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An analysis of flow was carried out in this study to find the effects of trees on the flow structure in a straight compound open channel. To resemble the vegetation stem, two rows of vertical rods were installed on the floodplain of an asymmetrical compound channel. Two-dimensional flow velocity was measured using Particle Image Velocimetry. The results showed a decreasing trend in turbulent kinetic energy and turbulent intensity along the floodplain. At low flow depth, a significant decrease was observed in mean velocity and kinetic energy. Similar reduction was observed at the intersection of floodplain and main channel and around the stems. Under this condition, the direction of velocity vectors changed from floodplain to the main channel. However, this trend was reversed at higher flow depths. This reversal was due to the changes in momentum transfer and shear stress at the intersection of floodplain and main channel. Additionally, a numerical analysis showed that vortices are formed at the junction of the floodplain and main channel. [ABSTRACT FROM AUTHOR]

Vegetation plays an important role in soil erosion control, but few studies have been performed to quantify the effects of vegetation stems on hydraulics of overland flow. Laboratory flume experiments were conducted to investigate the potential effects of vegetation stems on Reynolds number, Froude number, flow velocity and hydraulic resistance of silt-laden overland flow. Cylinders with diameter D of 2·0, 3·2 and 4·0 × 10−2 m were glued onto the flume bed to simulate the vegetation stems, and a bare slope was used as control. The flow discharge varied from 0·5 to 1·5 × 10−3 m3 s−1 and slope gradient was 9°. Results showed that Reynolds number on vegetated slope was significantly higher than that on bare slope because of the effect of vegetation stems on effective flow width. All the flows were supercritical flow, but Froude number decreased as D increased, implying a decrease in runoff ability to carry sediment. The mean flow velocity also decreased with D, while the velocity profile became steeper, and no significant differences were found in surface flow velocities among longitudinal sections on all slopes. Darcy-Weisbach friction coefficient increased with D, implying that the energy consumption of overland flow on hydraulic resistance increased. Reynolds number was not a unique predictor of hydraulic roughness on vegetated slopes. The total resistance on vegetated slopes was partitioned into grain resistance and vegetation resistance, and vegetation resistance accounted for almost 80% of the total resistance and was the dominant roughness element. Further studies are needed to extend and apply the insights obtained under controlled conditions to actual overland flow conditions. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The condition of emergent or submerged vegetation plays a vital role in affecting the flow behaviour in open channels. To understand the flow processes in free surface flows under emergent rigid vegetation condition, an extensive experimental investigation is performed in a laboratory channel with staggered combination of emergent rigid vegetation. Three‐dimensional velocities data are collected by ADV at various section and position of vegetated channel. The three‐dimensional flow properties such as velocity distribution, turbulence intensity, turbulent kinetic energy, Reynolds shear stress in vegetation, and non‐disturbed region are analysed and compared with the longitudinal and transverse length of vegetated zone. In the vegetation region, the longitudinal velocity in free stream region decreases as compared with non‐disturbed region. However, the vertical velocity also decreases in free stream region except at the centre line of channel cross‐section. Variations in velocity in the vegetation section were observed because of the local effect induced by the vegetation stems. Three‐dimensional Reynolds shear stress and turbulent intensity were weaker in magnitude at vegetation region. The presence of vegetation reduces the longitudinal profile of flow velocity, Reynolds shear stress and turbulence intensities, which means that vegetation can be used as an effective tool to reduce the flow resistance. The turbulent kinetic energy, skewness and kurtosis are also evaluated and compared between the vegetated and non‐vegetated zone. The outcomes of the results provide an important view to understand flow behaviour at immediately front and immediately behind of vegetation stem. [ABSTRACT FROM AUTHOR]

Vegetation stems and litter cover have different effects on sediment transport capacity under the same experimental conditions, which in essence, may be due to differences in their hydraulic properties, but the availability of comparative studies is limited. This study aimed to compare the hydraulic properties affected by litter and stem cover, compare differences in the drag forces exerted by litter and stems on overland flow, and develop new Manning's n and flow velocity equations for litter cover. Two series of flume experiments were conducted with the same slope gradients (8.8%, 17.6%, 26.8%) and flow discharge rates (0.5, 1.0 × 10−3 m3 s−1). Artificial Gramineae stems with a 0%–30% cover level and Pinus tabulaeformis litter with a 0%–70% cover level were used in series 1 and series 2, respectively. The flow velocity and depth were measured. The results showed that the Froude number and flow velocity affected by stem cover were much lower than those affected by litter cover, while the opposite trend was observed in the relative magnitude of the Reynolds number, flow depth and shear stress. The form resistance caused by stems was 22–57 times greater than that caused by litter for the same cover level, which suggests that stem cover contributes more than litter cover to increasing the flow resistance and reducing the flow's ability for sediment detachment and transport. Two new equations for calculating Manning's n and flow velocity under the influence of litter cover were developed, with R2 and NSE values of 0.96. The results of this study contribute to revealing the mechanisms of the differences of the effects of stem and litter cover on soil erosion. [ABSTRACT FROM AUTHOR]

