Application of the Tidal Random Walk Model to Calculate Water Dispersion Coefficients in the Ems Estuary

Abstract

Longitudinal and lateral dispersion coefficients are computed for the Ems Estuary using a modified version of the tidal random walk model developed by Zimmerman. This model relates the mixing properties of the flow to the velocity and length scales of the tidal current and the residual eddies. It is found that values of the longitudinal dispersion coefficient are 200–1200m2s−1, depending on the position in the estuary, and lateral dispersion coefficients are a factor ofc. 50 smaller. These results are three to four times larger than earlier reported values, which were determined from field observations. This difference is attributed to the fact that in the tidal random walk model, the dependence of the dispersion mechanism on the detailed kinematics of the flow is neglected, since a statistical distribution of residual eddies is assumed. However, the model predicts the spatial distribution of the dispersion coefficients in the estuary quite well. Particular attention is paid to the presence of three peaks in the dependence of these coefficients on the distance to the river mouth. Using the concepts underlying the tidal random walk model, these peaks are related to the presence of tidally induced residual circulations around sand banks. It is also concluded that the effect of shear dispersion on the horizontal mixing processes in the Ems Estuary can be neglected.