Bedform Kinematics in Modern Environments: Experimental Investigations
Spatiotemporal Bedload Transport Patterns over Bedforms
Collaborators: Dr. Mark Schmeeckle (Arizona State University)
Despite a rich history of studies investigating transport over bedforms and dunes in rivers, the spatiotemporal patterns of transport over bedforms and their link to complex bedform geometry remains poorly understood. Previous experiments assessing the effects of flow separation on downstream fluid turbulent structures and bedload transport suggest that localized, intermittent, high-magnitude transport events, called permeable splat events, play an important role in both downstream and cross-stream bedload transport near flow reattachment. Here, we report results from a set of flume experiments that assess the combined effects of flow separation/reattachment and flow reacceleration up the stoss side of the bedform. The flume was lined with 17 two-dimensional, concrete bedforms that had a 2 cm high crest and were 30 cm long. A high-speed camera observed bedload (d50=0.05 cm) transport along the entirety of the bedform at 250 f/sec. Grain trajectories, grain velocities, and grain transport direction were acquired from bedload images using semi-automated particle tracking techniques. Downstream and vertical fluid velocity was measured 3 mm above the bed using Laser Doppler Velocitmetry (LDV) at 15 distances along bedform profile.
As observed in the experiments of Leary and Schmeeckle (2017), mean downstream fluid velocity increases nonlinearly with increasing distance along the bedform (i.e. trough to crest). Observed bedload transport, however, increases linearly with increasing distance along the bedform with an exception at the crest of the bedform, where both mean downstream fluid velocity and bedload transport decrease substantially. If laterally consistent, this linear increase in bedload transport along the stoss side of the bedform is necessary for bedforms to retain their two-dimensional shape while translating downstream, but how do bedforms attain this pattern of bedload transport and when does it fail? Bedload transport time-series and manual particle tracking data show a zone of high-magnitude cross-stream transport near flow reattachment, suggesting that permeable splat events also play an important role in the region just downstream of flow-reattachment. A simple Exner Equation applied at the sub-bedform scale combined with our observations of particle motion, suggests two potential mechanisms that drive the transition from two-dimensional to three-dimensional bedform geometries: (1) the occurrence of splat events near flow reattachment and (2) localized, nonlinear increases in bedload transport rates along the stoss side of the bedform. These two processes may be genetically linked, and we suggest that (1) could drive (2).
The Importance of Splat Events on the Spatiotemporal Pattern of Bedload Transport over Bedforms: Laboratory Experiments Downstream of a Backward-Facing Step
Published at JGR: Earth Surface: Leary_Schmeeckle_2017
Project Summary: Despite numerous experimental and numerical studies investigating transport over bedforms 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. Experimentally observed sediment transport is compared 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. Localized, intermittent, high-magnitude transport events are more apparent near flow reattachment than farther downstream. These events are composed of downstream and cross-stream sediment transport of comparable magnitudes. Transport pattern and fluid velocity data are 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.