The sediment transport capacity plays a pivotal role in erosion research, and is usually predicted using hydraulic variables. The transport capacity and hydraulic variables are affected by vegetation cover. Our understanding of the effect of vegetation cover, including the size, density, and arrangement of vegetation stems, on the relationship between the sediment transport capacity and hydraulic variables were rather limited. The objectives of this study were to investigate the effect vegetation stem cover on the relationship between hydraulic variables and the sediment transport capacity and to derive an equation for predicting the sediment transport capacity in the presence of vegetation cover. Five data sets from 288 flume experiments with a wide range of discharge (0.25–2 × 10−3 m3 s−1), slope (8.8–42.3%), median sediment diameter (0.11–1.16 × 10−3 m), stem cover (0–30%), stem diameter (2–36 mm), and stem arrangement (bead, tessellation, zigzag, random, and banding) were compiled for this study. Extensive regression analysis has shown that the sediment transport capacity could be expressed as a power function of flow velocity, shear stress, stream power, or unit stream power. Predictors of the sediment transport capacity were ranked from the unit stream power as the strongest, followed by the stream power, flow velocity, and the shear stress. Vegetation stem cover had no apparent and direct effect on the relationship between hydraulic variables and the sediment transport capacity so long as the unit stream power or stream power was used as its predictor. Vegetation cover became a significant factor only when the shear stress was used to predict the sediment transport capacity. Finally, a new equation involving the slope gradient, flow velocity, and median sediment diameter in a nondimensional form was shown to be a superior predictor of the sediment transport capacity with the Nash–Sutcliffe coefficient of efficiency of 0.92. The product of slope and flow velocity, that is, the unit stream power, captures the effect of vegetation stem cover and surface roughness and was shown to be an effective predictor of the transport capacity in the presence of vegetation cover. [ABSTRACT FROM AUTHOR]

Spillways can present a way to control the overflowing of water during flood events and prevent damage from levee breaches. With increasing interest in nature-based solutions, the interaction between flow and vegetation parameters has to be understood. Aeration usually occurs during the overflow of sloped spillways, leading to the bulking of flow, alterations of flow characteristics, and energy dissipation. The influence of the vegetation parameter on aerated flow characteristics has not yet been investigated in greater detail; no systematic investigation of the effect of vegetation parameters has been conducted. This paper aims to systematically analyze the influence of different vegetation heights on air entrainment during the overflow of spillways. Therefore, a spillway model with a slope of 18° (1:3) was equipped with artificial turf of varying turf heights, and supercritical flows were investigated. The aeration was measured using double-tip conductivity probes, giving insights into air concentration profiles, bubble count rates, estimations of energy dissipation, and flow velocities. The results highlighted the significant influence of vegetation height on the aeration process. Higher air concentrations over the flow depth were observed for higher turf heights tested in this study. Also, the energy dissipation and flow velocity reduction increased with higher vegetation heights. Overall, the present study uncovers the effect of vegetated covers, thereby contributing to the fundamentals of aerated flows. [ABSTRACT FROM AUTHOR]

Zero-plane is an important reference level included in the logarithmic velocity profile. The prediction of the zero-plane displacement remains challenging, especially in open-channel flows with submerged vegetation. In this study, we first review the existing formulas for estimating the zero-plane displacement. Then, we conducted a series of laboratory experiments of open-channel flows with submerged rigid vegetation, by varying flow rate, water depth, vegetation density, and stem diameter. By utilizing particle image velocimetry (PIV) technique, the spatio-temporal averaged (double-averaged) flow velocity and Reynolds shear stress profiles were obtained. The data analysis shows that the zero-plane displacement is almost proportional to the surface layer depth for the shallow flow cases. As the flow depth increases, the zero-plane displacement tends to be constant and increases with decreasing vegetation density. This result substantiates the hypothesis that the zero-plane displacement is related to large-scale eddies. Lastly, a heuristic model is developed to explain possible variations of the large-scale eddies, which yields a formula for predicting the zero-plane displacement for flows subjected to submerged rigid vegetation. [ABSTRACT FROM AUTHOR]

Laboratory experiments were conducted to directly measure the drag force induced by vegetation patches under unidirectional flow conditions. The drag force was measured with a highly responsive load cell for six patch models, of which the vegetation stems were simulated using rigid cylindrical rods. The measured drag coefficient varies with the Reynolds number and the number of rods included in the patch model. It generally reduces with increasing Reynolds number, and the reduction appears more significant for a patch model with fewer rods. To take into account effects of the patch porosity, this study proposed to redefine the drag coefficient and Reynolds number based on new length and velocity scales, that is, the vegetation hydraulic radius and the average velocity for the flow through the patch. This change produces a monotonic relationship between the redefined drag coefficient and redefined Reynolds number, which is generally applicable for all patch models. [ABSTRACT FROM AUTHOR]

This work presents and discusses a series of experiments focusing on the transport of floating particles, mimicking seeds and propagules, within an array of randomly arranged cylinders mimicking emergent vegetation stems. The focus is on the temporary capture process by which particles colliding with a cylinder are trapped by surface tension for finite but relatively long retention times, thus promoting a large mechanical dispersion. Video analysis of the particle paths within the array shows that the probability of particles being captured, either temporarily or permanently, as well as the mean retention time, vary with flow velocity while being weakly affected by stem density. On the contrary, stem density plays a significant role in determining the frequency of the temporary captures; in particular, the probability of having temporary, rather than permanent, captures increases with vegetation density. We also propose some relationships to predict the probability of having temporary capture events and their mean duration based on experimental results. Key Points: The probability that floating particles are captured by emerging stems is weakly dependent on stem densityTemporary captures increase with stem density at the expense of permanent captures due to the turbulence generated by vegetationMean retention time of captured particles is inversely related to the mean flow velocity within the canopy [ABSTRACT FROM AUTHOR]

