Determination of Longitudinal Dispersion Coefficient of a Channel Having Rigid Vegetation

Abstract

In recent years, aquatic vegetation has become more accepted and important plans for restoration of rivers and preservation of river ecology. The purpose of this thesis is to investigate the effect of vegetation on longitudinal hydrodynamic dispersion and to have a better understanding the effect of vegetation on the mixing processes. To achieve this goal, we performed a series of experiments in an open channel with emergent rigid acrylic tubes of three different diameters in staggered configuration with three different spacing between tubes. The speed measurement of the tracer concentration for nine vegetation densities in 27 numbers of experiments. The coefficient of dispersion is depended on the diameter of rigid vegetation, when a rigid vegetation of different diameter is kept at a distance from point of injection the dispersion coefficient value increases in magnitude and it is inversely related with the density of vegetation till some extent, but it is directly proportional with the diameter of vegetation as the diameter increases the dispersion coefficient is also increases. In vegetation, the longitudinal dispersion is reduced as the vegetation density increases with a reduction of approximately 30% to 50% when the density is doubled. The percentage difference between the mean velocity and the cloud velocity is greater for the lower Reynolds number and this percentage decreases for the higher Reynolds number. The speed at which the cloud propagate that decrease in the maximum concentration and the resulting concentration profile along the stream are of great importance in pollution control. The experiment results obtained in the present study are compared with empirical relationship provided by various researcher. It is found that fischer routing equation has best fit to our present experiment results. Also, the equation is developed for predicting longitudinal dispersion coefficient having rigid vegetation in channel flow considering the various parameters and found that the coefficient of dispersion depends on the density of the vegetation but depends strongly on the diameter of the vegetation due to the increase of the surface area. The functional relationship between the various parameters obtained using IBM SPSS Statistics and the predicted equation is developed to determine the dispersion coefficient which found to correlate well with the observed dispersion coefficient.