Limits of sediment transfer in an alpine debris-flow catchment,Illgraben, Switzerland |
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| Authors: | Fritz Schlunegger Alexandre Badoux Brian W McArdell Corina Gwerder David Schnydrig Dirk Rieke-Zapp Peter Molnar |
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| Institution: | 1. Technische Universität Darmstadt, Institut für Angewandte Geowissenschaften, Schnittspahnstr. 9, 64287 Darmstadt, Germany;2. OMV Aktiengesellschaft, Trabrennstrasse 6–8, 1020 Vienna, Austria;3. Università di Firenze, Dipartimento di Scienze della Terra, Università di Firenze, Italy;1. Institute of Geology, University of Berne, CH-3012 Berne, Switzerland;2. School of Geography, Environment and Earth Science, Victoria University of Wellington, NZ-6140 Wellington, New Zealand;1. U.S. Geological Survey, P.O. Box 250466, MS966 DFC, Denver, CO, USA;2. East Carolina University, Greenville NC, USA;1. U.S. Geological Survey, Golden, CO, USA;2. National Weather Service, Los Angeles – Oxnard, CA, USA;3. U.S. Geological Survey, Albuquerque, NM, USA;4. Arizona Geological Survey, Tucson, AZ, USA |
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| Abstract: | The 9.5 km2 Illgraben catchment, located in the Rhône valley in the Central Alps of Switzerland, is one of the most active debris flow torrents in the Alps. In this paper we present sediment yield data collected in 2006 for segments where hillslopes and channels form a fully connected network and contrast these with sediment yields measured for disconnected hillslopes. The data reveal that sediment yields are 1–2 orders of magnitude larger in segments where hillslopes are connected with the channel network than on disconnected hillslopes. Support for this conclusion is provided by observations made on 1959, 1999 and 2004 aerial photographs that the vegetation cover in the disconnected segments is still intact, whereas denudation rates of several centimeters per year in the connected segments have inhibited the establishment of a stable vegetation cover. Furthermore, sediment supplied from hillslopes during the past 40 years has temporarily accumulated along the Illgraben channel, indicating that the channel aggraded over this period and has not yet recovered. An implication of this observation is that initiation of debris flows in the Illgraben catchment is limited more by the availability of intense rainfall than sediment. In contrast, on disconnected hillslopes, sediment flux does not appear to be driven by precipitation.The petrographic composition of the Illgraben fan deposits indicates two distinct sediment sources, one related to rockfall and the other to landslides and debris flows. The presence of clasts from both sources implies multiple processes of erosion, deposition, mixing and re-entrainment in the catchment before the material is exported to the Illgraben fan and to the Rhône River. In addition, delivery of large amounts of coarse-grained sediment to the Rhône causes a modification of the flow pattern from meandering or anastomosing upstream to braided downstream. Hence, the direct connectivity between hillslope and channelized processes in the Illgraben catchment causes not only rapid topographic modifications in the catchment, but also morphologic adjustment in the Rhône valley downstream. |
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