Field and numerical study on natural river mixing
Swick, William A.
MacMahan, Jamie H.
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Mixing in several natural rivers is investigated using comprehensive point-source dye experiments, Lagrangian GPS-equipped drifter observations and a validated three-dimensional hydrodynamic model. The high spatial and temporal observations provide estimates of mixing that were previously unobtainable. The presence of river irregularities are shown to greatly enhance the local streamwise and transverse diffusivity due to large-scale horizontal coherent flow structures. Tracer study transverse diffusivity was initially small and increased following channel features. Model simulations compared well with field observations. Idealized bathymetric features reveal transport is dominated by the mean flow and combined channel feature influence is nonlinear. Lagrangian analysis of continuous drifter observations describe fine-scale natural river processes and provides quantitative estimates of the mean flow field, pathways, and spatial variability of mixing in natural rivers. Near-field diffusivity estimates are shown to be independent of drifter deployment location and the effect of river bends on streamwise and transverse diffusivity is quantified. Single-particle streamwise diffusivity increased linearly associated with turbulence and velocity shear. Two-particle streamwise diffusivity scales as Richardson-like. Reaches with numerous bends resulted in anomalously small two particle diffusivities scaling with river bends owing to surface flow convergence. Transverse length scales >20m are predominantly random.
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