Laboratory analysis on silt infiltration into a gravel bed
Modélisation physique d'infiltration de sédiments fins dans un lit de graviers
Résumé
Fine sediment infiltration into a gravel bed may have significant impact on environmental, socioeconomic and river morphodynamics issues. Several human activities enhance fine sediment flux, either in the production zone due to changes in land use or within the river system due to reservoir flushes or dredging. Understanding the mechanisms that control this phenomenon can help to minimize impacts of such anthropic actions. A set of experiments has been performed in the new tilting flume (18 meter long and 1 meter wide) of Irstea-Lyon hydraulics laboratory. Silt sized silica (10-30 microns) and gravels (3-12 mm) are used as fine and coarse sediments, respectively. Fine sediments are introduced for a specified concentration in a flow with fixed discharge, water depth and bed slope. All the explored conditions correspond to fine sediment travelling in homogeneous suspension (wash load). Keeping the concentration constant during each test, the process of infiltration of fine sediments into the gravel bed is analyzed and the influence of flow velocity and fine sediment concentration is discussed. Fine sediment deposition takes place as a progressive filling of the bed starting from the base of the channel and ending one to three diameters below the bed surface. The thickness of the surface layer that remains unclogged in the equilibrium situation depends mainly on the bed shear stress. On the other hand, a positive correlation is found between the suspended sediment concentration and the rate of infiltration with time. Water mass exchange and turbulent structures that occur at the flow-bed interface seem to be the key mechanisms that control fine sediment infiltration.
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