Feasibility of Velocity-Based Method for Transverse Mixing Coefficients in River Mixing Analysis

The feasibility of using the velocity-based method for calculating the transverse mixing coefficient of the two-dimensional contaminant transport model was studied to substitute the concentration-based method in which the mixing coefficient is calculated from the concentration curves obtained via the costly tracer experiment. To calculate the transverse mixing coefficients, the hydraulic data and concentration data of the electric conductivity (EC) were collected at the confluence of tributary rivers in the Nakdong River, South Korea. The experimental results showed that the transverse mixing was controlled by secondary currents that were caused by the merging of the tributary rivers as well as the meandering of the main channel in the study area. The comparison of the flow-weighted transverse mixing coefficients estimated from velocity measurements with values estimated from concentration measurements revealed that a linear relationship with a fitted slope of 1.310 was found between the two values. This discrepancy occurred because the velocity-based method contains only shear effects due to the vertical velocity deviation, while the concentration-based method embraces other mixing effects, such as channel irregularities, storage zone effects, and turbulent diffusion, as well as the shear effect.