was to determine the event frequency and the spread of debris floods in the Gratzental (Tyrol, Austria). The analysis of growth disturbances in the tree-ring series of 227 Picea abies (L.) Karst. and Larix decidua Mill. allowed the reconstruction of 37 events for the last 200 years. The lateral spread and preferable avulsion locations of reconstructed debris-flood events were assessed based on the dating of the events and the spatial position of trees affected by an event on the fan. Results show that the Gratzentalbach preferentially avulsed to the east, but affected trees were evenly spread over the fan. Reconstructed data illustrates the high potential of dendrogeomorphology for hazard assessment of debris floods.