Deciphering debris flow activity in the lake sediment record of Plansee (Austria, Eastern Alps)

In many mountain environments worldwide, the number of rainstorms capable of triggering debris flows has significantly increased in the last century. An increase in debris flow frequencies in recent times is often postulated but poorly substantiated due to a lack of continuous records over relevant timescales of environmental forcing, especially in the Alpine environment. In this study, onshore processes and the sediment record of an Alpine lake are assessed to reconstruct debris flow activity. The continuous sedimentation regime in lake Plansee (Tyrol, Austria) provides a long-term archive of natural hazards in a setting of highly connective catchment morphology. In an on- and offshore investigation, we integrate LiDAR data, swath bathymetry and sediment core analyses. Two types of alluvial fan deltas are characterized, which differ in sediment delivery and their geomorphic landforms in the terrestrial and subaquatic realm. In a core transect from one fan delta towards the depocentre, we distinguish turbidites related to debris flows and earthquakes. 138 debris flow events were identified in the 4,000 year sedimentary record. The event chronology reveals four phases of different debris flow recurrence. Here a constant background sedimentation with low debris flow rates contrasts to i) debris flow frequency increases interpreted as post-seismic landscape response and ii) a drastic 7-fold increase in debris flow frequency between the periods ~1520 to 1920 CE and 1920 to 2018 CE. These recently enhanced rates may link to the nearly doubled frequency of heavy rainfall events from 1920 to 2010 CE in the Plansee area. We provide sedimentological evidence for a previous LiDAR-based observation of increased debris flow recurrence at Plansee in the 20^th century. These results indicate that variations in debris flow activity are mostly controlled by a few severe earthquakes and by climate forcing.

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