Sediment deposition within and around a finite patch of model vegetation over a range of channel velocity

The interaction between flow and vegetation creates feedbacks to deposition that vary with channel velocity. This experimental study describes how channel velocity and stem-generated turbulence influence the deposition within and around an emergent patch of model vegetation, with a particular focus on deposition within the patch. The Reynolds number threshold for stem-scale turbulence generation was determined using velocity spectra and flow visualization. At high channel velocity resuspension occurred in the bare regions of the channel and a nonuniform spatial distribution of net deposition was observed around and within the patch. In contrast, at low channel velocity there was no (or limited) resuspension and a uniform distribution of net deposition was observed around and within the patch. The deposition inside the patch was enhanced, relative to a bare-channel control, only when the following two criteria were met: (1) the absence of stem turbulence, and (2) the presence of sediment resuspension in the bare channel. Comparison to previous lab and field studies further support these criteria.National Science Foundation (U.S.) (Grant EAR-1414499

The onset of sediment transport in vegetated channels predicted by turbulent kinetic energy

This laboratory study advances our understanding of sediment transport in vegetated regions, by describing the impact of stem density on the critical velocity, Ucrit, at which sediment motion is initiated. Sparse emergent vegetation was modeled with rigid cylinders arranged in staggered arrays of different stem densities. The sediment transport rate, Qs, was measured over a range of current speeds using digital imaging, and the critical velocity was selected as the condition at which the magnitude of Qs crossed the noise threshold. For both grain sizes considered here (0.6–0.85 mm and 1.7–2 mm), Ucrit decreased with increasing stem density. This dependence can be explained by a threshold condition based on turbulent kinetic energy, kt, suggesting that near-bed turbulence intensity may be a more important control than bed shear stress on the initiation of sediment motion. The turbulent kinetic energy model unified the bare bed and vegetated channel measurements.National Science Foundation (U.S.) (NSF grant EAR 1414499

Sediment patterns near a model patch of reedy emergent vegetation

This laboratory study describes the sediment patterns formed in a sand bed around circular patches of rigid vertical cylinders, representing a patch of reedy emergent vegetation. The patch diameter was much smaller than the channel width. Two patch densities (solid volume fraction 3% and 10%) and two patch diameters (22 and 10 cm) were considered. For flows above the threshold of sediment motion, patterns of sediment erosion and deposition were observed around and within the patch. Scouring within the patch was positively correlated with turbulent kinetic energy in the patch. For sparse patches, sediment scoured from within the patch was mostly deposited within one patch diameter downstream of the patch. For dense patches, which experience greater flow diversion, sediment scoured from the patch was carried farther downstream before deposition along the patch centerline. Differences between the sparse and dense patch patterns of deposition are explained in the context of flow diversion and wake structure, which are related to a nondimensional flow blockage parameter. While sediment was redistributed near the patch, observations suggest that net deposition was not recorded at the reach scale.National Science Foundation (U.S.) (STC Center for Earth-surface Dynamics Agreement EAR-0120914)National Science Foundation (U.S.) (Grant EAR 0738352

Estimating the Instantaneous Drag–Wind Relationship for a Horizontally Homogeneous Canopy

The mean drag–wind relationship is usually investigated assuming that field data are representative of spatially-averaged metrics of statistically stationary flow within and above a horizontally homogeneous canopy. Even if these conditions are satisfied, large-eddy simulation (LES) data suggest two major issues in the analysis of observational data. Firstly, the streamwise mean pressure gradient is usually neglected in the analysis of data from terrestrial canopies, which compromises the estimates of mean canopy drag and provides misleading information for the dependence of local mean drag coefficients on local velocity scales. Secondly, no standard approach has been proposed to investigate the instantaneous drag–wind relationship, a critical component of canopy representation in LES. Here, a practical approach is proposed to fit the streamwise mean pressure gradient using observed profiles of the mean vertical momentum flux within the canopy. Inclusion of the fitted mean pressure gradient enables reliable estimates of the mean drag–wind relationship. LES data show that a local mean drag coefficient that characterizes the relationship between mean canopy drag and the velocity scale associated with total kinetic energy can be used to identify the dependence of the local instantaneous drag coefficient on instantaneous velocity. Iterative approaches are proposed to fit specific models of velocity-dependent instantaneous drag coefficients that represent the effects of viscous drag and the reconfiguration of flexible canopy elements. LES data are used to verify the assumptions and algorithms employed by these new approaches. The relationship between mean canopy drag and mean velocity, which is needed in models based on the Reynolds-averaged Navier-Stokes equations, is parametrized to account for both the dependence on velocity and the contribution from velocity variances. Finally, velocity-dependent drag coefficients lead to significant variations of the calculated displacement height and roughness length with wind speed.National Science Foundation (U.S.) (Grant AGS1005363

