The hydrodynamics of turbulent flow through submerged flexible vegetation is investigated in a flume using acoustic Doppler velocimetery (ADV) measurements. The flow characteristics such as the energetics and momentum transfer derived from conventional spectral and quadrant analyses are considered as the flow encounters a finite vegetation patch. Consistent with numerous canopy flow experiments, a shear layer and coherent vortex structures near the canopy top emerge caused by Kelvin–Helmholtz instabilities after the flow equilibrates with the vegetated layer. These instabilities are commonly attributed to velocity differences between non-vegetated and vegetated canopy layers in agreement with numerous experiments and simulations conducted on dense rigid canopies. The power-spectral density function for vertical velocity turbulent fluctuations at different downstream positions starting from the edge of the vegetation layer are also computed. For a preset water depth, the dominant dimensionless frequency is found to be surprisingly invariant around 0.027 despite large differences in vegetation densities. The ejection and sweep events significantly contribute to the Reynolds stresses near the top of the vegetation. The momentum flux carried by ejections is larger than its counterpart carried by the sweeps above the canopy top. However, the momentum flux carried by sweeps is larger below the top of the canopy.

The structure of turbulent flow through submerged flexible vegetation

The significance of riparian vegetation on river flow and material transport is not in dispute. Conveyance laws, sediment erosion and deposition, and element cycling must all be adjusted from their canonical rough-wall boundary layer to accommodate the presence of aquatic plants. In turn, the growth and colonization of riparian vegetation are affected by fluvial processes and river morphology on longer time scales. These interactions and feedbacks at multiple time scales are now drawing significant attention within the research community given their relevance to river restoration. For this reason, a review summarizing methods, general laws, qualitative cognition, and quantitative models regarding the interplay between aquatic plants, flow dynamics, and sediment transport in vegetated rivers is in order. Shortcomings, pitfalls, knowledge gaps, and daunting challenges to the current state of knowledge are also covered. As a multidisciplinary research topic, a future research agenda and opportunities pertinent to river management and enhancement of ecosystem services are also highlighted.

Flow dynamics and sediment transport in vegetated rivers: A review

Aquatic suspended canopies are porous obstacles that extend down from the free-surface but have a gap between the canopy and bed. Examples of suspended canopies include those formed by aquaculture structures or floating vegetation. The major difference between suspended canopies and the more common submerged canopies, which are located on the bottom boundary, is the influence of the bottom boundary layer beneath the suspended canopy. Data from laboratory experiments are presented which explore aspects of the flow through and beneath suspended canopies constructed from rigid cylinders. The experiments, using both acoustic Doppler and two-dimensional (2D) particle tracking velocimetry, give details of the flow structure that may be divided vertically into a bottom boundary layer, a canopy shear layer, and an internal canopy layer. The experimental data show that the penetration of the shear layer into the canopy is limited by the distance between the canopy and bottom boundary layer. Peaks in velocity spectra indicate an interaction between the bottom boundary and canopy shear layer. An analytical model is also developed that can be used to calculate a drag coefficient that includes the effect of both canopy drag and bed friction. This drag coefficient is suitable for use in 2D (depth-averaged) hydrodynamic modeling. The model also allows the average velocity within and beneath the canopy to be calculated, and is used to investigate the effect of canopy density and thickness on both total drag and bottom friction.

Closure of "Depth-Averaged Drag Coefficient for Modeling Flow through Suspended Canopies"

[1] The past decade witnessed rapid developments in remote sensing methods that now permit an unprecedented description of the spatial variations in water levels (Hw), canopy height (hc), and leaf area density distribution (a) at large spatial scales. These developments are now renewing interest in effective resistance formulations for water flow within and above vegetated surfaces so that they can be incorporated into simplified water routing models driven by such remote sensing products. The first generation of such water routing models linked the bulk velocity to gradients in Hw via a constant diffusion velocity that cannot be inferred from canopy properties (a and hc). The next generation of such hydrologic models must preserve the nonlinear relationship between the resistance value, canopy attributes (e.g., a and hc), and Hw without compromising model simplicity. Using a simplified scaling analysis on the depth-integrated mean momentum balance and a two-layer model for the bulk velocity, the Darcy-Weisbach friction factor (f) was shown to vary with three canonical length scales that can be either measured or possibly inferred from remote sensing products Hw, hc, and the adjustment length scale Lc = (Cda)−1, where Cd is the drag coefficient (of order unity). The scaling analysis proposed here reveals that these length scales can be combined in two dimensionless groups, Hw/hc and Lc/hc. The dependence of f on these two functional groups was then explored using a combination of first-order closure modeling and 130 experimental runs derived from a large number of flume experiments carried out for rigid and flexible vegetation. The results from the data and the model show a nonlinear decrease in f with increasing Hw/hc at a given Lc/hc and the nonlinear increase in f with decreasing Lc/hc. Furthermore, both model and data results did not exhibit any dependence on the bulk Reynolds number.

Hydraulic resistance of submerged rigid vegetation derived from first order closure models

Predicting the vertical low suspended sediment concentration in vegetated flow using a random displacement model

Turbulence structure in open channel flow with partially covered artificial emergent vegetation

Analytical solutions of velocity profile in flow through submerged vegetation with variable frontal width

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Jiao Zhang

Papers

The structure of turbulent flow through submerged flexible vegetation

Turbulence structure in open channel flow with partially covered artificial emergent vegetation

Analytical solutions of velocity profile in flow through submerged vegetation with variable frontal width

Turbulence and Particle Deposition Under Steady Flow Along a Submerged Seagrass Meadow

Evaluation of a random displacement model for predicting longitudinal dispersion in flow through suspended canopies