By using the ANSYS FLUENT numerical simulation software, the Navier-Stokes equation as the governing equation, based on the finite volume method, this paper adopts the standard k -- ε turbulence model to establish a numerical analysis model. Under the condition of constant density, numerical simulation study is carried out on the water flow characteristics of three kinds of rigidly submerged vegetation open channels with thick stems. The results show that the experimental results are compared with the numerical simulation calculation results, and the calculated velocity values are in good agreement with the experimental values, which verifies the correctness and feasibility of the numerical simulation method. By analyzing the flow velocity distribution changes in the flow field, the increase in the thickness of the vegetation stems is obtained. By analyzing the vertical distribution of the flow velocity in the flow field, it is concluded that the slope close to vegetation has a smaller range of velocity change, and the water flow is more turbulent. As the distance from vegetation continues to increase, the vertical velocity change range increases significantly. The mixed turbulence effect of vegetation on the point flow field is weakened. [ABSTRACT FROM AUTHOR]

The mean slope flow velocity is critical in soil erosion models but the mechanism of its variation under rigid vegetation cover remains unclear. On natural slopes, vegetation grows predominantly perpendicular to the horizontal plane (BH), with some growing perpendicularly to the slope surface (BS); however, current research often neglects the effects of these two growth directions on the mean flow velocity. We conducted simulation experiments using different coverage levels, rigid vegetation, slope angles, and flow rates and showed that the flow rate and slope significantly influenced the mean flow velocity. As the coverage of rigid vegetation increased, the mean flow velocity increased more under conditions perpendicular to the horizontal plane (BH) and those perpendicular to the slope (BS). A model for predicting mean flow velocity was developed using vegetation equivalent roughness and the Manning formula, which accurately predicted flow velocity in different conditions. This study contributes to the refinement of slope flow theory and provides data that support soil and water conservation efforts. [ABSTRACT FROM AUTHOR]

This study employs the XBeach surfbeat model (XBSB) to explore the effects of vegetation on wave attenuation and dune erosion in a case study of Mexico Beach during Hurricane Michael. The XBSB model was validated against laboratory experiments of wave-induced dune erosion and wave attenuation by vegetation. In the case study of vegetation on dunes in Mexico Beach during Hurricane Michael, different vegetation drag coefficients were evaluated to investigate the effects of vegetation on wave attenuation and dune erosion. LiDAR data of dune profiles before and after Hurricane Michael were used for model validation. The findings reveal that vegetation on dunes significantly affects wave attenuation and dune erosion. Under vegetated conditions, as the vegetation drag coefficient value increases, wave attenuation also increases, leading to a reduction of dune erosion. An increase in vegetation density enhances wave attenuation in the vegetated area, including reductions in significant wave height and flow velocity. However, the rate of change in attenuation decreases as the vegetation density increases. Through simulations under regular wave condition on Mexico Beach, an optimal vegetation density was identified as 800 units/m2. Beyond this density, additional vegetation does not substantially improve wave attenuation. Furthermore, the position of the dune crest elevation is related to the location where the alongshore flow velocity begins to decrease. The findings highlight the essential role of coastal vegetation in enhancing coastal resilience against hurricanes. [ABSTRACT FROM AUTHOR]

Standing flows in natural channels often cause phenomena that can be very serious, such as flooding, deformation of channel geometry, and destruction of infrastructure (dams, bridges, and culverts). This study focuses on the computation of gradually varying permanent flows (backwater curves) by two methods: direct integration (Chow) and successive approximation (depth variation). To solve the system of equations governing the problem of gradually varying one-dimensional stationary flows at a free surface, a large amount of data should be taken into account, namely, the flow rate, the water head, the mean flow velocity, the rugosity, and the slope. These parameters are very important, as they cause nonlinear behavior, making the problem and its mathematical solution complex. Digitizing these parameters can help to determine and visualize the longitudinal profile of the water line for known flow rates. This study aimed to: (1) determine the influence of rugosity on gradually varying steady flows and the overclassification of eddy curves in a prismatic channel, (2) study the effect of geometric shape on these flows, and (3) investigate and compare the effects of the calculation methods. The results reveal the great influence of rugosity on gradually varying permanent flows for four selected geometric shapes of the channel, as it has a direct influence on the normal depth and the critical slope. Each time the resistance of the bottom to the flow increases, these results increase. The influence of the geometric shape on these flows is less significant. The comparative study showed a difference between the results obtained. [ABSTRACT FROM AUTHOR]

In river research, forecasting flow velocity accurately in vegetated channels is a significant challenge. The forecasting performance of various independent and hybrid machine learning (ML) models are thus quantified for the first time in this work. Utilizing flow velocity measurements in both natural and laboratory flume experiments, we assess the efficacy of four distinct standalone machine learning techniques—Kstar, M5P, reduced error pruning tree (REPT) and random forest (RF) models. In addition, we also test for eight types of hybrid ML algorithms trained with an Additive Regression (AR) and Bagging (BA) (AR-Kstar, AR-M5P, AR-REPT, AR-RF, BA-Kstar, BA-M5P, BA-REPT and BA-RF). Findings from a comparison of their predictive capabilities, along with a sensitivity analysis of the influencing factors, indicated: (1) Vegetation height emerged as the most sensitive parameter for determining the flow velocity; (2) all ML models displayed outperforming empirical equations; (3) nearly all ML algorithms worked optimal when the model was built using all of the input parameters. Overall, the findings showed that hybrid ML algorithms outperform regular ML algorithms and empirical equations at forecasting flow velocity. AR-M5P (R2 = 0.954, R = 0.977, NSE = 0.954, MAE = 0.042, MSE = 0.003, and PBias = 1.466) turned out to be the optimal model for forecasting of flow velocity in vegetated-rivers. [ABSTRACT FROM AUTHOR]

