COMPUTATIONAL STUDY OF FLOW DYNAMICS IN A FIELD-SCALE EXPERIMENTAL MEANDERING STREAM WITH A POOL-RIFFLE SEQUENCE

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 flow in the pool 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 counter-rotating center region and outer bank cells at the water surface gives rise to a line of 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 flow convergence due to highly-anisotropic velocity fluctuations. Our results make a strong case that the line of flow convergence is generated by the collision of two counter-rotating secondary cells. 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.