The Influence of Debris Flow Activity on the Sediment of the Lake Plansee over 3.6 ka (Tyrol, Austria)

Debris flows represent a widespread geomorphological hazard in mountainous regions. Understanding the long-term dynamics of debris flow activity in view of climate change is crucial for the prevention and mitigation of future events. The activity of debris flows is evidently linked to the magnitude of rainstorms. Dietrich & Krautblatter (2017) found an increase in debris flow volumes after 1980 by a factor of 2 compared to the period 1947-1980 and by a factor of 3 compared to the mean Lateglacial/Holocene debris flow volumes by investigating aerial photos of the surroundings of lake Plansee (Reutte, Austria) and estimating debris flow cone volumes with geophysical methods.

In this study, the terrestrial observations of increasing debris flow volumes were compared with the subaquatic deposits from the deepest basin of the lake. The debris flow volume within a three-month period on a large debris cone was monitored by Terrestrial Laserscanning (TLS) and the debris flow activity over the last 3 600 years was reconstructed using sediment cores. Four short cores of up to 145 cm depth were recovered in a transect from the shallow subaquatic debris cone area to the deepest basin of the lake. The grain size, density, Magnetic Susceptibility as well as the d¹³-C, d¹⁵N- and C/N-ratios of the sediment were analyzed.

The Terrestrial Laserscans revealed a sediment delivery ratio of 30% for the steep debris cone bordering the lake. In the four correlated short cores, 52 debris flow events were differentiated within the last 3 600 years of sedimentation. The proportion of event layers in the cores ranges between 34% and 57% of the total section thickness. The sedimentation rates from a dated core confirm the increase of debris flow activity that was observed with terrestrial methods by Dietrich & Krautblatter (2017). The sedimentation rates show an 11-fold increase after 1930 compared to the rates before 1930 and a 5-fold to 12-fold increase compared to the average Holocene sedimentation rates in lake Plansee. Three types of event deposits were distinguished according to sedimentological criteria: flood-triggered debris flows, earthquake-induced subaquatic suspension flows and mega-events. The TOC/TN ratios of the sediment reveal a permanent influence of terrestrial carbon on the lake sediment and a mixed source of allochthonous and autochthonous organic matter. Large debris flow events can be distinguished from background sediments by increased d¹³C isotope ratios.

The results of this study reveal further scientific proof for the increase of debris flow activity in conjunction with increasing rainstorm activity. Here we show one of the first long-term archives of debris flow activity in the Northern Alps spanning the last 3 600 years and revealing cyclic shifts in debris-flow transport volumes by one order of magnitude.