Infiltration of fine sediment into a coarse mobile bed: a phenomenological study

International audienceExperiments were undertaken to study the nature of granular interaction in running water by examining the influence of fine grain inputs to a coarser sediment bed with a mobile surface. Video recordings of grain sorting by both kinetic sieving and spontaneous percolation are used to diagnose the critical processes controlling the overall bed response. Kinetic sieving takes place in the mobile bed surface, with the finer sediment moving to the bottom of the bedload transport layer at the interface with the underlying quasi-static coarse bed. We show that the behavior at this interface dictates how a channel responds to a fine sediment input. If, by spontaneous percolation, the fine sediment is able to infiltrate into the underlying quasi-static bed, the total transport increases and the channel degrades. However, if the fine sediment input rate exceeds the transport capacity or is geometrically unable to infiltrate into the underlying bed, it forms a quasi-static layer underneath the transport layer that inhibits entrainment from the underlying bed, resulting in aggradation and an increase in bed slope

Are Results in Geomorphology Reproducible?

International audienceThere recently has arisen substantial concern for the reproducibility of scientific findings, but the discussion has not significantly impacted Earth science. We consider repetition, replication, and reproducibility in Earth science, using an example from geomorphology. Repetition repeats the program of observations in the same exercise to establish precision of results. Replication is duplication of observations using similar resources but in an independent program. Reproduction is confirmation of scientific principles using different resources in an independent program. We conclude that results will mainly be limited to reproduction-confirmation of principles-and that this is the essential goal for advancing the science. We illustrate these concepts by review of our experiments on the infiltration of fine grains in flowing water into a bed of coarser grains, conducted using glass beads in a laboratory flume.Plain Language Summary Recent years have witnessed a rapid increase in concern for the confidence that can be placed in scientific results. The key to establishing such confidence is reproduction of the result under independent circumstances. The issue has been less discussed in Earth science than in other disciplines. In this commentary we draw attention to the issue using an example from geomorphology involving the infiltration of fine sediments in flowing water into a bed of coarser sediments

Infiltrations de grains fins en charriage: la transition vers un nouveau equilibre

International audienceSimplified experiments on fine grain inputs to a coarse bed in mobile equilibrium were undertaken in a small, steep, narrow flume using spherical glass particles to study the influence upon the channel response of the size ratio between the bed (Dc) and the input fines (Df). Size ratios (Dc/Df) between 7.14 and 1.25 were tested, with a constant flow and coarse sediment feed rate, and a variety of fine and total feed rates. Transition to a new, two-size equilibrium occurs through slope adjustment (aggradation/degradation), accompanying a change in sediment mobility created by the addition of the fine material. Previous work has documented superior mobility following a fine grain input; the present experiments identify limits of this behaviour related to the fine grain size, the proportion of fines introduced, and the total sediment supply. The mechanistic reasons for these limits are examined with respect to the grain sorting behaviour, leading to the development (or not) of a quasi-static layer of the fine material at the base of the transport layer. Despite the variation in bed slope response depending upon these factors, the slope transitions consistently follow an exponential profile

Influence du ratio de taille et de la concentration des fines sur l'évolution du lit par tri granulométrique de mélanges bimodaux

International audienceGrain size sorting in bed material has distinct implications for sediment transport in gravel-bed rivers. As a consequence, the behavior of mixtures differs from that of uniform material. It is essential that understanding of grain size sorting, and its influence upon sediment transport is deepened due to implications for channel stability, ecology and stratigraphy. Previous work has shown how the addition of finer material to a coarse channel bed can enhance the mobility of the coarser sediment due to a reduced entrainment threshold. This change in mobility has been indexed using the change in equilibrium slope within the channel. However, it is not yet known how variations in the grain size ratio (diameter of coarse/diameter of fine), along with the concentration of fine material, influences this behavior. New experimental research has been undertaken which, firstly identifies that degradation can occur when fine sediment is added to a coarse bed, and then shows the grain size ratios and fine sediment feed concentration at which this arises. Additionally the amount of degradation under varying conditions is quantified using the change in equilibrium bed slope. Futhermore, this research also shows that under certain conditions, aggradation can also occur due to the addition of finer sediment to a coarse channel bed. This aggradation, which occurs under given grain size ratios and fine sediment concentrations, is also quantified using the change in equilibrium bed slope. This experimental work was undertaken using bimodal mixtures of spherical glass particles in a relatively narrow sediment-feed-flume. This experimental arrangement allows the control of input conditions, and permits observation of the individual and bulk particle motion