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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]

Systematical downslope‐turbidity‐current experiments were performed to clarify the relationship between sediment‐transport modes and current propagation patterns caused by rigid vegetation, as well as adjustments in turbulence characteristics of current. The equations for predicting the front velocities of downslope turbidity currents with emergent vegetation were proposed and validated via experimental data. Experimental results reveal that sediment deposition causes the lower turbulent kinetic energy (TKE) peaks of turbidity currents to decrease or even disappear without affecting their upper TKE peaks, while the rigid vegetation has the opposite effect. Vegetation stems destroy the longitudinal low‐high speed streaks associated with the quasi‐streamwise eddies in the near‐bed region and increase the proportions of the outward and inward interactions. In addition, sediment deposition severely suppresses the turbulent bursting events within turbidity currents but does not influence the relative proportions of the four bursting types. Rigid vegetation and sediment deposition both degrade the absolute values of the third‐order moments of velocity fluctuations without changing their signs to reduce the generation frequency of sweep events. Emergent rigid vegetation accelerates the formation of the reflected bore to provide energy for the deposited sediment to move downstream continuously. On the other hand, the dense submerged vegetation makes turbidity currents easily form the two‐head propagating mode, which allows part of the sediments carried by the upper current head to be transported downstream rather than deposited within or upstream of the vegetation region. Furthermore, the sloping boundary provides favorable conditions to initiate these specific modes for sediment transport adjusted by rigid vegetation. Key Points: Equations are developed and validated to predict the front velocities of turbidity currents flowing slopes with emergent rigid vegetationRigid vegetation and sediment gradation are key factors in controlling the turbulence characteristics of downslope turbidity currentsRigid vegetation distinctly changes the sediment‐transport modes of downslope turbidity currents by adjusting their propagation patterns [ABSTRACT FROM AUTHOR]

Aquatic plants play a crucial role in the hydrodynamic and material transport processes within the aquatic environments due to the additional flow resistance induced by vegetation stems. In this study, high-resolution numerical experiments were performed to investigate the drag characteristics of circular vegetation patches fully immersed in a turbulent open channel flow. The submerged vegetation patch was modeled as a rigid cylinder array with a diameter D composed of N cylinder elements with a diameter d. The effects of vegetation density Φ (0.023 ≤ Φ ≤ 1) and relative diameter d/D (d/D = 0.051 and 0.072) were tested. The simulation results show that Φ and d/D affect the flow resistance exerted by the vegetation patch by modifying the bleeding flow intensity. With the increase in Φ, the drag forces acting on the individual cylinder elements decrease, whereas the total drag forces of the patch increase. The oscillation strength of the drag force of individual cylinders depends on Φ and the fixed positions within the patch. The presence of the free end of submerged cylinder array leads to enhanced wake entrainment with the increase in Φ. The drag coefficient of the submerged patch is smaller than that of the emergent patch when the dimensionless frontal area aD > 3. However, the two patches exhibit comparable drag coefficients for smaller aD values. [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]

Enhanced understanding of flow resistance in open channels with emergent vegetation is essential for flood management and river ecosystem restoration. The presence of vegetation can significantly alter bed resistance, leading to a challenge in accurately predicting flow discharge, water levels, sediment transport, and bed deformation. Previous studies on vegetated flows have focused on vegetation resistance, on which the impact of vegetation has been ignored or poorly estimated. This study proposes a new analytical model, built upon the momentum conservation law, to predict flow resistance to vegetated zones in a plain bed without bed forms, explicitly quantifying bed resistance and vegetation resistance in a corollary manner. The proposed model is benchmarked against five typical sets of laboratory experiments. It is demonstrated that the present model using a modified logarithmic velocity distribution performs best, whereas that assuming a uniform velocity profile considerably overestimates the vegetation resistance and neglects the effect of vegetation on bed resistance. The ratio of bed resistance to the total resistance is shown to range between 5% and 40%, and it decreases with increasing vegetation density and decreases with water depth. Therefore, bed resistance cannot be ignored when modelling shallow water flow with sparsely distributed vegetation. It is also revealed that vegetation arrangements significantly affect flow resistance, and therefore a model incorporating the effect of vegetation arrangement performs better. Overall, the present model facilitates a viable and promising tool for quantifying flow resistance in emergent vegetated channels. [ABSTRACT FROM AUTHOR]

