Erosion des sédiments cohésifs en auto-consolidation

Pollutants and suspended matters of a river can accumulate into the sedimentary column. Once deposited, they are submitted to self-weight consolidation processes, ageing and burying, leading to an increase of their erosion resistance. Pollutant fluxes can be related to sedimentary fluxes, determined by threshold laws. In this work, an erosion threshold model is suggested by introducing a cohesion force into the usual force balance. A model of cohesion is developed on the basis of interactions between argileous cohesive particles (clays), particularly the Van der Waals force, whose parameterization is ensured by means of granulometry and porosity. Artificial erosion experiments were performed in a recirculating erosion flume with natural cored sediments where critical shear stress measurements were performed. Other analyses provided granulometry and porosity. The results obtained constitute a good database for the literature. The model is then applied to the experimental conditions and gives good agreement with measurements. An example of the accounting for self-weight consolidation processes is finally suggested, before finishing on a Mohr like diagram dedicated to soft cohesive sediment erosion.Les polluants et les matières qui transitent dans les cours d'eau peuvent constituer des stocks dans la colonne sédimentaire. Ces dépôts, soumis aux processus d'autoconsolidation, sont sujets au vieillissement et à l'enfouissement, qui augmentent leur temps de résidence dans la colonne sédimentaire par augmentation de leur résistance à l'érosion. Les flux de polluants peuvent être ramenés aux flux de matières sédimentaires, dont l'érosion est décrite par des lois à seuil. Nous proposons une modélisation du seuil d'érosion d'un sédiment cohésif. Ce modèle considère les propri étés cohésives des matières qui composent les couches sédimentaires, en s'appuyant sur la force attractive électrodynamique de contact de Van der Waals. Cette force, que nous avons paramétrée par la granulométrie et la porosité, vient s'ajouter au bilan des forces généralement considéré pour modéliser le seuil d'érosion. Des expériences d'érosion artificielle ont été menées dans un canal à érosion, avec des sédiments naturels prélevés dans la partie en aval du Rhône, pour en évaluer le seuil d'érosion, la granulométrie et la porosité. Les résultats des expériences alimentent la littérature en données expérimentales sur l'érosion de sédiments cohésifs consolidés. Le modèle d'érosion ainsi développé est appliqué dans les conditions expérimentales. Il donne un bon accord avec les mesures. Nous illustrons sa prise en compte dans un modèle de consolidation, permettant de construire des diagrammes du type Mohr pour l'érosion d'un sédiment cohésif faiblement consolidé

Considering the sedimentary diagenesis in models of radionuclide transfers in rivers

When radioactive effluents are released into water, the pollutant is dispersed by the turbulent flow and it is distributed between a dissolved phase and a phase fixed on the suspended matter and the bottom sediment. Transfer between these two phases depends on flows of solid matter resulting from the sedimentary dynamics within the river. Particularly, the processes of deposition and erosion condition the formation of stocks of matter (and their associated pollutants) in some places of the river. These processes depend on both flow conditions and sedimentary diagenesis, i.e., on the degree of cohesion of the sediments which increases with the quantity and the age of the settled matter. The subject of this work concerns the resuspension processes of the pollutant confined in the bottom sediment of rivers. It deals with the middle- and long-term becomings of the stocks. Which quantity of matter (and thus of pollutants) can be resuspended towards the water column, and under which conditions

Two stable methods with numerical experiments for solving the backward heat equation

International audienceThis paper presents results of some numerical experiments on the backward heat equation. Two quasi-reversibility techniques, explicit filtering and structural perturbation, to regularize the ill-posed backward heat equation have been used. In each of these techniques, two numerical methods, namely Euler and Crank-Nicolson (CN), have been used to advance the solution in time. Crank-Nicolson method is very counter-intuitive for solving the backward heat equation because the dispersion relation of the scheme for the backward heat equation has a singularity (unbounded growth) for a particular wave whose finite wave number depends on the numerical parameters. In comparison, the Euler method shows only catastrophic growth of relatively much shorter waves. Strikingly we find that use of smart filtering techniques with the CN method can give as good a result, if not better, as with the Euler method which is discussed in the main text. Performance of these regularization methods using these numerical schemes have been exemplified

Criteria of Erosion for Cohesive Soils

International audienceThis paper focus on three types of erosion: the suffusion, the backward erosion, and the piping erosion processes. A new triaxial device to quantifying suffusion and backward erosion, and a new model for interpreting the Hole Erosion Test, are presented. It is recommended to use these erosion tests in order to evaluate the erosion parameters on any sample of cohesive soil from a site

Observation, analysis and modelling in complex fluid media - Settling classes of fine suspended particles

International audienceThe modelling of pollutant transfer in freshwater systems can be refined by considering the heterogeneities of the sedimentary dynamics and of the chemical reactivities of fine suspended particles. One of the first steps is the fractionation of these fine particles into effective settling classes. Although several methods exist, most of them are based on either granulometric considerations and/or arbitrary threshold criteria. This article presents the bases of an experimental method focusing on the direct measurement of the settling velocities without considering the granulometry and/nor any threshold criteria. The experimental work consists in recording the temporal evolution of the vertical distribution of the suspended solid concentration in a settling tank. A mathematical analysis provides the number of particle groups, and the mass contribution and the settling velocity for each This procedure is described and applied for validation, as a first step, to calibrated suspensions. Additional work is needed for a further analysis of the physical constraints involved in the model, as well as for more extensive experimental validation