Turbulent mixing of floating pollutants at the surface of the river

Tracer tests were conducted using floaters equipped with a global positioning system (GPS) to investigate the turbulent mixing of the floating pollutant at the surface of natural rivers. The position data were collected by tracking the floaters at different reaches of the Nakdong River in Korea, and the diffusion coefficients were calculated by applying the moment method to the floater distribution data collected. While applying the moment method, the diffusion coefficient was calculated from the time evolution of variance of the particle distribution, which becomes the distance from the centroid of the particle cloud, based on the homogeneous turbulence assumption and the ergodic hypothesis of the turbulent mixing. Analysis of the trajectories of GPS floaters showed that the horizontal mixing in the initial mixing stage was hampered by the interaction and collision between floaters. The experimental results revealed that the dimensionless streamwise diffusion coefficients were larger, in the range of 0.11-0.56, while the spanwise component was in the range of 0.02-0.16. The simulation results using the Lagrangian particle tracking model of the Environmental Fluid Dynamics Code program showed that the discrepancy between the predicted values and the observed data in the initial time gradually decreased as time elapsed.