Spatiotemporal Structure and Covariance of Bedload Motion and Near-Bed Fluid Velocity over Bedforms: Laboratory and Numerical 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. Details are compared to a coupled large eddy simulation and distinct element simulation (LES-DEM) of the same geometry.

Two synchronized high-speed video cameras simultaneously observed bed load 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, there is a strong positive correlation between sediment flux and near-bed fluid velocity. Sediment flux was determined by manually tracking grains that passed over a 6 cm long line in the middle of the field of view on the bedload images. Sediment flux increased monotonically downstream of flow reattachment. Localized, intermittent, high-magnitude transport events were more apparent near flow reattachment than further downstream. Often, these high-magnitude events were seen to have significant cross-stream particle velocities. These events are consistent with permeable “splat events” visualized in the LES-DEM numerical simulations, wherein a volume of fluid moves toward and impinges on the bed. Fluid impingement and penetration of the bed results in outward flow and sediment motion from the center of the splat. Work is ongoing to quantify spatial and temporal autocorrelations and covariances of the fluid velocity and sediment motions.