Marine vegetation is increasingly viewed as a living shoreline that protects coastal communities and ecosystems from the damaging effects of wave energy. Many studies have explored the potential of marine vegetation in terms of reducing wave height, but more work is needed. Here, we used particle image velocimetry, fluid–structure interaction simulation, and multiple regression analysis to estimate the bending behaviors of flexible marine vegetation in water flow, and we predicted the wave height reduction in the downstream vegetation meadow. We considered different vegetation types and water flow velocities, constructed a total of 64 cases, and derived a multiple regression equation that simply estimates the vegetation bending height with a tolerance of ~10%. When the bending height rather than the vegetation height was applied, wave height reduction was alleviated by 1.08–9.23%. Thus, flexible vegetation reduced wave height by up to ~10% less than rigid vegetation in our investigation range. This implies that the impact of bending behavior becomes more pronounced with a larger vegetation meadow. The relative % decrease in wave height reduction was greater for fully submerged vegetation compared to partially submerged vegetation. [ABSTRACT FROM AUTHOR]

Coastal flood risk is expected to increase due to climate change and population growth. Much of our coastlines is protected by “grey” infrastructure such as a dike. Dike maintenance and strengthening requires ever increasing capital and space, putting their economic viability in question. To combat this trend, more sustainable alternatives are explored, also known as Nature based Solutions. A promising option has shown to be tidal marshes. Tidal marshes are coastal wetlands with high ecological and economic value. Also, they protect dikes through wave attenuation and in case of a dike breach reduce its development. However, the effectiveness of a tidal marsh on reducing dike breach development rates highly depends on the stability of the tidal marsh itself. Not much is known about the stability of a tidal marsh under dike breach conditions, which are accompanied with flow velocities that can reach 4–5ms−1. In this study we tested the vegetation response and erodibility of a mature tidal marsh, in-situ, under high flow velocities ( > 0.5 m s−1). Our results confirm that tidal marshes similar to the one tested in this study are highly erosion resistant with low erodibility. More research is necessary to confirm this for tidal marshes with different soil and vegetation properties. For tidal marshes similar to what is tested thus far, erosion under dike breach conditions is negligible and other erosion mechanisms such as headcut erosion probably dominate the erosion process [ABSTRACT FROM AUTHOR]

The prediction of changes in velocity distribution and channel bed morphology is significant in open channel management and design. This paper implements experimental work to realize and quantify the effect of rigid aquatic bank weeds on vertical velocity profiles and channel bed morphology. In the experimental work, weeds were given a staggered distribution using three distances of 25, 50, and 75 mm, unilaterally and bilaterally, with Froude numbers ranging from 0.11 to 0.30, achieving 168 scenarios. Results for the tested weed cases showed that the average velocity was directly proportional to the weed density and approached the Froude number. By comparing the smooth and weeded velocities, it was found that the velocity inside the infested reach was close to the downstream velocity and exceeded the upstream velocity by about 10% and 41%, respectively. Scour depths along the centerline of the vegetated reach for the bilateral weeds were higher by 11% to 33% than those for the unilateral weeds. The maximum observed depth of the scour holes along the smooth bank was about 30% to 60% of the maximum scour depth at the middle line. Finally, to quantify the results, multiple regression analysis was performed to develop empirical equations to assist in the water management process. [ABSTRACT FROM AUTHOR]

Vegetation filter strips (VFSs) have always been an important measure to control agricultural soil erosion, especially in mountainous and hilly areas with more sloping farmland. To investigate the mechanism of the sediment-trapping process by VFSs, a series of tests were conducted with four gradients of flow rate, 7.5–45 L min−1 m−1, and two different sediment concentrations of 40 and 120 g L−1. The whole process of overland flow was monitored, and sediment and particle size samples from the inflow and outflow were collected and measured. The results showed that the changes in sediment concentration did not significantly affect the corresponding coefficients in the power function relationship between overland flow rate and velocity. Using the Reynolds number alone cannot effectively indicate the flow pattern of overland flow on vegetation hillslopes. The peak particle size and linear function were effective in describing the relationship between sediment particle composition and delivery rate during the sediment-trapping process by VFSs. During the sediment-trapping process, the sediment-trapping capacity of VFSs continued to decrease. The increase in sediment discharge was accompanied by a higher proportion of coarse particles. Under the same flow rate conditions, when the sediment concentration was higher, the coarse particles and their proportion also increased faster. Therefore, using only a certain particle size threshold to distinguish suspended and transported sediment may lead to inaccurate estimation of the sediment-trapping performance of VFSs. This study deepened the understanding of the mechanism of water–sediment processes on vegetation hillslopes and promoted the widespread and efficient application of VFSs management technology. [ABSTRACT FROM AUTHOR]

