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

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

Development of a laboratory system and 2D routing analysis to determine solute mixing within aquatic vegetation

A laser induced fluorometry (LIF) system was developed to quantify mixing within spatially variable aquatic vegetation. A comparison is made between intrusive fluorometry techniques and the application of LIF, to quantify mixing in real vegetation in the laboratory setting. LIF provides greater spatial resolution when compared to point fluorometry. Furthermore, LIF is non-intrusive. A two-dimensional routing procedure is used to calculate the longitudinal and transverse velocities and mixing coefficients from a single pulse injection of tracer within a vegetation patch

Logjams With a Lower Gap: Backwater Rise and Flow Distribution Beneath and Through Logjam Predicted by Two-Box Momentum Balance

AbstractLogjams with a gap at the bed form naturally in small channels and are used in engineering practice to maintain river connectivity at base flow. Limited understanding of a jam's effect on backwater rise and flow velocity limits assessment of geomorphic and ecological impacts of natural logjams, design of river restoration interventions, and representation in flood models. The distribution of flow through and beneath the jam satisfies a two‐box, momentum‐based model constrained by drag generated in the jam, momentum loss in flow through the lower gap, and net pressure force. The model was validated with 68 flume experiments. Backwater rise is predicted from discharge beneath the jam following established models for solid sluice gates. As a result, backwater rise increases with jam resistance, which forces a greater discharge beneath the jam. Below‐jam velocity and Shields parameter increased with ratio of friction coefficient to slope and decreasing gap height.</jats:p

Gravity Currents in Aquatic Canopies

A lock exchange experiment is used to investigate the propagation of gravity currents through a random array of rigid, emergent cylinders which represents a canopy of aquatic plants. As canopy drag increases, the propagating front varies from the classic profile of an unobstructed gravity current to a triangular profile. Unlike the unobstructed lock exchange, the gravity current in the canopy decelerates with time as the front lengthens. Two drag-dominated regimes associated with linear and nonlinear drag laws are identified. The theoretical expression for toe velocity is supported by observed values. Empirical criteria are developed to predict the current regime from the cylinder Reynolds number and the array density.National Science Foundation (U.S.) (grant EAR0309188)National Science Foundation (U.S.) (grant EAR0509658)Massachusetts Institute of Technology (Presidential Graduate Fellowship

Retention time and dispersion associated with submerged aquatic canopies

[1] The shear layer at the top of a submerged canopy generates coherent vortices that control exchange between the canopy and the overflowing water. Unlike free shear layers, the vortices in a canopy shear layer do not grow continuously downstream but reach and maintain a finite scale determined by a balance between shear production and canopy dissipation. This balance defines the length scale of vortex penetration into the canopy, δe, and the region of rapid exchange between the canopy and overflow. Deeper within the canopy, transport is constrained by smaller turbulence scales. A two-box canopy model is proposed on the basis of the length scale δe. Using diffusivity and exchange rates defined in previous studies, the model predicts the timescale required to flush the canopy through vertical exchange over a range of canopy density and height. The predicted canopy retention times, which range from minutes to an hour, are consistent with canopy retention inferred from tracer observations in the field and comparable to retention times for some hyporheic regions. The timescale for vertical exchange, along with the in-canopy velocity, determines the minimum canopy length for which vertical exchange dominates water renewal. Shorter canopies renew interior water through longitudinal advection. Finally, canopy water retention influences longitudinal dispersion through a transient storage process. When vertical exchange controls canopy retention, the transient storage dispersion increases with canopy height. When longitudinal advection controls water renewal, dispersion increases with canopy patch length

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

Retention time and dispersion associated with submerged aquatic canopies

Author Posting. © American Geophysical Union, 2007. 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 43 (2007): W04422, doi:10.1029/2006WR005362.The shear layer at the top of a submerged canopy generates coherent vortices that control exchange between the canopy and the overflowing water. Unlike free shear layers, the vortices in a canopy shear layer do not grow continuously downstream but reach and maintain a finite scale determined by a balance between shear production and canopy dissipation. This balance defines the length scale of vortex penetration into the canopy, δ e , and the region of rapid exchange between the canopy and overflow. Deeper within the canopy, transport is constrained by smaller turbulence scales. A two-box canopy model is proposed on the basis of the length scale δ e . Using diffusivity and exchange rates defined in previous studies, the model predicts the timescale required to flush the canopy through vertical exchange over a range of canopy density and height. The predicted canopy retention times, which range from minutes to an hour, are consistent with canopy retention inferred from tracer observations in the field and comparable to retention times for some hyporheic regions. The timescale for vertical exchange, along with the in-canopy velocity, determines the minimum canopy length for which vertical exchange dominates water renewal. Shorter canopies renew interior water through longitudinal advection. Finally, canopy water retention influences longitudinal dispersion through a transient storage process. When vertical exchange controls canopy retention, the transient storage dispersion increases with canopy height. When longitudinal advection controls water renewal, dispersion increases with canopy patch length.This material is based upon work supported by the National Science Foundation under grant EAR0309188

Hydraulic & Design Parameters in Full-Scale Constructed Wetland & Treatment Units: Six Case Studies

The efficiency of pond and constructed wetland (CW) treatment systems, is influenced by the internal hydrodynamics and mixing interactions between water and aquatic vegetation. In order to contribute to current knowledge of how emergent real vegetation affects solute mixing, and on what the shape and size effects are on the mixing characteristics, an understanding and quantification of those physical processes and interactions was evaluated. This paper presents results from tracer tests conducted during 2015-2016 in six full-scale systems in the UK under different flow regimes, operational depths, shapes and sizes, and in-/outlet configurations. The aim is to quantify the hydraulic performance and mixing characteristics of the treatment units, and to investigate the effect of size and shape on the mixing processes. Relative comparison of outlet configuration, inflow conditions, and internal features between the six different treatment units showed variations in residence times of up to a factor of 3. A key outcome of this study, demonstrated that the width is a more important dimension for the efficiency of the unit compared to the depth. Results underlined the importance of investigating hydrodynamics and physics of flow in full-size units to enhance treatment efficiency and predictions of water quality models

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

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