Longitudinal dispersion coefficient for mixing in open channel flows with submerged vegetation

Abstract

An experimental investigation was conducted to analyze the flow and pollutant mixing characteristics in the vegetated channel, in which the velocity data was collected by micro acoustic Doppler velocimeter, and pollutant dispersion data was obtained through tracer tests in an open channel with submerged vegetation. The effects of submergence ratio (S-r = 2.0-3.5), and stem density (M = 0.4-5.9) on the vertical distribution of the stream-wise velocity and the turbulent shear stress were analyzed. The results of the flow experiments showed that the intensity of velocity deviation became larger with increasing stem density and decreasing submergence ratio. The turbulent shear stress at the exchange zone also increased with submergence ratio, as well as with increasing stem density. The calculation results of the longitudinal dispersion coefficient based on the vertical velocity profile data showed that as the submergence ratio and stem density increased, the dispersion coefficients increased linearly. This monotonic increase of the velocity-based dispersion coefficient differed from former research, in which concentration-based dispersion coefficient values approached a constant value in a higher submergence ratio of over 2. Comparison of the velocity-based longitudinal dispersion coefficients with concentration-based coefficients in this research demonstrated that the velocity-driven coefficient had a linear relation with the concentration-driven coefficients, with smaller values than the concentration-based dispersion. This difference can be explained by the fact that concentration-driven dispersion coefficient included the storage effects due to the submerged vegetation, while the velocity-driven coefficient only accounted for shear flow effects.