Increasing debris-laden flow volumes at Plansee (AT): Comparing the lacustrine sedimentary archive with the terrestrial sediment budget from historical aerial imagery

Climate forcing influences the frequency and magnitudes of debris-laden flows (including hyperconcentrated flows and debris flows), which are controlled by intense local precipitation events. Reconstructing debris flow activity is challenging due to incomplete archives, a lack of historical evidence, and unrepresentative precipitation data of local rainfall intensity.  To reconstruct past activity and infer future precipitation thresholds, we need robust archives that include a wide range of temporal and spatial scales. Plansee (Tyrol, Austria) is a relatively pristine and virtually closed source to sink system. Sediment is transferred from 54 transport-limited Hauptdolomit catchments to the lake-adjacent fans. Debris-laden flows regularly enter the lake as underflows, creating turbidites in the basin. Lacustrine sediment cores taken from the fan delta towards the depocenter offer a 4000-year archive with 138 debris-flow-induced turbidites. The photogrammetric reconstruction of historical aerial imagery since 1952 enables the quantification of elevation changes in the active debris flow channel, allowing for the estimation of the sediment budget at each fan. In this contribution, we compare the in-lake deposition rates from the sediment cores with the terrestrial erosion, deposition, and net volumes at each surrounding fan, to conclude that both calculations match.

The frequency of debris-laden flows has increased significantly since 1920, with a most recent peak after the 1980s observed in the terrestrial record. The terrestrial inventory reveals varying activity over the decades on all fans, providing a better understanding of the mechanisms and controlling factors of sediment propagation in pre-alpine catchments. This contribution presents for the first time the integration of lacustrine and terrestrial records to reconstruct historical debris flow activity and outline trends in frequencies and magnitudes.

This research is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation – Project number 558963977) and the Austrian Science Fund (FWF, grant https://doi.org/10.55776/PIN7180424)