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Theses

Influence des aménagements et des contaminations sur les héritages sédimentaires des fleuves anthropisés : le cas du Rhône

Abstract : Although complex interactions between humans and rivers have existed for several millennia, anthropogenic pressures on hydrosystems have increased sharply over the last two centuries. It is therefore essential to gain a better understanding of the recent evolution of river corridors in relation to the human activities that have developed there. The investigation of the sedimentary compartment is especially relevant for this type of retrospective approach. This thesis is based on the study of the Rhône River, a major European river subject to strong anthropogenic pressures, and is particularly focused on two of these pressures: contamination and river engineering. More specifically, its aims are to 1) prove the existence of legacy sediments caused by the river infrastructures implemented on the Rhône River and characterise those sediments, 2) develop a methodology for the interpretation of sedimentary archives sampled in a highly anthropised river environment subject to historical and emerging contamination, and 3) demonstrate the relevance of the concept of legacy sediments in a European context and in relation to river engineering. The methodology developed in this work combines a diachronic analysis of the study sites, their investigation using geophysical methods and the sampling of sediment cores that allows the quantification of many sedimentary parameters (e.g., grain-size, total organic carbon, contaminants and radionuclides content). Its implementation allowed to prove the existence of two types of legacy sediments on the Rhône River, that are related to the two engineering phases to which the river has been subjected (river training and bypassing for hydroelectricity production). In both cases, the legacy sediments result from a loss of connectivity between the main channel and the depositional environments investigated, and grain-size is the main parameter allowing to distinguish the legacy sediments from the underlying deposits. This leads us to propose an evolution in the definition of legacy sediments to take these observations into account. This study also proves that the recording capacity of river sediments is strongly influenced by the depositional environments studied and by the modifications of their hydrodynamic conditions induced by river engineering. Nevertheless, the proposed methodology makes it possible to interpret these archives in a robust manner and to reconstruct the recent trajectory of the Rhône River, as well as the temporal contamination trends of historical (metallic elements, polychlorinated biphenyls) or emerging (brominated flame retardants) pollutants, using cores from various environments. Such a retrospective approach is essential to better understand the many anthropogenic impacts on fluvial systems, to relate those impacts with present river management issues, and to suggest policies and monitoring and/or restoration measures consistent with the evolution of the river over time.
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Sophia Vauclin. Influence des aménagements et des contaminations sur les héritages sédimentaires des fleuves anthropisés : le cas du Rhône. Ecosystèmes. Université de Lyon, 2020. Français. ⟨NNT : 2020LYSET016⟩. ⟨tel-03448844⟩

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