The present study investigates flow turbulence and dispersion processes in the presence of flexible and dense vegetation on the bed. The turbulent dispersion coefficients and the terms of the turbulent kinetic energy equation are determined by using data collected in a straight laboratory channel with living vegetation on the bed. Results show that the turbulent integral lengths assume an order of magnitude comparable to the stems’ characteristic dimension independently by the direction and the turbulence assumes an isotropic behavior. The coefficients of dispersion have a trend similar to that of the turbulent lengths and assume low values in the longitudinal, transversal and vertical directions. Results also show that, in the mixing layer, the shear and wake turbulence production terms balance the dissipation; the turbulent diffusion term also assumes low values and its sign varies along the vertical indicating a transport of turbulent energy both from the vegetation to the free surface and from the free surface to vegetation.
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The paper addresses the problem of the resistance due to vegetation in an open channel flow, characterized by partially and fully submerged vegetation formed by colonies of bushes. The flow is characterized by significant spatial variations of velocity between vertical profiles that make the traditional approach based on time averaging of turbulent fluctuations inconvenient. A more useful procedure, based on time and spatial averaging (Double-Averaging Method) is applied for the flow field analysis and characterization. The vertical distribution of mean velocity and turbulent stresses at different spatial locations has been measured with a 3D Acoustic Doppler Velocimeter (ADV) for two different vegetation densities where fully submerged real bushes (salix pentandra) have been used. Velocity measurements were completed together with the measurements of drag exerted on the flow by bushes at different flow depths. The analysis of velocity measurements allows depicting the fundamental characteristics of both the mean flow field and turbulence. The experimental data show that the contribution of form-induced stresses to the momentum balance cannot be neglected. The mean velocity profiles and the spatially averaged turbulent intensity profiles allow inferring that the vegetation density is a driving parameter for the development of a mixing layer at the canopy top in the case of submerged vegetation. Moreover, the net upward turbulent momentum flux, evaluated with the methodology proposed by Lu and Willmarth (1973), appears to be damped for increased vegetation density; this finding can rationally explain the reduction of the suspended sediment transport capacity typically observed in free surface flows over a vegetated bed.
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