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Utilisation de la morphogranulométrie dans le suivi source to sink du transfert particulaire dans les bassins versants

Abstract : Source to sink is one of the main concepts in Earth Sciences for a better knowledge of hydrosystems dynamics. Regarding this issue, one challenge consists in the characterization by in-situ measurements of the sources of suspended sediment, or sediment source fingerprinting. Few methods can fully cover such requirements and among them, the methodology using the grain shape deserves to be developed. Indeed, morphometry of particles is widely used in sedimentology to identify different sedimentary stocks, source-to-sink transport and sedimentation mechanisms. Currently, most grain shape analyses are carried out with a scanning electron microscope coupled to image analysis to measure various size and shape descriptors on particles like elongation, circularity, bluntness or fractal dimension. However, complexity and time of analysis are the main limitations of this technique for a long-term monitoring of SPM transfers. Here we present an experimental morphometric approach using a morphogranulometer (a CCD camera coupled to a peristaltic pump). The camera takes pictures while the sample is circulating through a flow cell, leading to the analysis of numerous particles in a short time. The image analysis provides size and shape information discriminating various particles stocks by statistical analyses. The aim of this thesis is (1) to calibrate the device and understand how the size and shape data can discriminate pure sediment samples; (2) use those results to evaluate the contribution of contrasted sedimentary sources in a mix; (3) to establish a measurement and data analysis protocol; and (4) use those protocol and methodology for several case studies of sediment transfers in various hydrosystems. The first case study presented concern the watershed of the Thio river, on which there is mining activity. A sediment fingerprinting study using size and shape characterization was carried on. The sediment sources of the different tributaries of the Thio river were analyzed as sources and the sediments of the main river itself were used as mixes of these sources. A model of the contributions of the sources in the mixes was achieved and its results show great coherence. It brings information on the transfers of the sediments between the different tributaries and the Thio river which will be later combined to other studies using different fingerprinting methods to evaluate the impact of the mining activity in these transfers. The second study focus on the karstic watershed of the Hannetot spring. Measurements were carried out on various field samples, from suspended sediments to core samples. Several experiments allowed to identify the varying contributions of different sediment sources through time. Several proxies were identified, seasonal and meteorological, as controlling factor for these variations. The last study is also a fingerprinting study of a karstic system. Its aim is to evaluate the origin of sediment sampled during turbidity peaks in the drinking water well of the town of Gisors. The source contribution model identified several potential sources of sediments and shows the increased contributions of certain sources during the flood period. All those studies shows that size and shape measured with the morphogranulometer are a good tracer for fingerprinting studies. They also demonstrate that this technique performs better in combination to other methods and brings the need to carry more studies combining morphogranulometry with several other fingerprinting techniques.
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David Viennet. Utilisation de la morphogranulométrie dans le suivi source to sink du transfert particulaire dans les bassins versants. Sciences de la Terre. Normandie Université, 2020. Français. ⟨NNT : 2020NORMR016⟩. ⟨tel-02966950⟩

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