Constraining centennial to sub-annual sediment dynamics on alpine alluvial fans – first insights from the Friedergries (Germany)

Alpine alluvial fans are important storages in the alpine sediment cascade. It is to be expected that climate change and the resulting changes in precipitation will have a massive impact on the dynamics of alpine alluvial fans. In order to differentiate between short-term and long-term dynamics, we compile a dataset quantifying sediment redistribution of a small mountain river and its alluvial fan on centennial, decadal and sub-annual scales. Our dataset comprises historical topographic maps from 1826 to 1912, 25 sets of historical aerial images collected between 1945 and 2024, and 17 high-resolution UAV-campaigns collected between September 2018 and October 2024. We identify the spatial changes in the sediment body, quantify the sediment redistribution and relate both to precipitation.

On centennial timescales, our data show a shift from presumably low geomorphic activity that persisted for at least 100 years (1820s-1930s) in the eastern sector of the fan, to high geomorphic activity with rapid channel migration across the central fan within the past 60 years. The onset of intense geomorphic activity may be contemporaneous to the increase in debris flow activity at nearby lake Plansee in the 1920s (Kiefer, Oswald et al., 2021). Decadal changes to the active area are largely explained by median precipitation (r² = 0.66, p < 0.002) measured at a weather station c. 10 km east. Since the 1960s, incision at the apex and deposition at the toe of the fan can be observed. Sub-annual change detections show that for most epochs, erosion and deposition balance out within the uncertainty margin and the main channel gradually shifts its position by bank erosion and gravel bar construction. However, following an extreme deposition event between August and September 2019 with a net deposition of 8000 ± 3500 m³, the course of the main channel abruptly shifted. Our preliminary results show that while historical maps and aerial images are useful to reconstruct long-term trends, repeat topographic surveys with a close temporal spacing are needed to understand the processes behind these trends.

Kewords: alpine alluvial fan, sediment redistribution, geomorphic change detection, multiscale investigation

Kiefer, C., Oswald, P. Moernaut, J., Fabbri, S.C., Mayr, C., Strasser, M. & Krautblatter, M. (2021): A 4000-year debris flow record based on amphibious investigations of fan delta activity in Plansee (Austria, Eastern Alps). – Earth Surface Dynamics, 9: 1481–1503. DOI: 10.5194/esurf-9-1481-2021