The flow in arid and semi-arid regions changes significantly during seasons, letting many vegetation patches develop in different parts of rivers. In the presence of aquatic plants in streams, different flow structures have resulted. When the water level increases in these rivers, the presence of vegetation patches influences the turbulent flow structures, which may considerably change the estimation of key hydraulic parameters. The results of earlier investigations indicated that a wide range of submerged and non-submerged vegetation influences the hydrodynamic features of flows in rivers and streams. In the present investigation, two pools with various slopes of entry and exit sections were used to conduct eight independent experiment runs. In addition, a vegetation patch over the entire pool section has been set up to investigate the effects of the vegetation patch on flow structures in pools. The effect of two slopes of 5 and 10 degrees for both entrance and exit of the pools on flow structure has been investigated. Considering two aspect ratios of 2.0 and 2.7, the distributions of flow velocity, Reynolds normal and shear stresses, turbulence intensities, turbulent kinetic energy (TKE), quadrant analysis, and spectral analysis have been studied at the trailing edge of the vegetation patch along an artificial pool. Results show that, for large entrance and exit slopes (10 degrees), the TKE distribution profiles have no specific form. However, the TKE values have a convex-shaped distribution pattern with the maximum TKE value near the bed when the slopes of the entrance and exit sections of the pool are small (5 degrees). Results showed that both ejections and sweeps govern the turbulence structures and coherent motions at the trailing edge of the vegetation patch along the pool. The geometry, entrance, and exit slopes of the pool have no effect on the validation of power spectral function compared to the presence of a vegetation patch in a flatbed. [ABSTRACT FROM AUTHOR]

Accurate estimation of mean flow velocity is crucial for hydraulics and soil erosion studies. A correction factor (α) is required to convert flow surface velocity to mean velocity when using the dye tracer method. However, the determination of an appropriate α value is challenging and little information is available about how α varies with progressive inundation conditions, especially for rough surfaces with protruding roughness elements (e.g., stones). Therefore, laboratory experiments were conducted in a flume with sandpaper (SD) and stones (ST) surfaces to investigate the impact of roughness elements on the correction factor with varying inundation conditions. The results showed that using a single α value is inappropriate for rough surfaces. The correction factor was positively correlated with the Reynolds number and is controlled by flow regimes as well. Moreover, the impacts of inundation conditions as well as protruding roughness elements should be taken into account for determining α. Correction factor varied non‐monotonically with inundation ratio, depending on the progressive inundation status of protruding roughness elements. The developed equations introducing the inundation ratio performed well in predicting α, with R2 = 0.86 and NSE = 0.91. The results contribute to our understanding of measuring mean flow velocity using the dye tracer method. [ABSTRACT FROM AUTHOR]

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Floodplain vegetation is of great importance in velocity distribution and turbulent coherent structure within compound open channel flows. As the large eddy simulation (LES) technique can provide detailed instantaneous flow dynamics and coherent turbulent structure predictions, it is of great importance to perform LES simulations of compound open channel flows with floodplain vegetation. In the present study, a wall-modeled large eddy simulation (WMLES) method was employed to simulate the compound open channel flows with floodplain vegetation. The vegetation-induced resistance effect was modeled with the drag force method. The WMLES model, incorporating the drag force method, was verified against flume measurements and an analytical solution of vegetated open channel flows. Numerical simulations were conducted with a depth ratio of 0.5 and four different floodplain vegetation densities (frk = 0, 0.28 m−1, 1.13 m−1 and 2.26 m−1). The main flow velocity, secondary flow, bed shear stress and vortex coherent structure, based on the Q criterion, were obtained and analyzed. Based on the numerical results, the influences of floodplain vegetation density on the flow field and turbulent structure of compound open channel flows were summarized and discussed. Compared to the case without floodplain vegetation, the streamwise velocity in the main channel increased by 10.8%, 19.9% and 24.4% with the frk = 0.28 m−1, 1.13 m−1 and 2.26 m−1, respectively. The results also indicated that, when the floodplain vegetation density increased, the following occurred: the velocity increased in the main channel, while the velocity decreased in the floodplain; the transverse momentum exchange was enhanced; and the strip structures were more concentrated near the junction area of compound open channel flows. [ABSTRACT FROM AUTHOR]

In this study, we present a three-dimensional numerical model for the interaction of flow with submerged flexible vegetation, based on a large-eddy simulation and the immersed boundary method. The model innovatively realises the interaction between the flow and highly flexible vegetation with clustered leaves. Besides being a three-dimensional model of motion with full degrees of freedom, this study improves the consideration of the motion of the vegetation in all directions, and in addition the energy and momentum transfer in the spanwise direction. Furthermore, we perform a flume experiment for the flow with submerged flexible vegetation, the results of which are used to validate the simulation effects of the numerical model. It is found that the numerical model can effectively simulate the velocity profiles and the movement of vegetation induced by the flow. Using the model to analyse the flow–vegetation interaction, we find that the movement of vegetation is closely related to the flow velocity. As the flow velocity increases, both the offset angle and the vegetation swaying amplitude increase. Compared to vertical rigid vegetation, the tilting of flexible vegetation does not significantly change the velocity difference and the magnitude of the turbulent kinetic energy between the inside and the outside of the vegetation canopy, but it does weaken the disturbance to flow, thus reducing the resistance to flow. However, the swaying of vegetation dose significantly increase the velocity difference between the inside and the outside of the canopy. It forms Kelvin–Helmholtz hairpin vortices intensifying the turbulence production, and enhancing the disturbance and resistance to flow. [ABSTRACT FROM AUTHOR]

