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The presence of vegetation in rivers, streams and riparian zones affects significantly the flow field and consequently the resistance, pollutant dispersion, sediment transport and ecological habitat. In this paper, a comparative study is undertaken to investigate the effect of an array of simple rigid or compound semi-flexible elements, resembling submerged small plants, on key features of the flow field. . Measurements were taken by means of a 3-D ADV instrument at selected locations within the vegetation array and downstream of it. Vertical profiles of the (temporal mean) velocities and turbulent shear stresses were obtained and compared.

Numerous studies focus on flow and mixing within cylinder arrays because of their similarity to vegetated flows. Randomly distributed cylinders are considered to be a closer representation of the natural distribution of vegetation stems compared with regularly distributed arrays. This study builds on previous work based on a single, fixed, cylinder diameter to consider non-uniform cylinder diameter distributions. The flow fields associated with arrays of randomly distributed cylinders are modeled in two dimensions using the ANSYS Fluent Computational Fluid Dynamics software with Reynolds Stress Model turbulence closure. A transient scalar transport model is used to characterize longitudinal and transverse mixing (Dx and Dy) within each geometry. The modeling approach is validated against independent laboratory data, and the dispersion coefficients are shown to be comparable with previous experimental studies. Eight different cylinder diameter configurations (six uniform and two non-uniform) are considered, each at 20 different solid volume fractions and with seven different transverse positions for the injection location. The new dispersion data cover a broad range of solid volume fractions, for which simultaneous estimates of Dx and Dy have not been available previously. There are no systematic differences in non-dimensional Dx and Dy between uniform and non-uniform cylinder diameter distributions. When non-dimensionalized by cylinder diameter, both dispersion coefficients are independent of solid volume fraction. When non-dimensionalized by cylinder spacing, both longitudinal and transverse dispersion can be described as linear functions of the ratio of cylinder diameter to cylinder spacing.

A set of laboratory experiments were conducted to study the impact of vegetation on bed form resistance and bed load transport in a mobile bed channel. Vegetation stems were simulated by using arrays of emergent polyvinyl chloride (PVC) rods in several staggered configurations. The total flow resistance was divided into bed, sidewall, and vegetation resistances. Bed resistance was further separated into grain and bed form (i.e., ripples and dunes) resistances. By analyzing experimental data using the downhill simplex method (DSM), we derived new empirical relations for predicting bed form resistance and the bed load transport rate in a vegetated channel. Bed form resistance increases with vegetation concentration, and the bed load transport rate reduces with vegetation concentration. However, these conclusions are obtained by using experimental data from this study as well as others available in the literature for a vegetated channel at low concentration.