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Erosion des galets des rivières de montagne au cours du transport fluvial : étude expérimentale et application aux réseaux hydrographiques d'orogènes actifs

Abstract : Terrestrial landscapes result from the competition between two opposing processes: tectonic uplift and erosion. At mountain range scale, rivers are the most important agents that shape the landscape: in response to uplift, they incise into bedrock and control the progressive lowering of base level for hillslope erosion, in particular by landslides. Moreover, they evacuate the erosional products of the range as dissolved, suspended and bed load. The latter interacts with river bedrock and controls the mode and rate of its incision. A thorough understanding and quantification of abrasion processes linked to pebble – bedrock interactions is therefore necessary to understand the evolution of mountainous landscapes. To this aim, we have realized an experimental study based on a new device in which we reproduce as realistically as possible the abrasion processes that act in natural rivers. With regard to previous studies, we confirm the control exerted by pebble lithology and particle velocity, with in particular a relationship between abrasion rate and particle kinetic energy. The dependence on particle size is, however, complex to describe by a single law; for example, the trend observed by using material that is heterogeneous in size is opposite to the one obtained by using graded material: in the first case, the smaller the particles, the faster they are abraded. These first results, which need to be characterized more finely by further experiments, indicate that pebble and bedrock abrasion strongly depends on numerous variables and that the bedload exerts a major control on the geomorphology and evolution of the fluvial network. Parallel to this experimental study, a field study has been carried out in an active orogenic setting (Marsyandi valley, Himalaya), where the downstream evolution of sediment characteristics can be interpreted in terms of abrasion during fluvial transport. Combining the two studies leads to new insights in sediment supply and transport modalities and in interactions between sediments and bedrock. We show, in particular, that the downstream evolution in grain size reflects mainly the influence of the sources and the transport modalities. Therefore, fining rates measured in the field cannot be compared directly with experimental abrasion rates. On the other hand, pebble abrasion can be revealed by the downstream evolution of lithologic proportions, the latter being less sensitive to source and transport effects. By using a simple numerical model, we achieve to reconcile experimental abrasion rates with those deduced from the observations along the Marsyandi River. Our experimental and field results emphasize the importance of the spatial and temporal distribution of supply zones and of their characteristics, and confirm the major role played by bedrock and bedload lithology in terms of controlling tectonic and denudational processes, and consequently in terms of the long term evolution of mountain ranges.
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https://tel.archives-ouvertes.fr/tel-00004097
Contributor : Mikaël Attal <>
Submitted on : Monday, January 5, 2004 - 11:08:14 PM
Last modification on : Thursday, November 19, 2020 - 3:54:26 PM
Long-term archiving on: : Wednesday, September 12, 2012 - 12:15:44 PM

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Mikaël Attal. Erosion des galets des rivières de montagne au cours du transport fluvial : étude expérimentale et application aux réseaux hydrographiques d'orogènes actifs. Géomorphologie. Université Joseph-Fourier - Grenoble I, 2003. Français. ⟨tel-00004097⟩

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