We investigate the entrainment, deposition and motion of coarse spherical particles within a turbulent shallow water stream down a steep slope. This is an idealization of bed-load transport in mountain streams. Earlier investigations have described this kind of sediment transport using empirical correlations or concepts borrowed from continuum mechanics. The intermittent character of particle transport at low-water discharges led us to consider it as a random process. Sediment transport in this regime results from the imbalance between entrainment and deposition of particles rather than from momentum balance between water and particles. We develop a birth-death immigration-emigration Markov process to describe the particle exchanges between the bed and the water stream. A key feature of the model is its long autocorrelation times and wide, frequent fluctuations in the solid discharge, a phenomenon never previously explained despite its ubiquity in both nature and laboratory experiments. We present experimental data obtained using a nearly two-dimensional channel and glass beads as a substitute for sediment. Entrainment, trajectories, and deposition were monitored using a high-speed digital camera. The empirical probability distributions of the solid discharge and deposition frequency were properly described by the theoretical model. Experiments confirmed the existence of wide and frequent fluctuations of the solid discharge, and revealed the existence of long autocorrelation time, but theory overestimates the autocorrelation times by a factor of around three. Particle velocity was weakly dependent on the fluid velocity contrary to the predictions of the theoretical model, which performs well when a single particle is moving. For our experiments, the dependence of the solid discharge on the fluid velocity is entirely controlled by the number of moving particles rather than by their velocity. We also noted significant changes in the behaviour of particle transport when the bed slope or the water discharge was increased. The more vigorous the stream was, the more continuous the solid discharge became. Moreover, although 90 % of the energy supplied by gravity to the stream is dissipated by turbulence for slopes lower than 10 %, particles dissipate more and more energy when the bed slope is increased, but surprisingly, the dissipation rate is nearly independent of fluid velocity. A movie is available with the online version of the paper.

A longstanding problem in the study of sediment transport in gravel-bed rivers is related to the physical mechanisms governing bed resistance and particle motion. To study this problem, we investigated the motion of coarse spherical glass beads entrained by a steady shallow turbulent water flow down a steep twodimensional channel with a mobile bed. This experimental facility is the simplest representation of sediment transport on the laboratory scale, with the tremendous advantages that boundary conditions are perfectly controlled and a wealth of information can be obtained using imaging techniques. Flows were filmed from the side by a high-speed camera. Using image processing software made it possible to determine the flow characteristics such as particle trajectories, their state of motion ͑rest, rolling, or saltating motion͒, and flow depth. In accordance with earlier investigations, we observed that over short time periods, sediment transport appeared as a very intermittent process. To interpret these results, we revisited Einstein's theory on sediment and derived the statistical properties ͑probability distribution and autocorrelation function͒ of the key variables such as the solid discharge and the number of moving particles. Analyzing the autocorrelation functions and the probability distributions of our measurements revealed the existence of long-range correlations. For instance, whereas theory predicts a Binomial distribution for the number of moving particles, experiments demonstrated that a negative binomial distribution best fit our data, which emphasized the crucial role played by wide fluctuations. These frequent wide fluctuations stemmed particle entrainment and motion being collective phenomena rather than individual processes, contrary to what is assumed in most theoretical models.

Substantial variations in the particle flux are commonly observed in field measurements on gravel-bed rivers and in laboratory experiments mimicking river behavior on a smaller scale. These fluctuations can be explained by the natural variability of sediment supply and hydraulic conditions. We conducted laboratory experiments of particle transport down a two-dimensional inclined channel, for which the boundary conditions were properly controlled. Most flow variables and the features of particle trajectories were measured using a high-speed camera. The particles were 6-mm glass beads entrained by a rapid, turbulent, supercritical water flow. Even under these well-controlled experimental conditions and despite steady supply, solid discharge exhibited significant variations with time. The objective of this paper was to pinpoint the origins of these fluctuations by investigating different flow conditions. Two experiments were done with a fixed (smooth or corrugated) channel bottom and two others were run with a mobile bed (involving layers of closely packed particles lying along the channel base, which could be entrained by the stream); in the latter case, two particle arrangements were tested. It was found that, to a large extent, fluctuations reflected the finite size of the observation window. For fixed beds, the characteristic time scale of fluctuations and their probability distribution can be predetermined by evaluating the mean and fluctuating velocities of a single particle. Solid-discharge fluctuations were exacerbated when the bed was mobile because (i) the moving solid phase and the stationary bed exchanged particles and (ii) collective entrainment of particles occurred.

Polyacrylonitrile (PAN) is one of the few waterproof polymers that can be spun from relatively safe solvents, facilitating the use of PAN nanofi bre mats in diverse medical and biological applications, such as tissue engineering and cell growth promotion. PAN, on the other hand, is signifi cantly harder to use in electrospinning than polyethylene glycol and other water-soluble biopolymers. In our recent study, we thus varied spinning and material parameters for PAN dissolved in dimethyl sulfoxide (DMSO) and studied spinnability, as well as the resulting nanofi bre mat morphologies, using a "Nanospider Lab" needleless electrospinning machine. The results were examined using confocal laser scanning microscopy and atomic force microscopy. On the one hand, the images show that the relative humidity in the chamber plays a signifi cant role: excessively high values may cause undesired fi bre connections between oppositely charged parts of the Nanospider, creating cotton-candy-like structures that impede the free fl ow of fi bres to the substrate and thus the creation of the desired nanofi bre mat. On the other hand, the PAN concentration in the spinning solution is crucial: similar to the electrospinning of other (bio-)polymers, no fi bres are formed if the polymer concentration is too low. Third, the PAN material itself aff ects the nanofi bre creation process, illustrating that not every PAN is ideal for electrospinning.

Bedload sediment transport of two-size coarse spherical particle mixtures in a turbulent supercritical flow was analyzed with image and particle tracking velocimetry algorithms in a two-dimensional flume. The image processing procedure is entirely presented. Experimental results, including the size, the position, the trajectory, the state of movement (rest, rolling, and saltation), and the neighborhood configuration of each bead, were compared with a previous one-size experiment. Analysis of the solid discharge along the vertical displayed only one peak of rolling in the two-size bed, whereas three peaks of rolling appeared in the one-size case due to a larger collective motion. The same contrast is evidenced in spatio-temporal diagrams where the two-size mixtures are characterized by the predominance of saltation and a smaller number of transitions between rest and rolling. The segregation of fine particles in a bed formed by larger particles was analyzed taking into account the neighborhood configurations.