| 25 Oct 2026
Massive debris flow activity and connectivity shift in the 1980s deciphered by combined terrestrial and lacustrine record in a changing climate
Abstract. Recent climatic change affects sediment transfer in Alpine catchments beyond historical experience, increasing the need for robust quantification of sediment cascades to better anticipate the frequency and magnitude of future climate-driven natural hazards. Despite widespread projections of increased natural hazard activity, many studies lack quantitative volume estimates over more than a decade. Lake Plansee, located in the Northern Calcareous Alps (Tyrol, Austria), represents a quasi-closed geomorphological system in which previous studies have identified an increase in debris flow activity since the 1920s. Here, we revisit historical aerial imagery using recent photogrammetric techniques to quantify sediment transfer on 54 debris fans to the lake between 1952 and 2020, and to establish a first-order quantitative reconciliation between terrestrial sediment-transfer estimates and sediment accumulation inferred from the lacustrine sink archive within a common source-to-sink framework. Fans surrounding Plansee are grouped based on their connection to the lake inferred from the co-occurrence of active channel linkage to the lake and net sediment export (i.e., negative net change). Twenty-one debris flow channels are connected to the lake and actively deliver sediment. Sixteen channels indicate progression towards the lake and are expected to connect in the future. Rainstorm intensity analysis shows that, since 1980, the number of days with precipitation intensities capable of initiating debris flows (>10 mm h-1) has increased by 36%, accompanied by a 12% increase in total erosion volumes and a 47% decrease in deposition volume at connected fans, suggesting an overall decrease in internal storage at the fans and enhanced sediment transport to the lake. The decadal response of sediment dynamics on the fans suggests that the increase in precipitation events results in an overall increase in sediment connectivity between catchments, fans, and the lake, driven by episodic events and diverse individual fan responses. These findings are reflected in land-cover changes: the increase in erosion is accompanied by an increase in non-vegetated areas on 70% of the active fans after the 1980s. Event-layer deposition from the lacustrine record reveals a first-order debris flow volume estimate of 3.69 ± 0.42 × 105 m3 deposited between 1952 and 2018. In comparison, a total of 3.42 ± 0.43 × 105 m3 of sediment was transferred from the fans to the lake in the last seven decades. Here, we provide the first direct agreement between terrestrial sediment-volume estimates derived from historical aerial imagery and lacustrine sediment-core reconstructions over multi-decadal timescales. While reconciliation of debris flow activity over time from sediment cores and terrestrial sediment dynamics is complex, both records agree on the overall increase of debris flow activity over the last decades coinciding with a shift in climatic conditions.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Earth Surface Dynamics.
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