Sweep and clean : A laboratory investigation on fine-sediment erosion in immobile coarse-grained beds

In river systems characterized by sediment-supply limited conditions, such as mountain streams or river-reaches below damns, fine-sediments are transported over a coarse bed of cobble or gravel, immobile for most of the flow conditions. The prediction of sediment transport is particularly challenging, under these conditions, as it is strongly correlated to the level of protrusion of the immobile grains. The aim of this work is to investigate the influence of immobile grains on the fine-sediment erosion at the bedform- and grain-scale on the basis of an experimental approach in a laboratory flume. The immobile gravel bed was modelled with a staggered pattern of spheres (2 cm in diameter) with different roughness densities. Two types of fine sediments were used. Experiments were performed for varying Shields parameter, aspect ratio and relative submergence. The bed topography and the sediment transport were measured during the whole duration of the experiment (until a stable bed elevation was reached), while simultaneous stereo-PIV measurements were performed at selected staged of the bed development. A novel approach is developed to extend the stereo-PIV measurements to obtain data deep in between the roughness elements down to the level of the fine-sediments. The morphodynamic measurements at the reach scale showed the development of ridges with a lateral inter-ridge spacing of 1.3 times the water depth, which defines the characteristic ridge wavelength. The number of ridges is given by the integer number of wavelengths which fit into the effective width of the channel. It is shown that the reason for faster erosion over the troughs than over the ridges is connected to the local downflow that pushes the sweeps towards the bed. Both over the ridges and over the troughs, the analysis shows that the erosive behaviour changes with the protrusion level of the spheres $P$. Four stages take place: reduced erosion for $P$ 0.7. The first transition at $P_{c1}$ = 0.2 is connected to the development of a shear layer at the top of the spheres which induces enhanced fine-sediment shear stresses through generation of sweep events. The second transition transition for $P_{c2}$ = 0.4 is connected to the dominance of strong upflow conditions in the lee side of the spheres which locally prevent the shear layer to reach the fine sediments, damping the shear stress. The flow there is characterized by isotropic turbulent conditions. In the stoss side of the spheres, the shear layer is still able to reach the fine-sediment bed. Here the flow is characterized by the predominance of sweeps which effectively originate in the shear layer. The interaction between the shear layer and the sphere downstream suggests wake-interference regimes. The third transition for $P_{c3}$ = 0.7 is characterised by the switch to skimming flow conditions, as the upflow extends up to the top of the spheres pushing the shear layer out of the bed interstices. At the level of the bed critical conditions of motion are observed. In the lee side of the spheres, inward interactions are predominant, while in the stoss side isotropic turbulence is observed. The influence of the roughness density, Shields parameter, sediment size and relative submergence on the fine-sediment erosion were also investigated. For a generic rough bed, the second and third transitions can be identified by the protrusion levels for which the planar density of the roughness elements reach 0.3 and 0.5. The first transition is expected when the protrusion level corresponds to the angle of attack of approximately 20° for which the shear layer starts to develops