[1] Predicting the erosion resistance of saturated natural sediments requires taking into account cohesion, which results from interactions between clay particles. The current paper describes a combined experimental and theoretical examination of the threshold conditions for a mixture of clays and sands. Erosion threshold measured values are larger than those predicted from noncohesive models. Beyond the usual dependence on grain size, a significant correlation between erosion threshold and porosity measurements is confirmed for heterogenous mixtures of grains with particle Reynolds number lower than 5, for pH values in the range of 6-8, and under freshwater conditions. A model of the erosion criterion is proposed. First, a cohesion force between two spherical particles is introduced into the usual erosion criterion. This reveals a specific function of the cohesion force, called cohesion function. The force we consider is the long-range van der Waals interaction. Then, multiple interactions between one particle and those surrounding it are counted and modeled on the basis of both the coordination number and the porosity. Finally, the overall erosion threshold for the sediment bed is inferred from the average of the multiple interactions over the grain size distribution. The model highlights that cohesion comes from clay particles (about 2 10 À6 m) and can affect the entire grain size range (from clays to sands) by means of coordination. The model relevance is assessed by comparison with experimental thresholds obtained from resuspension campaigns. The results show that the proposed cohesion model offers good agreement with experimental data.

The present Note reports a detailed analysis of some of the specific flow properties which are associated with the development region of circular pipes and ducts of square or rectangular cross-sections in turbulent conditions. Indeed, there are no data presently available to determine a priori the acceleration effect which results from the progressive thickening of the boundary layers on the pipe walls. In addition, published results concerning the longitudinal extension of this development region are rather confusing. Through our study, on the one hand, the litigious points are clarified, and, on the other hand, the pertinent grouping of variables (which include the hydraulic diameter D h and the boundary layer displacement thickness δ 1 ) providing the parameter relevant for obtaining similarity between all the situations is displayed. To cite this article: F.

The 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.

Abstract. This paper presents a dynamic box model for the radionuclide behaviour in rivers on medium-and long-term periods (several days to several years). The river is described as a succession of boxes representative of its different reaches. In each reach, the compartments are the water column and three bottom sediment layers. Called interface, the first layer plays a fundamental role for the vertical exchanges of solid radionuclide phases between the water column and the sediment. The second layer results from the consolidation of the previous one. Its interstitial water is mobile and the dissolved radionuclide phases can be exchanged with the water column. It is called active. The last layer results from the consolidation of the active layer. Its interstitial water is slightly mobile and it is assumed that its dissolved radionuclide phases cannot be exchanged. It is called passive. In each compartment, the model computes the temporal evolution of the radionuclide activities in the main abiotic and biotic components. The abiotic components are the water and different matter classes classified according to their deposit kinetics. The biotic components are phytoplankton, zooplankton, and fish distributed in planktonivorous and omnivorous species, in water column and macrobenthos in bottom sediment.

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