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[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.

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Flume tanks are becoming increasingly important research tools in aquatic ecology, to link biological to hydrodynamical processes.There is no such thing as a ''standard flume tank'', and no flume tank is suitable for every type of research question. A series of experiments has been carried out to characterise and compare the hydrodynamic characteristics of 12 different flume tanks that are designed specifically for biological research. These facilities are part of the EU network BioFlow. The flumes could be divided into four basic design types: straight, racetrack, annular and field flumes. In each facility, two vertical velocity profiles were 2006 measured: one at 0.05 m s -1 and one at 0.25 m s -1 . In those flumes equipped with Acoustic Doppler Velocimeters (ADV), time series were also recorded for each velocity at two heights above the bottom: 0.05 m and 20% of the water depth. From these measurements turbulence characteristics, such as TKE and Reynolds stress, were derived, and autocorrelation spectra of the horizontal along-stream velocity component were plotted. The flume measurements were compared to two sets of velocity profiles measured in the field.Despite the fact that some flumes were relatively small, turbulence was fully developed in all channels. Straight and racetrack flumes generally produced boundary layers with a clearly definable logarithmic layer, similar to measurements in the field taken under steady flow conditions. The two annular flumes produced relatively thin boundary layers, presumably due to secondary flows developing in the curved channels. The profiles in the field flumes also differed considerably from the expected log profile. This may either have been due the construction of the flume, or due to unsteady conditions during measurement. Constraints imposed by the different flume designs on the suitability for different types of boundary layer research, as well as scaling issues are discussed.

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This study concerns turbulent jets with density variations. The local statistical properties, obtained for the velocity field and the mean mass fraction, are needed both for better understanding of the density effects and for modeling of such flows. Special attention is paid to the near-field region (x/Dj≤20), where there is a lack of experimental results. A vertical axisymmetric turbulent jet emitted from a fully developed pipe flow, weakly confined in an air coflow, is investigated. Apart from the constant density case (air/air), two situations with variable density are treated with a density ratio varying from 0.14 [He/air] to 1.5 [CO2/air], with the momentum flux Mj maintained constant. Even though there is no exact similarity for jets when density varies, some pseudosimilarity laws can be established, and they are experimentally well confirmed. It is also found that the values of the correlations 〈u2v〉, 〈uv2〉, and 〈uw2〉 are negative in the central region (r/Lu<0.5), and this is valid for the three gases considered here. On the other hand, the use of the results for calculating balances of some of the flow conservation equations (i.e., continuity, momentum, and kinetic energy) allows us to confirm some measurements and particularly to estimate some terms that are not, and cannot be, measured at present in variable density flows, such as the dissipation rate of turbulent kinetic energy.