Use of drag coefficient to predict dispersion coefficients in emergent vegetation at low velocities

Existing functions for predicting transverse and longitudinal dispersion coefficient within vegetation are dependent on the drag coefficient. This paper focuses on comparing drag coefficients and on comparing predictions of dispersion coefficient derived from drag coefficient to estimates of dispersion coefficient from laboratory measurements. Three drag coefficients have been considered: drag coefficient estimated from turbulent kinetic energy measurements; drag coefficient predicted from vegetation characteristics; and an assumed drag coefficient of 1. Predicted dispersion coefficients have been compared to estimated dispersion coefficients in two types of artificial vegetation and two types of real vegetation (Carex and Typha). Dispersion coefficients in Carex were not well predicted. All three drag coefficients produced reasonable predictions of dispersion coefficients in the artificial vegetation and Typha, showing the functions that predict dispersion coefficient to be relatively insensitive to drag coefficient. The use of a drag coefficient of 1 is shown to be reasonable for predicting dispersion coefficient.