Hyporheic exchange, or the exchange of water and solutes between surface and subsurface water at the sediment‐water interface, regulates water quality and biogeochemical cycle in aquatic ecosystems. Vegetation, which is ubiquitous in nature, is known to impact hyporheic exchange; yet how vegetation impacts hyporheic exchange remains to be characterized. Here, we show that at the same spatially and temporally‐averaged flow velocity U $U$, vegetation increases the rate of hyporheic exchange by a factor of four. By tracking the movement of fluorescent dye in a flume with the refractive‐index‐matched sediment and translucent vegetation dowels, we demonstrate that the vegetation‐induced hyporheic exchange can be characterized by an effective hyporheic exchange velocity, VH ${V}_{H}$. We further demonstrate that VH ${V}_{H}$ correlates with the total near‐bed turbulent kinetic energy kt ${k}_{t}$ rather than U $U$, when kt<6×10−4m2/s2 ${k}_{t}< 6\times {10}^{-4}\,{\mathrm{m}}^{2}/{\mathrm{s}}^{2}$, indicating that turbulent kinetic energy is a better metric than flow velocity for predicting hyporheic exchange in regions with vegetation. Plain Language Summary: The exchange of contaminants and nutrients between surface and subsurface water in the hyporheic zone of rivers and wetlands controls water quality as well as the metabolism of benthic microbes and the associated biogeochemical cycle. Vegetation, which is ubiquitous in aquatic ecosystems, has been found to affect the surface‐subsurface exchange and as such impact water quality and stream biogeochemical cycle. However, how vegetation impacts this exchange remains unclear, making it difficult to predict the contaminant transport and biogeochemical cycle in streams, lakes, and coastal areas with vegetation. In this study, we directly visualized the release of a fluorescent dye from the transparent sediment into the surface water in a water‐recirculating tank filled with translucent vegetation. We discovered that vegetation can significantly increase the exchange in the hyporheic zone. Furthermore, we proposed a model to predict the impacts of vegetation on hyporheic exchange. We believe that this finding will help improve predictions of contaminant transport and biogeochemical cycle in streams and other aquatic ecosystems. The results of this study will also help ecologists design stream restoration projects that use vegetation to increase the retention and degradation of contaminants in sediment. Key Points: Emergent vegetation increases the exchange of water and solutes between surface and subsurface water in the hyporheic zoneThe release of solute from the sediment bed can be approximated by a mass transfer equation with an effective hyporheic exchange velocityThe impacts of vegetation on the hyporheic exchange velocity can be characterized by the near‐bed turbulent kinetic energy kt [ABSTRACT FROM AUTHOR]

The steep slopes of actively developed gullies are extremely sensitive to erosion in the Mollisol region of Northeast China; however, decision making on the optimal configuration for gully bank protection is under discussion. Here, five vegetation communities were established on regraded gully banks with bare slopes as the control to identify their controlling effects on flow hydraulics and soil erosion through in situ scouring experiments. A hydraulic flume experiment was conducted to clarify the ability of roots to reduce erosion. For vegetated slopes, flow velocity, stream power, and unit stream power decreased by 42–67%, 0–18%, and 44–68%, respectively, while flow depth, shear stress, the Darcy–Weisbach friction coefficient, and Manning coefficient increased by 55–150%, 69–156%, 5–22‐times and 2–5‐times, respectively. Changes in the seven flow hydraulic parameters were significantly influenced by vegetation coverage, aboveground biomass, root mass density and litter biomass (p <.05). Consequently, the five vegetation communities experienced decreased runoff and soil loss by 19–30% and 78–97%, respectively. Vegetation with tap‐roots and rich roots of 1.0–2.0 mm in diameter was more effective in controlling gully bank erosion. Thus, the vegetation community consisting of Dactylis glomerata, Festuca arundinacea, Uraria crinitaandMedicago sativa (25:25:25:25) is perfectly acceptable for gully bank protection in the Mollisol region of Northeast China. The results provide a scientific reference for selecting optimal vegetation configuration for gully bank rehabilitation. [ABSTRACT FROM AUTHOR]

Traditional bed shear stress‐based models (e.g., Rouse model) derived from the classic parabolic profile of eddy viscosity in open‐channel flows fail to accurately predict suspended sediment concentration (SSC) in flows with aquatic vegetation. We developed a two‐layer, turbulence‐based model to predict SSC profiles in emergent vegetated flows. Turbulence generated from vegetation, bed, and coherent structures caused by stem‐bed‐flow interaction are considered into the near‐bed turbulent kinetic energy (TKE) to calculate the effective bed shear velocity, ubeff*. The model, validated by experimental data, further showed that the thickness height of the near‐bed layer (effective bottom boundary layer), Hb, varies with flow velocity and canopy density. Two additional models are provided to estimate Hb and ubeff*. The model is expected to provide critical information to future studies on sediment transport, landscape evolution, and water quality management in vegetated streams, wetlands, and estuaries. Plain Language Summary: Aquatic vegetation serves important roles in wetlands, estuaries, riverine, and coastal areas. It provides shelter for living organisms, protects coastal and riverbank regions from waves, currents, and floods, and makes a contribution to the global carbon circulation. However, under various scenarios of environmental change, the presence of aquatic vegetation continuously decreased in these areas during recent decades. To improve restoration efforts and protect vegetation habitats, understanding vegetation‐sediment dynamics is a paramount and will lead to more accurate models of landscape evolution and water quality management. Traditional suspended sediment models widely applied in open‐channel flows, do not work in regions with aquatic plants. We propose a modified model to predict suspended sediment concentration in vegetated flows, validated with experimental data. The new model showed that near‐bed turbulence generated by bed‐vegetation‐flow interaction is as important as the bed‐ and vegetation‐generated turbulence considered separately. The model is expected to provide critical information for future studies on sediment transport, landscape evolution, and water quality management in vegetated streams, wetlands, and estuaries. Key Points: We present a two‐layer, turbulence‐based model to predict suspended sediment concentration (SSC) in flows with emergent vegetationWe include coherent structures generated by stem‐bed‐flow interactions to calculate near‐bed turbulent kinetic energyWe derive models for effective boundary layer thickness and effective bed shear velocity to predict SSC [ABSTRACT FROM AUTHOR]

