| 13 Apr 2026
Disentangling the contributions of external forcings and internal factors to future alpine sediment fan dynamics
Abstract. Alpine alluvial fans and debris flow cones are central components of mountainous sediment cascades. The projected increase in heavy precipitation due to ongoing climate warming has been shown to intensify sediment redistribution dynamics under transport-limited conditions; however, sediment fan response to heavy precipitation has been shown to be strongly system-specific. The relative importance of external forcings and internal factors for sediment dynamics and their implications for sediment fan evolution in a future changing climate have not been assessed systematically so far. In this study, we compare decadal-scale planimetric dynamics of a mature alpine alluvial fan (“Friedergries”, 5 km2 catchment area) to juvenile debris flow cones (Lake Plansee, catchment areas mostly < 0.5 km2) in the Main Dolomite region of the Northern Calcareous Alps. In both areas, planimetric sediment redistribution dynamics are governed by external forcing by heavy rainfall. Internal system variables such as catchment morphometry transform the external forcing: the juvenile cones corresponding to small and steep catchments are susceptible to moderate precipitation extremes (< 1 year return interval) while floodplain dynamics on the mature fan are only susceptible to extreme precipitation events with supra-regional extent (3–20 years return interval). A detailed analysis of volumetric changes at Friedergries from 2018 to 2024 reveals that total erosion and deposition are best explained by heavy precipitation, while the location and timing of incision and backfill are determined by small-scale autogenic cycles. Based on current projections of climate change in the European Alps, we suggest that future sediment fan dynamics will be characterized by (i) an intensification of sediment redistribution in response to rainfall intensification, (ii) an increase of geomorphic work per rainstorm event in response to rainfall concentration, (iii) an increase in likelihood of system state changes, and (iv) a catchment-dependent seasonal shift of sediment redistribution: Activity of juvenile cones with small, steep catchments will shift towards spring and autumn while mature fans with larger, gentler catchments will continue to experience sediment redistribution mainly in summer. Here we show that catchment morphology and fan maturity control future susceptibility to rainstorms and thus sediment fan evolution in the coming decades.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Earth Surface Dynamics.
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