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

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

Shear instability and coherent structures in shallow flow adjacent to a porous layer

Author Posting. © Cambridge University Press, 2007. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 593 (2007): 1-32, doi:10.1017/S0022112007008415.Results are presented from an experimental study of shallow flow in a channel partially obstructed by an array of circular cylinders. The cylinder array is a model for emergent vegetation in an open channel, but also represents a simple sparse porous medium. A shear layer with regular vortex structures forms at the edge of the array, evolving downstream to an equilibrium width and vortex size. The vortices induce nearly periodic oscillations with a frequency that matches the most unstable linear mode for a parallel shear flow. The shear layer is asymmetric about the array interface and has a two-layer structure. An inner region of maximum shear near the interface contains a velocity inflection point and establishes the penetration of momentum into the array. An outer region, resembling a boundary layer, forms in the main channel, and establishes the scale of the vortices. The vortex structure, educed by conditional sampling, shows strong crossflows with sweeps from the main channel and ejections from the array, which create significant momentum and mass fluxes across the interface. The sweeps maintain the coherent structures by enhancing shear and energy production at the interface. A linear stability analysis is consistent with the experimental results and demonstrates that the instability is excited by the differential drag between the channel and the array.This material is based upon work supported by the National Science Foundation under Grant 0125056

Flow and Transport in Regions with Aquatic Vegetation

This review describes mean and turbulent flow and mass transport in the presence of aquatic vegetation. Within emergent canopies, the turbulent length scales are set by the stem diameter and spacing, and the mean flow is determined by the distribution of the canopy frontal area. Near sparse submerged canopies, the bed roughness and near-bed turbulence are enhanced, but the velocity profile remains logarithmic. For dense submerged canopies, the drag discontinuity at the top of the canopy generates a shear layer, which contains canopy-scale vortices that control the exchange of mass and momentum between the canopy and the overflow. The canopy-scale vortices penetrate a finite distance into the canopy, δe, set by the canopy drag. This length scale segregates the canopy into two regions: The upper canopy experiences energetic turbulent transport, controlled by canopy-scale vortices, whereas the lower canopy experiences diminished transport, associated with the smaller stem-scale turbulence. The canopy-scale vortices induce a waving motion in flexible blades, called a monami.National Science Foundation (U.S.) (EAR 0309188)National Science Foundation (U.S.) (EAR 0125056)National Science Foundation (U.S.) (EAR0738352)National Science Foundation (U.S.) (OCE0751358

A joint velocity-intermittency analysis reveals similarity in the vertical structure of atmospheric and hydrospheric canopy turbulence

Turbulent fow through and over vegetation continues to draw signifcant research attention given its relevance to a plethora of applications in earth and environmental science. Canopy fows are characterized by three-dimensional coherent vortical motions not directly accessible from single-point measurements, which pose a challenge to formalizing links between vegetation structure and turbulent motion. A joint velocity-intermittency technique is applied to velocity data collected within and above aquatic vegetation in a hydraulic fume and above a forested canopy. The approach reveals behavior that provides greater insight into canopy fow dynamics than may be inferred from the vertical profles of mean velocity, turbulence intensity and Reynolds stresses, which are the quantities usually studied. There is a remarkable similarity in the structure of such fows between the forest canopy and the fume study despite large diferences in morphology and stem rigidity. In particular, these results determine an outer fow type arising above 1.5 canopy heights, while turbulent in-rushing events are most signifcant at the zero-plane displacement. The approach also implies ways in which improved models for canopy turbulence may be developed

Effects of Added Vegetation on Sand Bar Stability and Stream Hydrodynamics

Vegetation was added to a fully developed sandy point bar in the meander of a constructed stream. Significant changes in the flow structure and bed topography were observed. As expected, the addition of vegetative resistance decreased the depth-averaged streamwise velocity over the bar and increased it in the open region. In addition, the secondary circulation increased in strength but became confined to the deepest section of the channel. Over the point bar, the secondary flow was entirely outward, i.e., toward the outer bank. The changes in flow led to changes in bar shape. Although the region of the bar closest to the inner bank accumulated sediment, erosion of the bar and the removal of plants by scouring were observed at the interface between the planted bar and the open channel.National Science Foundation (U.S.) (Grant No. EAR 0738352

Laboratory investigation of lateral dispersion within dense arrays of randomly distributed cylinders at transitional Reynolds number

Published versio

Hydrodynamics of vegetated channels

This paper highlights some recent trends in vegetation hydrodynamics, focusing on conditions within channels and spanning spatial scales from individual blades, to canopies or vegetation patches, to the channel reach. At the blade scale, the boundary layer formed on the plant surface plays a role in controlling nutrient uptake. Flow resistance and light availability are also influenced by the reconfiguration of flexible blades. At the canopy scale, there are two flow regimes. For sparse canopies, the flow resembles a rough boundary layer. For dense canopies, the flow resembles a mixing layer. At the reach scale, flow resistance is more closely connected to the patch-scale vegetation distribution, described by the blockage factor, than to the geometry of individual plants. The impact of vegetation distribution on sediment movement is discussed, with attention being paid to methods for estimating bed stress within regions of vegetation. The key research challenges of the hydrodynamics of vegetated channels are highlighted.National Science Foundation (U.S.) (Grant No. EAR0309188)National Science Foundation (U.S.) (EAR0125056)National Science Foundation (U.S.) (EAR0738352)National Science Foundation (U.S.) (OCE0751358

Exchange flow between open water and floating vegetation

This study describes the exchange flow between a region with open water and a region with a partial-depth porous obstruction, which represents the thermally-driven exchange that occurs between open water and floating vegetation. The partial-depth porous obstruction represents the root layer, which does not penetrate to the bed. Initially, a vertical wall separates the two regions, with fluid of higher density in the obstructed region and fluid of lower density in the open region. This density difference represents the influence of differential solar heating due to shading by the vegetation. For a range of root density and root depths, the velocity distribution is measured in the lab using PIV. When the vertical wall is removed, the less dense water flows into the obstructed region at the surface. This surface flow bifurcates into two layers, one flowing directly through the root layer and one flowing beneath the root layer. A flow directed out of the vegetated region occurs at the bed. A model is developed that predicts the flow rates within each layer based on energy considerations. The experiments and model together suggest that at time- and length-scales relevant to the field, the flow structure for any root layer porosity approaches that of a fully blocked layer, for which the exchange flow occurs only beneath the root layer.National Science Foundation (U.S.) (grant EAR0509658