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

Abstract

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