Deciphering increasing debris flow activity as a composed signal of 54 contributing catchments over the last 70 years: combined terrestrial and lake record (Plansee, AT)

Debris flow activity is expected to show a nonlinear response under different climate change scenarios. The Plansee (Tyrol, Austria) in the Northern Calcareous Alps is one of the few catchments where the strong increase in debris flow activity could be evidenced over the last 70 years (terrestrially) and 4000 years (lake sediments). The latter study (Kiefer et al. 2021) shows a 9-fold recent (since 1920) increase in debris flow volumes. The 54 alluvial fans bordering the lake are connected to heavily jointed Dolostone catchments with constant debris production and form an archive for the evolution of debris flow activity over the Holocene. By photogrammetric analysis of historical and digital aerial images starting in 1952, we capture a 7-decade period of terrestrial hillslope erosion. The volumes of debris flow-induced sediment deposition in the lake since 1952 derived from turbidite deposits match the yearly cumulative net change in the catchments calculated from Digital Surface Models derived from historical aerial images. An increase in rainfall days since the 1980s corresponds to an increase in mean erosion over all catchments. We compare the sediment yields of these catchments over the last 7 decades to find out whether varying catchment characteristics control the activity on each fan or the variation in rockfall activity and local intense precipitation over time outweighs differences between catchment morphometry. We aim to analyze (i) how the longterm increase in debris flow activity since 1920 is reflected in the short term sediment dynamics of multiple alluvial fans, (ii) whether we can observe trends or heterogeneous activity on all fans within this phase of overall enhanced activity, (iii) how the vegetation cover changed within 7 decades, (iv) which sedimentation patterns we can reveal with geophysical methods and (v) the amount of geomorphic work carried out in the catchments over the last 7 decades.