Sediment resting, entrainment and depositional patterns in a gravel bed: Connecting bedload transport with turbulence structure

This research is founded on the consideration that bedload transport in gravel-bed rivers is interrelated in many ways with turbulence structure. As such, the research includes semi-theoretical modeling and open-channel flume experimental studies that improve our understanding of bedload transport by incorporating knowledge about turbulence, in addition to improving our fundamental knowledge about turbulence.

The first study focused on sediment resting time, which is a minimally understood component of sediment virtual velocity. A semi-theoretical model for resting time that considers momentum transfer from near-bed turbulence was developed, calibrated, and verified. The formulation also included parameters that consider sediment size nonuniformity and bed characteristics. Simulated resting times compared well with experimental data, making the model a practical and physics-based prediction tool.

The second study focused on the effects of boulders in mountainous gravel-bed streams and examined timescales of bedload pulsation in their vicinity. Using existing data, two distinct timescales were identified in bedload time series obtained at the exit of a laboratory boulder array. The largest pulsation timescale is suggested to relate to local bedload deposition and mobilization cycles around boulders, which are believed to result from the interaction of eddy structures forming around boulders with mobile sediment.

Motivated by observations that boulder submergence affects the location of sediment deposition around boulders, the third study utilized laboratory volumetric particle image velocimetry (PIV) measurements to document the effects of submergence on flow upstream of boulders. Multiple flow field analyses were performed and showed delayed flow separation, suggested flow intrusion into the porous bed upstream of the boulder, and identified atypical characteristics of the horseshoe vortex system. Comparing partially and fully submerged boulder experiments, stronger flow deceleration was observed for partial submergence and is believed to promote upstream deposition of sediment for these conditions.

The fourth study characterized Reynolds stress anisotropy around fully submerged boulders via volumetric PIV data. Spatially organized features of anisotropy were identified, such as streamwise-aligned anisotropy bands in the wake and strongly anisotropic behavior occurring near typical vortex structure locations. This unique volumetric characterization of turbulence anisotropy provides a target condition for future testing and validation of numerical models.