The Influence of Splat Events on the Spatiotemporal Pattern of Bedload Transport over Bedforms: Laboratory Experiments Downstream of a Backward-Facing Step

Abstract:

Despite numerous experimental and numerical studies investigating transport over ripples and dunes in rivers, the spatiotemporal details of the pattern of transport over bedforms remain largely unknown. Here we report turbulence-resolving, simultaneous measurements of bedload motion and near-bed fluid velocity downstream of a backward facing step in a laboratory flume. Two synchronized high-speed video cameras simultaneously observed bedload motion and the motion of neutrally buoyant particles in a laser light sheet 6 mm above the bed at 250 frames/s downstream of a 3.8 cm backward-facing step. Particle imaging velocimetry algorithms were applied to the laser sheet images to obtain two-dimensional field of two-dimensional vectors while manual particle tracking techniques were applied to the video images of the bed. As expected, the experiments exhibit a strong positive correlation between sediment flux and near-bed fluid velocity. The effect of flow separation on the pattern of sediment flux is explored by comparing experimentally observed sediment transport to sediment transport modeled as a function of boundary shear stress using a Meyer-Peter Müller type equation. Modeled sediment transport underestimates observed sediment transport near flow reattachment. This region of underestimated transport corresponds to an increase in the variance of near-bed vertical fluid velocity. Localized, intermittent, high-magnitude transport events are more apparent near flow reattachment than farther downstream. Often, these high-magnitude events are seen to have significant cross-stream particle velocities. In addition, downstream and cross-stream sediment transport events are of comparable magnitudes near reattachment. In contrast, farther downstream of the zone of underestimated transport, cross-stream transport events are small compared to downstream transport. The pattern of intermittent, high-magnitude cross- and downstream transport events near flow reattachment is consistent with the existence permeable “splat events”, wherein a volume of fluid moves toward and impinges on the bed. The substantial effects of splat events on transport over bedforms cannot be modeled using simple bedload transport equations and must be included in future models of bedform evolution.