Interaction between neighboring vegetation patches : impact on flow and deposition

Flow and sedimentation around patches of vegetation are important to landscape evolution, and a better understanding of these processes would facilitate more effective river restoration and wetlands engineering. In wetlands and channels, patches of vegetation are rarely isolated and neighboring patches influence one another during their development. In this experimental study, an adjacent pair of emergent vegetation patches were modeled by circular arrays of cylinders with their centers aligned in a direction that was perpendicular to the flow direction. The flow and deposition patterns behind the pair of patches were measured for two stem densities and for different patch separations (gap widths). The wake pattern immediately behind each individual patch was similar to that observed behind an isolated patch, with a velocity minimum directly behind each patch that produced a well-defined region of enhanced deposition in line with the patch. For all gap widths (Δ), the velocity on the centerline between the patches (Uc) was elevated to a peak velocity Umax that persisted over a distance Lj. Although Umax was not a function of Δ, Lj decreased with decreasing Δ. Beyond Lj, the wakes merged and Uc decayed to a local minimum. The merging of wakes and associated velocity minimum produced a local maximum in deposition downstream from and on the centerline between the patches. If this secondary region of enhanced deposition promotes new vegetation growth, the increased drag on the centerline could slow velocity between the upstream patch pair, leading to conditions favorable to their merger

Influence of particle size and density, and channel velocity on the deposition patterns around a circular patch of model emergent vegetation

National Science Foundation (U.S.) (EAR-1414499

Flow, deposition, and erosion near finite patches of vegetation

Keynote Lecture

A vortex-based model of velocity and shear stress in a partially vegetated shallow channel

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Water Resources Research 44 (2008): W01412, doi:10.1029/2006WR005651.This paper presents a method for predicting the distributions of velocity and shear stress in shallow channels with a boundary of emergent vegetation. Experiments in a laboratory channel with model vegetation show that the velocity profile exhibits a distinct two-layer structure, consisting of a rapidly varying shear layer across the vegetation interface and a more gradual boundary layer in the main channel. In addition, coherent vortices are observed which span both layers, and are the dominant contributors to lateral momentum fluxes. From these observations, we propose a model for the vortex-induced exchange and find expressions for the width of momentum penetration into the vegetation, the velocity and shear stress at the vegetation edge, and the width of the boundary layer in the main channel. These variables, along with a momentum balance in the main channel, comprise a modeling framework which accurately reproduces the observed velocity and shear stress distributions. The predictions for the velocity and shear stress can provide a basis for modeling flood conveyance, overbank sediment transport, and scalar residence time in the vegetated layer.This material is based upon work supported by the National Science Foundation under grant 0125056

The motion of kelp blades and the surface renewal model

We consider how the flapping of kelp blades may enhance the flux of nutrients to a blade, by stripping away the diffusive sub-layer and renewing the fluid at the blade surface. The surface renewal model explains the degree of flux enhancement observed in previous studies under different flow and flapping conditions. We measured the motion of real kelp blades of Laminaria saccharina, Macrocystis pyrifera, and Nereocystis luetkeana under uni-directional current in a laboratory flume. Observed flapping frequencies coupled with the renewal model, suggest that the flapping of blades in the field has the potential to significantly enhance flux to the blade surface at low current speed, but has little effect on flux at high current speeds.National Science Foundation (U.S.) (Ocean Sciences Division, No. 0751358