Deciphering Alpine hazard frequencies using lake sediments

Recent observations and model projections suggest that natural hazards in the Alpssuch as landslides, debris flows, and floodsare becoming more frequent and intense, largely due to ongoing climate change. However, a clear, data-driven understanding of how these hazard frequencies have evolved over time remains limited due to the scarcity of long-term, high- resolution records. The ALPHA project seeks to address this gap by reconstructing Alpine hazard patterns and magnitudes under varying climatic conditions, thereby improving future hazard predictions. The project focuses on analyzing sediment layers preserved in four Alpine lakes located in the Bavarian-Tyrolean region, a globally recognized hotspot for late Holocene hazard activity. Two of these lakes (Plansee and Achensee) serve as supersites for building long-term event chronologies, while two others (Walchensee and Eibsee) offer insights into the mechanisms of extreme events. These sites were selected based on extensive preliminary data, including onshore/offshore geomorphological mapping, geophysical surveys, LiDAR-based surface change detection, and detailed lake sediment analyses. By systematically analyzing lake sediments and comparing them to terrestrial processes, ALPHA aims to: (i) distinguish the types of hazard events that produced specific sediment layers, (ii) estimate the terrestrial processes and magnitudes involved, and (iii) identify periods of increased hazard frequency linked to intensified climatic forcing, such as more frequent and severe rainstorms. Special emphasis is placed on warmer periods like the Holocene Climatic Optimum and the Medieval Warm Period, which may offer relevant parallels to future climate scenarios. A novel aspect of ALPHA is its application of a 4D approach to subaquatic sediment dynamicscombining repeated geophysical surveys, sediment core sampling, and calibration with sediment trapsto model sediment transport and deposition processes in space and time. Furthermore, the project integrates terrestrial and lacustrine data to establish mass and momentum balances, linking sediment volumes in lake basins with upstream erosion and mass movement processes across annual to millennial timescales. Through this comprehensive, process-based framework, ALPHA aims to enhance our understanding of (i) environmental drivers, (ii) sediment sources, (iii) transport pathways, and (iv) deposition patterns under changing climatic conditions. The outcomes will contribute to more robust, scenario-based projections of the frequency, intensity, and seasonal distribution of Alpine natural hazards, with implications for regional risk assessment and climate adaptation strategies.