Near-bed hydrodynamics controlling fine-sediment erosion over immobile coarse-grained beds

The erosion of fine sediments from an immobile coarse-grained bed depends on the degree of exposure of the coarse grains. In an earlier study (Trevisson & Eiff, Water Resour. Res. , 2022, vol. 58, issue 11, p. e2021WR031837), we showed that, under sediment-supply limitation, erosion evolves through four regimes of alternating enhancement and sheltering. However, the hydrodynamic processes governing these transitions are still unresolved. Here, we provide the first direct experimental link between erosive regimes and canopy-scale flow dynamics via simultaneous measurements of bed topography, bedload transport and flow within the canopy as the fine sediments are eroded from a two-layer array of spheres under uniform flow. The four erosive regimes correspond to systematic changes in near-bed flow structure as the spheres’ protrusion level upper P P P increases. The initial reduction in erosion during the drag-sheltering regime ( upper P less than 0.2 P < 0.2 P< 0.2 ) is attributed to increased form drag without boundary-layer separation. In the turbulent enhancement regime ( 0.2 less than upper P less than 0.4 0.2 < P < 0.4 0.2< P< 0.4 ), a shear layer develops at the canopy top, whose sweep and ejection events penetrate to the fine-sediment bed, enhancing shear stress and sediment transport. The wake-interference sheltering regime ( 0.4 less than upper P less than 0.8 0.4 < P < 0.8 0.4< P< 0.8 ), characterised by erosion-rate damping, is associated with an upflow region in the spheres’ lee which inhibits shear-layer penetration, producing quasi-isotropic turbulence and reduced near-bed shear stresses. In the spheres’ stoss, weaker shear-layer turbulence persists, sustaining the erosion. In the skimming-flow sheltering regime ( upper P greater than 0.8 P > 0.8 P> 0.8 ), upflow expands across the canopy and lifts the shear layer above the canopy top, leading to reduced turbulence anisotropy and damped streamwise-velocity fluctuations near the bed. The resulting skimming flow shelters the fine-sediment bed.