Fine-sediment erosion in coarse-grained beds

In sand-gravel bed rivers, sand is transported over gravel which are immobile for most flow conditions. While it has been observed that the erosion rate of fine sediments can be enhanced or dampened depending on the level of exposure of immobile coarse grains, the hydrodynamics behind these two competing erosive behaviours are poorly understood. To this end, a flume study was performed to analyse the erosion of an initially uniform fine-sediment bed covering a two-layered staggered array of immobile spheres. Stereo photogrammetric measurements of the bed topography and stereo-PIV measurements of the flow deep within the canopy were performed simultaneously as the bed is eroded. The topographic measurements reveal four different regimes as the protrusion of the spheres increases: (i) erosion rate reduction, (ii) erosion rate enhancement, (iii) erosion rate damping and finally, (iv) bed stabilization. The PIV measurements show that the first transition from erosion rate reduction (i) to enhancement (ii) is due to the development of a shear layer at the canopy crest. The onset of erosion rate damping (iii) is due to the evolving upflow conditions in the spheres’ lee leading to wake-interference flow conditions. Finally, in regime (iv), the upflow dominates the interstitial canopy flow, pushing the shear layer out of the canopy, which leads to skimming flow conditions. The fine sediment is then completely sheltered with critical conditions of motion.