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.

EXPERIMENTAL INVESTIGATIONS ON LONGITUDINAL DISPERSION CHARACTERISTICS OF TIDAL RIVERS

The longitudinal dispersion characteristics of tidal rivers areexperimentally investigated in a water channel. The longitudinal dispersion features and influentialfactors on pollutant in various stages of a tidal period in natural rivers are studied; the valueranges and variation trends of the longitudinal dispersion coefficient are obtained by means ofconcentration measurement. The results can provide important parameters for establishing the waterquality mathematical models in tidal rivers.

Effect of vegetation on flow structure and dispersion in strongly curved channels

The effect of vegetation on the flow structure and the dispersion in a 180 o curved open channel is studied. The Micro ADV is used to measure the flow velocities both in the vegetation cases and the non-vegetation case. It is shown that the velocities in the vegetation area are much smaller than those in the non-vegetation area and a large velocity gradient is generated between the vegetation area and the non-vegetation area. The transverse and longitudinal dispersion coefficients are analyzed based on the experimental data by using the modified N- zone models. It is shown that the effect of the vegetation on the transverse dispersion coefficient is small, involving only changes of a small magnitude, however, since the primary velocities become much more inhomogeneous with the presence of the vegetation, the longitudinal dispersion coefficients are much larger than those in the non-vegetation case.