Méthodologie d'analyse de l'enfoncement du lit mineur d'un fleuve : approche combinée modélisation hydraulique-géochimie. Application à la Loire Forézienne (France)

Abstract : Currently, the Loire river, sees its geodynamic increasingly deteriorated. The deficit of sediment transport causes erosion of the bed on several areas of the basin. This results in a gradual destabilization of structures (dams, bridges), erosion or clogging of banks or a decline in piezometric level alluvial.Treating these problems requires a better understanding of the processes of erosion and river incision, the second succeeding the first, once the alluvial material of the bottom was completely removed, revealing a bed outcrop.This complex phenomenon is currently found in various parts of the Loire river, in the plain of Forez.We propose three approaches for this analysis.The first approach is based on numerical modeling using the equations of Barre-de-Saint-Venant, for flow, coupled with the equations of Exner and Meyer-Peter Müller for sediment transport. This monodimensional model allows the simulation of riverbed changement and sediment discharge, right each calculation point of grid mesh.The second approach is mechanistic and consists of determining the rate of marly bedrock incision by a semi-empirical method by the use of two major variables: the total hydraulic power and an abrasion coefficient. The hydraulic power is calculated using a hydraulic model, while the abrasion coefficient is a mechanical property of the marl which is determined from laboratory tests.The third approach is the geochemistry. It consists in determining the provenance of the materials from tributaries and in the main chanel by analyzing their geochemical signatures. This has been the subject of laboratory tests on samples taken across the linear from Grangent to Balbigny.The results obtained show that these independent approaches are complementary and provide both a qualitative and quantitative description of the incision of the Loire river in the study area.
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Mamy Tombozafy. Méthodologie d'analyse de l'enfoncement du lit mineur d'un fleuve : approche combinée modélisation hydraulique-géochimie. Application à la Loire Forézienne (France). Autre. Ecole Nationale Supérieure des Mines de Saint-Etienne, 2011. Français. ⟨NNT : 2010EMSE0596⟩. ⟨tel-00666996⟩

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