Longitudinal dispersive coefficient in channels with aquatic vegetation: A review

The determination of longitudinal dispersion coefficient in rivers is necessary for pollution control, environmental risk assessment, and management. In rivers with aquatic vegetation, the flow field is remarkably modified by canopies, which affects velocity profiles and dispersion characteristics dominated by the heterogeneity of the velocity field. The dispersion is deduced from lateral and vertical longitudinal velocity gradients for compound channels with vegetated floodplains and rectangular channels with river-wide vegetation, respectively. Although many efforts have been exerted to clarify the dispersion process in different conditions and predict the diffusion of contaminants in vegetated rivers, no studies have introduced it systematically. This study reviews the dispersion coefficient characteristics, including magnitude, main impacted factors, and relationships with flow and vegetation features, in channels with aquatic canopies considering the variation of impact factors changing with the different vegetation and river morphology scenarios. Several typical methodologies for determining longitudinal dispersion coefficients are also summarized to understand the dispersion processes and concepts. Apart from the pioneer outcomes of previous studies, the review also emphasizes the deficiency of existing studies and suggests possible future directions for improving the theory of dispersion in vegetated channels.