Preprints
https://doi.org/10.5194/esurf-2021-23
https://doi.org/10.5194/esurf-2021-23

  19 Mar 2021

19 Mar 2021

Review status: this preprint is currently under review for the journal ESurf.

A 4,000 year debris-flow record based on amphibious investigations of fan delta activity in Plansee (Austria, Eastern Alps)

Carolin Kiefer1,, Patrick Oswald2,, Jasper Moernaut2, Stefano Claudio Fabbri3, Christoph Mayr4, Michael Strasser2, and Michael Krautblatter1 Carolin Kiefer et al.
  • 1Chair of Landslide Research, Technical University of Munich, Arcisstraße 21, 80333 Munich, Germany
  • 2Department of Geology, University of Innsbruck, Innrain 52 f, 6020 Innsbruck, Austria
  • 3Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland
  • 4Institute of Geography, Friedrich-Alexander University Erlangen-Nuremberg, Glückstraße 5, 91054 Erlangen, Germany
  • These authors contributed equally to this work.

Abstract. The frequency of debris flows is hypothesized to increase in recent decades with enhanced rainstorm activity. Geological evidence to test this tendency for prehistoric times is scarce due to incomplete sediment records, complex stratigraphy, and insufficient age control especially in Alpine environments. In lacustrine archives, the link between onshore debris-flow processes and the depositional record in lake depocentres is poorly investigated. We present an amphibious characterization of alluvial fan deltas and a continuous 4,000 year debris-flow record from Plansee (Tyrol, Austria) combining Light detection and ranging (LiDAR) data, swath bathymetry, and sediment core analyses. The geomorphic investigation of two fan deltas in different developmental stages revealed a sediment delivery ratio of 7.9 % for the juvenile fan and no sediment transport into the lake on the mature fan within a 3-month summer period (May 2019–August 2019). Event deposits were dated and categorized according to their causal mechanism in a transect of four sediment cores. Debris flow-induced turbidites feature a more gradual fining-upward grain-size trend and higher TOC and δ13C values compared to earthquake-induced turbidites. Over the last 4,000 years, the record containing 138 debris flow-induced turbidites reveals four different debris-flow activity phases. Phase 1 (2050–1960 before the common era; BCE) depicts the second highest observed event frequencies. Phase 2 (1960 BCE–1550 common era; CE) shows large recurrence intervals. Phase 3 (1550–1905 CE) displays a gradual increase of event frequency. Phase 4 (1905–2018 CE) exhibits a debris-flow frequency increase between 1908 and 1928 CE, followed by the overall highest debris-flow frequency between 1928 and 1978 CE, and lower debris-flow frequencies since 1978 CE, which still exceed those of phase 1 to 3. Most remarkably, we find a ~7-fold increase of debris-flow frequency compared to the reference period 1700–1900 CE. The triggering of debris flows is more controlled by short intense rainstorms than for any other mass movement process and we demonstrate that lacustrine debris-flow records provide a unique inventory of hazard-relevant rainstorm frequencies over decades, centuries, and millennia. In a calibration period of 7 decades, we can show that the debris flow-induced turbidite record matches with the previously published debris-flow volume increase derived from aerial photography coincident to a pronounced rainstorm frequency increase. Here we show a millennium-scale debris-flow record that documents a ~7-fold increase in debris-flow frequencies in the 20th and 21st century coincident to 2-fold enhanced rainstorm activity in the Northern European Alps and provide a novel basis for systematic non-stationary estimation of future debris-flow frequencies in a changing climate.

Carolin Kiefer et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on esurf-2021-23', Anonymous Referee #1, 23 Apr 2021
  • RC2: 'Comment on esurf-2021-23', Anonymous Referee #2, 28 May 2021

Carolin Kiefer et al.

Carolin Kiefer et al.

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Short summary
This study provides amphibious investigations of debris-flow fans. We characterize active debris-flow fans combining laser scan and sonar surveys at the Alpine lake Plansee. We discover a 4,000 year debris-flow record in sediment cores providing evidence for a ~7-fold debris-flow frequency increase in the 20th and 21st century coincident with 2-fold enhanced rainstorm activity in the Northern European Alps. Our results indicate climate change as the main factor controlling debris-flow activity.