Lateral dispersion in deflected emergent aquatic canopies

Understanding the mechanism of dispersion within plants is essential for proper operation of natural and constructed treatment wetlands. Plant deflection induced by the current drag modifies the flow and dispersion within an aquatic canopy. In this study, we look at the effect of the deflection angle of the plants on transverse dispersion. Experiments were carried out in a current flume on an array of inclined solid rods representing a deflected emergent canopy. The stem Reynolds number varied between 90 and 360, normalized stem densities between 0.011 and 0.033, and the inclination angles between 0° and 45°. The plume development was recorded using image processing technique. A special post-processing analysis was proposed to handle the discontinuities in the light intensity profile as a result of the light blocking effect of inclined cylinders. The results indicate that the dispersion coefficient increases with the inclination angle as well as vegetation density. However, the effects disappear at Reynolds numbers higher than 240. The effect of inclination angle on lateral dispersion coefficient is effectively described by the associated increase in the actual vegetation density, so the inclination angle and stem density can be combined into one single parameter. Based on a regression analysis, a compact equation is proposed for dispersion coefficient as a function of Reynolds number, stem angle, and vegetation density.