Debris-laden flows as indicators of Holocene natural hazard variability in alpine regions: First results from Lake Heiterwang (Northern Calcareous Alps, Austria)

Natural hazards in alpine regions show an increase in frequency over recent decades, often linked to climate change. In particular, debris-laden flows (such as hyperconcentrated flows and debris flows) have become more frequent in response to intensified precipitation events. However, such observations are largely limited to short instrumental or historical records. To understand long-term hazard behavior and improve assessments of future impacts on infrastructure and human safety, hazard activity must be investigated over extended timescales. Lake sediments offer continuous, high-resolution archives that reveal debris-laden flow activity over millennia, providing insight into long-term hazard dynamics.

Here, we present preliminary results from a 13 m long sediment core retrieved from Lake Heiterwang, an alpine lake located in the Northern Calcareous Alps of Tyrol, Austria. The lake has a surface area of approximately 1.37 km² and a maximum depth of 61 m and is surrounded by steep slopes composed of intensely jointed dolomite rock, providing abundant sediment supply and making the catchment highly susceptible to debris-laden flow activity during intense rainstorms. This setting is therefore well suited for reconstructing the frequency and magnitude of these processes throughout the Holocene. The sediment core was analyzed using a multi-method approach, including X-ray fluorescence (XRF) scanning, multisensor core logging (MSCL), computed tomography (CT) scanning, grain-size measurements, sedimentological descriptions, and dated by ¹⁴C and ²¹⁰Pb/¹³⁷Cs.

Background sedimentation consists of dark, laminated, fine-grained sediments. Correlated Fe and S enrichments within the background sediments suggests in situ precipitation of Fe-sulphides under anoxic conditions. Event layers are typically light brown to brown with higher density and locally contain coarser grain sizes. Some deposits show fining-upward grading from fine sand to silt with occasionally bright clay caps. A total of 57 event deposits thicker than 2 cm were identified macroscopically, most of which are 2–5 cm thick. High resolution scanning data reveals numerous mm-scale event deposits. Multivariate analysis (e.g., PCA, cluster analysis) will be used to identify the event deposits. With only five paleo-earthquake-related deposits identified at nearby Lake Plansee, most event layers reflect non-seismic, climate driven processes.

Comparisons with sediment cores from nearby Lake Plansee and Lake Achensee will further refine the regional context of debris-laden flow activity during the Holocene. These preliminary results show the potential of lacustrine sediment archives to create links between debris-laden flow activity and Holocene climate variability.

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).