Longitudinal and transverse dispersion coefficients of 2D contaminant transport model for mixing analysis in open channels

Abstract

In this study, longitudinal and transverse dispersion coefficients of the two-dimensional advection-dispersion model were calculated using both the velocity and concentration data obtained through tracer tests in large-scale meandering channels. The velocity-based dispersion coefficients calculated by applying the theoretical equations to the velocity data were compared with concentration-based dispersion coefficients that were calculated by applying the routing procedure to the measured concentration curves. The results showed that the cross-sectional averaged values of the dimensionless longitudinal dispersion coefficient, DL/HU∗, calculated from the velocity profile data ranged 4.1–6.5, which corresponded to the theoretical value, whereas DL/HU∗ by concentration data ranged 14.7–35.5, which was 4–6 times larger than the velocity-based coefficient. The cross-sectional maximum values of the dimensionless transverse dispersion coefficient, DT/HU∗, by the velocity-based method ranged 0.1–3.3, whereas DT/HU∗ by concentration-based method was 0.9–2.9. These results revealed that the velocity-based longitudinal dispersion coefficient is a fractional value of the concentration-based result, which contained mixing caused by channel irregularities throughout the reach, in addition to shear dispersion by vertical velocity variations, while transverse dispersion coefficients by both methods represented the same dispersion mechanism which was dominated by the secondary currents developed in highly sinuous channels.