Hydrodynamics of partially vegetated channels; analyticaland experimental studies

Channel vegetation plays an important role in the aquatic-ecosystem health of rivers, streams, and constructed water courses. Vegetation can occupy the entire width or just part of the stream, leading to different features of the flow disturbances. In a natural environment, the aquatic vegetations have different characteristics. They appear as submerged or emerged, rigid or flexible, leafed or leafless, have branches or rods, and with high or low density. Obviously, the additional drag due to the vegetation presence increases the resistance to the channel flow and consequently the risk for flooding increases. Therefore, understanding of the flow dynamic of vegetated channel is of crucial importance to ensure successful implementation of the stream conception and management. According to previous studies it was observed that flow around large patches of vegetation is characterized by the formation of a shear turbulent mixing layer at the interface between the vegetated and open channels. Despite the many studies on flow in partly vegetated open channels, this issue remains of fundamental importance in order to better understand the interaction between the flow behavior and the canopy structure. In this study we propose a new theoretical approach, based on flow momentum equations, which are capable of modeling the flow pattern within the shear layer in the unobstructed domain, adjacent to the canopy area. Details regarding the evolution of the shear layer and the turbulence structures are presented. New observations on the flow momentum exchange between the obstructed and unobstructed domains are illustrated. To validate the proposed theoretical model, many experiments were carried out on a physical model of a very large rectangular channel (4x15x0.4m) with the presence of an array of vertical, rigid and circular steel cylinders. The array of cylinders was partially mounted on the bottom of the channel, in the central part, leaving two lateral areas of free flow circulation near the walls. The three-dimensional flow velocity components were measured using a 3D Acoustic Doppler Velocimeter ADV. In contrast to the complexity of the flow distribution within canopies, in the unobstructed flow area, independently on the canopy characteristics, it was observed that the flow distribution always resembles a boundary layer feature. This implies the possibility of an easy flow interpretation at this area, which was the aim of this study. In this study, a simple expression of the main equilibrium flow velocity, at the interface between both domains, was determined as a function of the lateral positions. This expression was derived using the proposed theoretical approach and then experimentally proved. Based on the analysis of the experimental data, in this model we take into consideration the additional contribution of the secondary flow velocity component on the flow momentum balance, which was neglected in previous studies. Since it was observed that the secondary velocity component is closely associated with the depth-average primary (streamwise) velocity but with an unknown exact relationship, in the present study, for the sake of simplicity we have adopted a linear relationship, proved also experimentally, between the secondary and the primary velocity components. After some reasonable assumptions, an analytical solution depicting the lateral mean flow velocity was obtained. A comparison of the measured and predicted data of the present study with those obtained in other previous studies, carried out with different canopy density, show a non-dependence of this analytical solution on the array density. This confirms well the validity and stability of the proposed theoretical approach.