The Wimbach Observatory – Alpine sediment dynamics in transport-limited systems (Wimbach Valley, Berchtesgaden National Park)

Alpine sediment transport from rockslopes to rivers occurs along cascades and by Earth surface processes of varying frequency and magnitude. Modulated by ongoing climate change, they pose hazards as well as matter fluxes that impact the hydrological, ecological and economic system. Typical source-to-sink sediment routing in non-glaciated alpine headwater systems include mechanical weathering, rockfall, debris flows or avalanches, and fluvial transport. Characterized by a highly active sediment cascade, the Wimbach Valley in the Berchtesgaden National Park (A = 36 km²; δz = 2086 m) is the largest "Alpine Gries Landscape" in Europe and a textbook test site to study transient sediment dynamics in transport limited systems. Frequent rockfall from the strongly fractured, dolomitic rock faces supply huge amounts of sediments transported via debris flows and avalanches through numerous steep gullies towards the valley bottom. Fluvial transport only occurs when short-term surface runoff is triggered by heavy rainfall of variable thresholds, lending their non-linearity from the state of the water table within the sediment body. Runoff occurs on very limited spatial and temporal scales but is highly effective and controls this last link to the conveyor belt-like sediment routing system. This setting results in a massive valley fill that is frequently reshaped by different processes and led to a unique scenery with a complex pattern of so-called “Schuttströme” between vegetated areas of multiple successional stages after disturbance. Since the frequency and magnitude of precipitation and sediment transport events will increase with climate change, constraints on sediment transport will become disproportionally important: Several groynes and a dam delimiting the sediment body towards the valley outlet are already filled with sediments, which, if reaching the Wimbach Gorge, might affect tourism, infrastructure, and drinking water supply. We thus currently establish a comprehensive monitoring system to decode the entire sediment cascade based on cutting edge technologies covering all relevant processes with a high spatial and temporal resolution: Mechanical weathering, sediment production, rockfall, debris flow and avalanche dynamics as well as fluvial transport will be assessed using a multi-sensor approach, including rock temperature/humidity sensors, rockfall nets, annual airborne and event-based terrestrial Lidar data, SfM point cloud modelling based on historical aerial imagery and repeated UAV flights, stereo-webcams at neuralgic points, and a dense passive seismic network. The latter enables to detect, locate, track, and quantify major geomorphic processes and allow access to timing, magnitude, trajectory and coupling patterns amongst these processes, representative for many other mountainous landscapes subject to environmental change. The analyses of trigger mechanisms and variable thresholds will be based on dense climate data available to the project. Preliminary remote sensing analyses of multi-temporal orthophotos and aerial imagery using manual and deep learning mapping approaches as well as Lidar based difference modelling are recently in progress. Here we present the entire monitoring system, which is currently under implementation, as well as first mapping and modelling results showing drastic amounts of sediment transport in almost the entire catchment and an increase in geomorphological activity since 2003.