Flow analysis in a channel with flexible vegetation using double-averaging method

Abstract

A b s t r a c tThe 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.

Velocity profiles in a real vegetated channel

“…7a). As shown in previous studies [17,27,32], the position of this maximum corresponds to the top of the submerged canopy, denoted h p . Since this point is far enough from the bottom, the viscous stress is small, so this maximum is very close to the total shear stress qu Ã2 .…”

Hydrodynamics of vegetated channels

“…Thus, the flow resistance provided by the patch should be represented by the patch-scale geometry, that is, C D A p U 2 , with U being the channel velocity. This idea is supported by measurements of flow resistance produced by sparsely distributed bushes (Righetti 2008). A bush consists of a distribution of stems and leaves and so is a form of vegetation patch.…”

On the Relationship between Aquatic Plant Stem Characteristics and Drag Force: Is a Modeling Application Possible?

“…The model may also be applied to laboratory measurements of hydrodynamic processes in vegetated channels, which were previously conducted with the use of real [7,[61][62][63][64] or artificial plants [42,[65][66][67][68][69][70][71]. However, the mechanical characteristics of the materials that were used for hydrophyte analogs and imitations are much different from the mechanical characteristics of natural plants, e.g., flexible PVC elements used to imitate E. canadensis have a modulus of elasticity [70] that is several dozen times higher than that of E. canadensis [35].…”