There is considerable interest in how headwater management may influence downstream flood peaks in temperate humid regions. However, there is a dearth of data on flow velocities across headwater hillslopes and limited understanding of whether surface flow velocity is influenced by seasonal changes in roughness through vegetation cycles or management. A portable hillslope flume was used to investigate overland flow velocities for four common headwater grassland habitats in northern England: Low‐density Grazing, Hay Meadow, Rank Grassland and Juncus effusus Rush pasture. Overland flow velocity was measured in replicate plots for each habitat, in response to three applied flow rates, with the experiments repeated during five different periods of the annual grassland cycle. Mean annual overland flow velocity was significantly lower for the Rank Grassland habitat (0.026 m/s) followed by Low‐density Grazing and Rushes (0.032 and 0.029 m/s), then Hay Meadows (0.041 m/s), which had the greatest mean annual velocity (examples from 12 L/min flow rate). Applying our mean overland flow velocities to a theoretical 100 m hillslope suggests overland flow is delayed by >1 hr on Rank Grassland when compared to Hay Meadows in an 18 mm storm. Thus grassland management is important for slowing overland flow and delaying peak flows across upland headwaters. Surface roughness was also strongly controlled by annual cycles of vegetation growth, decay, grazing and cutting. Winter overland flow velocities were significantly higher than in summer, varying between 0.004 m/s (Rushes, November) and 0.034 m/s (Rushes, June); and velocities significantly increased after cutting varying between 0.006 m/s (Hay meadows, July) and 0.054 m/s (Hay meadows, September). These results show that seasonal vegetation change should be incorporated into flood modelling, as cycles of surface roughness in grasslands strongly modify overland flow, potentially having a large impact on downstream flood peak and timing. Our data also showed that Darcy‐Weisbach roughness approximations greatly over‐estimated measured flow velocities. [ABSTRACT FROM AUTHOR]

The time-averaged flow dynamics of a suspended cylindrical canopy patch with a bulk diameter of D is investigated using large-eddy simulations (LES). The patch consists of Nc constituent solid circular cylinders of height h and diameter d, mimicking patchy vegetation suspended in deep water (H/h » 1, where H is the total flow depth). After validation against published data, LES of a uniform incident flow impinging on the canopy patch was conducted to study the effects of canopy density (0.16 ≤ ø = Nc(d/D)2 ≤ 1, by varying Nc) and bulk aspect ratio (0.25 ≤ AR =D h/D ≤ 1, by varying h) on the near-wake structure and adjustment of flow pathways. The relationships between patch geometry, local flow bleeding (three-dimensional redistribution of flow entering the patch) and global flow diversion (streamwise redistribution of upstream undisturbed flow) are identified. An increase in either ø or AR decreases/increases/increases bleeding velocities through the patch surface area along the streamwise/lateral/vertical directions, respectively. However, a volumetric flux budget shows that a larger AR causes a smaller proportion of the flow rate entering the patch to bleed out vertically. The global flow diversion is found to be determined by both the patch geometrical dimensions and the local bleeding which modifies the sizes of the patch-scale near wake. While loss of flow penetrating the patch increases monotonically with increasing ø, its partition into flow diversion around and beneath the patch shows a non-monotonic dependence. The spatial extents of the wake, the flow-diversion dynamics and the bulk drag coefficients of the patch jointly reveal the fundamental differences of flow responses between suspended porous patches and their solid counterparts. [ABSTRACT FROM AUTHOR]

The influence of rigid unsubmerged vegetation on flow structure and turbulent kinetic energy of gradually varied flow are experimentally investigated in this research. Natural reed stems of different densities are employed to examine the effects of the rigid unsubmerged vegetation on the flow in rivers. The results reveal that the vegetation existence significantly changes the gradually varied flow state from type M1 to type M2 in the vegetation section. The traditional power law describing the vertical flow velocity profile is evidently invalid when the vegetation density becomes high. With the irregularity index proposed in the research, the irregularity of vertical flow velocity profile in vegetated reach can be exponentially described in relation to the vegetation density. Furthermore, the turbulent kinetic energy is found to increase and reach a maximum value near the end of the vegetation section that is a potential localized erosion area. The results of the research have significances in river ecological restoration applications utilizing aquatic vegetation. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A single-layer velocity model is proposed in this study to predict the cross-sectional average velocity for open channel flows with submerged rigid cylinders. This model is preferable to previous two-layer models in that it does not involve evaluations of vegetation drag coefficient and hydrodynamic height of roughness elements. In the development of the single-layer model, the concept of the hydraulic radius is applied to determine the length scale of the entire cross-section affected by the submerged vegetation. The resulting friction factor scales largely with the ratio of stem height to flow depth with exponent 3=2, and also decreases slightly with an increasing Reynolds number. Comparisons of the single-layer model with experimental data show that its performance is comparable to the previous two-layer models, and the model is also applicable to open channel flows subject to submerged flexible vegetation. However, the proposed model cannot be extended to two extreme conditions including flows over a smooth channel bed without vegetation and flows passing through emergent vegetation. [ABSTRACT FROM AUTHOR]

