Tree-Ring Reconstruction of Debris-Flow Events Leading to Overbank Sedimentation on the Illgraben Cone (Valais Alps, Switzerland)

Markus Stoffel,Michelle Bollschweiler,Astrid Leutwiler,Patrick Aeby

doi:10.2174/1874262900802010018

Abstract

Dendrogeomorphological analyses of trees affected by debris flows have regularly been used to date past events. However, while previous studies in the Swiss Alps have focused primarily on granitic and gneissic debris-flow material and on Larix decidua Mill. and Picea abies (L.) Karst. trees, they have - at the same time - widely disregarded torrents dominated by finely fractured calcareous and dolomitic lithologies and forests populated with Pinus sylvestris L. In this paper, we report on results obtained from a debris-flow cone in the Rhone valley (Valais, Switzerland) where very fine and muddy material (limestone, dolomite, quartzite and calcite) occasionally affects P. sylvestris trees. Based on the results of a geomorphic map, 1004 increment cores from 451 disturbed P. sylvestris, 37 L. decidua and five P. abies trees were sampled, allowing reconstruction of 15 events between AD 1793 and 2005 as well as the determination of breakout locations of events. From the data, it also appears that debris-flow material only rarely left the incised channel over the last 200 years and that overbank sedimentation events did not occur after 1961, when a rockslide delivered large amounts of erodible material and subsequent debris flows caused an important incision of the canyon on the cone.

Highlights

Debris flows are among the most common and widespread geomorphic processes in mountain regions, where their repeated occurrence may result in characteristic landforms with cones, channels and lo-bate deposits
In order to gather the greatest amount of information on growth disturbances (GD) caused by past events, increment cores were preferably sampled at the height of the visible damage or within the segment of the stem tilted during past events
In the study we report here, dendrogeomorphological analyses of 451 Pinus sylvestris L., 37 Larix decidua Mill. and 5 Picea abies (L.) Karst. trees have been used to identify outbreak locations and overbank sedimentation events on the cone of the Illgraben torrent in the Rhone valley (Valais, Switzerland)

Summary

Introduction

Debris flows are among the most common and widespread geomorphic processes in mountain regions, where their repeated occurrence may result in characteristic landforms with cones, channels and lo-bate deposits. In inhabited areas, they may cause damage to transportation corridors and buildings or even lead to the loss of lives [1, 2]. The reconstruction and appraisal of past activity seems essential for the understanding of current debris-flow dynamics in mountain torrents as well as possible future evolutions

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