Flexible patchy vegetation in a meandering bend: Effect on main flow and dispersion processes

Abstract

The presence of submerged vegetated islands on riverbeds influences hydrodynamics in aquatic ecosystems. A better understanding of interactions between the vegetated islands and the neighboring flow is crucial, especially in river restoration. While most studies in this field refer to straight channels—which are relatively rare in nature—and to emergent rigid vegetation, in this study we focus on river bends with finite vegetation patches, particularly flexible and submerged. The analysis is performed with the aid of data collected in a high-sinuosity meandering flume with squared patches (percentage of coverage of 10%) of fully submerged flexible vegetation (Cauchy number Ca=14.5) on the bed. Attention is focused on the bend-apex area of the flume. Results indicate the formation of a reduced-velocity region downstream of each patch's exit edge, along with a reverse flow on the vertical plane. In contrast to what observed in straight channels, the extension of this region decreases as the local radius of curvature decreases. High values of the Reynolds stress and the turbulent kinetic energy clearly occur especially close to the lateral edges of the patches, where the vegetation becomes a sink of turbulent motion. The presence of vegetated patches alters the typical curvature-induced circulation motion and the streamwise velocity distribution at the apex section. Two cores of high velocity form in the central part of the water depth and accelerated streamwise flow velocities are clearly visible between the two patches. This pattern influences the exchange and dispersion processes not only within the vegetated areas but also in the lateral patch-adjacent area.