The distribution of aquatic vegetation within conveyance channels plays a key role in the determination of their hydraulic characteristics, especially where the vegetation is patchy, as is commonly found in nature. This paper reports the results of a laboratory flume study on flow and turbulence conditions caused by patches of the emergent rush . Acoustic Doppler velocimeter measurements were taken within and downstream of single- and multiple-patch configurations of this plant, and the effects of varying incident flow rate (0·16-0·32 m/s) and stem-scale porosity (12-4% solid volume fraction) were investigated. The results showed that flow encountering a single patch formed a turbulent wake downstream of the vegetation. Within this wake, the Reynolds stress increased downstream initially, and then decayed. When a second patch was positioned within the region of maximum Reynolds stress, the Reynolds stress decreased by between 25 and 50%. The amount of this reduction was dependent on the porosity of the vegetation and the flow rate, and was greater at lower flow rates and porosities. These changes in turbulent flow fields around patches of emergent vegetation are likely to be important in determining sediment budgets in their vicinity. [ABSTRACT FROM AUTHOR]

Soil erosion is a major problem in the Loess Plateau (China); however, it can be alleviated through vegetation restoration. In this study, the overland flow on a slope during soil erosion was experimentally simulated using artificial grass as vegetation cover. Nine degrees of vegetation coverage and seven flow rates were tested in combinations along a 12° slope gradient. As the coverage degree increased, the water depth of the overland flow increased, but the flow velocity decreased. The resistance coefficient increased with increasing degree of coverage, especially after a certain point. The resistance coefficient and the Reynolds number had an inverse relationship. When the Reynolds number was relatively small, the resistance coefficient decreased faster; however, when it exceeded 600, the resistance coefficient decreased at a slower rate. A critical degree of vegetation cover was observed in the relationship between the resistance coefficient and submergence degree. When the degree of coverage was greater than 66.42%, the resistance coefficient first decreased and then increased with a higher submergence degree. Finally, the formula for the resistance coefficient under vegetation coverage was derived. This formula has a relatively high accuracy and can serve as a reference for predicting soil erosion. [ABSTRACT FROM AUTHOR]

The body of literature seeking to evaluate particle interception in vegetated, aquatic environments is growing; however, comparing the results of these studies is difficult due to large variation in flow regime, particle size, vegetation canopy density, and stem configuration. In this work, we synthesize data from these studies and develop a functional form of particle interception efficiency (η) as a function of stem Reynolds number ( Re c ), stem diameter, vegetation frontal area, particle–collector diameter ratio, flow velocity, and kinematic viscosity. We develop this functional relationship based on a dimensional analysis and hypothesize that the coefficients would exhibit regimes within different Re c ranges. We test this hypothesis by synthesizing data from 80 flume experiments reported in the literature and in-house flume experiments. Contrary to our hypothesis, data from different Re c ranges follow a single functional form for particle interception. In this form, η varies strongly with collector density and particle–collector diameter ratio, and weakly with Re c and particle–fluid density ratio. This work enables more accurate modeling of the flux terms in sedimentation budgets, which can inform ongoing modeling and management efforts in marsh environments. For example, we show that by integrating the new functional form of particle interception into established models of marsh elevation change, interception may account for up to 60% of total sedimentation in a typical silt-dominated marsh ecosystem with emergent vegetation. [ABSTRACT FROM AUTHOR]

Suspended sediment collected by vegetation in marshes and wetlands contributes to vertical accretion, which can buffer against rising sea levels. Effective capture efficiency (ECE), a parameter quantifying the fraction of incoming suspended particles directly captured by underwater vegetation surfaces, plays a key role in determining the significance of direct interception in morphodynamic models. The ways in which physical characteristics of collectors and transitionally turbulent flows affect ECE are not yet thoroughly understood. We conducted a set of 12 experiments at three flow velocities and three stem densities (plus equivalent zero-collector control experiments), plus four experiments where biofilm was allowed to accumulate. We determined that ECE decreases with increasing collector Reynolds number (study range: 66 to 200; p < 0.05 for two of three treatments) and increasing collector density (solid volume fraction: 0.22% to 1.17%; p < 0.05 for two of three treatments). Adding biofilm increased ECE in all cases, by a multiplicative factor ranging from 1.53 to 7.15 at different collector densities and biofilm growth durations. In some cases, the impact of biofilm on ECE far outweighed that of collector Reynolds number and density. By combining our data with those of one similar study, we present a preliminary model quantitatively assessing the effect of collector density on ECE. [ABSTRACT FROM AUTHOR]

Research of interactions between in-channel vegetation and flow structure is important for the restoration of aquatic ecosystems. This study aims to investigate the impact of the vegetation patch density on the wake structure. We used uniform fiberglass circular cylinders to simulate the non-submerged rigid plant community. In addition, a wide range of vegetation patch densities was considered and a 3D acoustic Doppler velocimeter (ADV) was used to measure local flow velocities. High-density vegetation patches correlated with a high maximum turbulent kinetic energy and a double-peak phenomenon for the lateral distribution. In conclusion, differences between Reynolds shear stresses near the bed surface upstream and downstream of vegetation patches correlate with the vegetation density. [ABSTRACT FROM AUTHOR]