Flow phenomena and mechanisms in a field-scale experimental meandering channel with a pool-riffle sequence: Insights gained via numerical simulation

Abstract

Large-eddy simulation of turbulent flow through a natural-like meandering channel with pool-riffle sequences installed in the St. Anthony Falls Laboratory Outdoor StreamLab is carried out to elucidate the hydrodynamics at bankfull flow condition. It is shown that the shallow flow in the riffle is dominated by the presence of large-scale roughness elements that enhance turbulent mixing; increase turbulence anisotropy; and induce multiple, streamwise secondary cells driven by turbulence anisotropy. The flow in the pool, on the other hand, is dominated by the formation and interaction of the center region and outer bank secondary flow cells and the large horizontal recirculation regions along the inner bank. The collision of the counterrotating center region and outer bank cells at the water surface gives rise to a line of three-dimensional separation (flow convergence) in the time-averaged streamlines at the surface and the associated strong downward flow toward the bed that redistributes streamwise momentum and increases the bed shear stress along the channel thalweg. Intense turbulence is produced along the line of separation due to highly anisotropic velocity fluctuations. Our results make a strong case that the center region cell is driven by the curvature effects while the outer bank cell is driven by the combined effects of turbulence anisotropy and the curvature-induced centrifugal force. The inner bank horizontal recirculation zone consists of multiple eddies, which collectively span the entire point bar. A striking finding is that the center of the primary eddy is located directly above the crest of the point bar.