Bend sections are ubiquitous in natural sandy river systems. This study employs Computational Fluid Dynamics–Discrete Phase Model (CFD-DPM) methodology to analyze particle transport dynamics in U-bend channel flows, focusing on the distinctions between partially vegetated (Case No.1) and non-vegetated (Case No.2) scenarios. The research aims to unravel the intricate relationships among bending channel-induced secondary flow, vegetation blockage, and particle aggregation, employing both quantitative and qualitative approaches. (I) The key findings reveal that vegetation near the inner walls of curved channels markedly diminishes the intensity of secondary circulation. This reduction in circulation intensity is observed not only within vegetated areas but also extends to adjacent non-vegetated zones. Additionally, the study identifies a close correlation between vertical vortices and particle distribution near the channel bed. While particle distribution generally aligns with the vortices' margin, dynamic patch-scale eddies near vegetation patches induce deviations, creating wave-like patterns in particle distribution. (II) The application of the Probability Density Function (PDF) provides insights into the radius-wise particle distribution. In non-vegetated channels, particle distribution is primarily influenced by secondary flow and boundary layers. In contrast, the presence of vegetation leads to a complex mixing layer, altering the particle distribution pattern and maximizing PDF values in non-vegetated free flow subzones. (III) Furthermore, the research quantifies spatial–temporal sediment heterogeneity through PDF variance. The findings demonstrate that variance in non-vegetated channels increases towards the outer wall in bending regions. Vegetation-induced turbulence causes higher variance, particularly in the mixing layer subzone, underscoring the significance of eddy size in sediment redistribution. (IV) The study of vertical concentration profiles in vegetated U-bend channels offers additional insights, while secondary flow in non-vegetated channels facilitates upward sediment transport and vegetation presence, although increasing the Turbulent Kinetic Energy (TKE), restricts channel space, and impedes secondary flow, thereby reducing vertical particle suspension. Sediment concentrations are found to be higher in the lower layers of vegetated bends, contrary to the pattern in non-vegetated bends. These findings highlight the complex interplay between vegetation, secondary flow, and sediment transport, illustrating the reduced effectiveness of secondary flow in promoting vertical particle transportation in bending channels due to the vegetation obstruction. [ABSTRACT FROM AUTHOR]

This study presents results from an experimental campaign to explore how different riparian zone characteristics may facilitate the transport or capturing of plastics floating through the fluvial system. Specifically, following field observations for the transport of plastics through fluvial vectors, a substantial number of flume experiments has been designed to assess the effect of floating macro-plastics and riparian zone characteristics. The results from flume experiments were analyzed using particle tracking velocimetry techniques to derive transport metrics (such as transport velocities) of macro-plastics of different sizes and shapes, released at five locations across a wide channel with distinct distance from the vegetated riverbank. The findings are discussed while considering the trapping mechanisms along the vegetated riverbank, which include a range of vegetation densities and arrangements, aiming to identify and quantify the degree of impact of each of the control parameters on the transport of floating plastics. The flow velocimetry records obtained at locations near and within the riverbank correlate well with the transport velocities of the floating plastics. Macro-plastic litter carried downstream away from the riverbank can have up to nine times the transport velocity, compared to those found within the riverbank. The change from a low to a high average density can result in about three times decrease in the transport velocity of floating macro-plastic litter within the riparian zone. These outcomes can help inform better practices for the management of riparian vegetation to maximize the trapping efficiency of macro-plastics, adapted to different flow conditions and river morphologies. [ABSTRACT FROM AUTHOR]

Aquatic vegetation and morphology are integral components of fluvial systems, study of which is important due to ecological and engineering projects. Often, studies in fluvial systems are carried out in the laboratory as collecting data in the field is expensive and difficult, and even risky. A field study was carried out to measure the hydraulic and geometric parameters of the bed and the banks of a gravel bed river with instream emergent vegetation in northern Iran. The aim of this study was to gain insight into the effect of bed forms and vegetation patches on flow resistance and distribution of velocity and bed shear stress. The boundary layer characteristics method was employed to calculate the shear velocity and friction factor, the resulted friction values are compared with that of Manning's equation. The friction of bed forms was estimated using a linear superposition of friction. Results showed that the boundary layer characteristics method was suitable for calculating the shear velocity under various conditions, moreover, the distribution of velocity and bed shear stress were discussed. The findings of this study will be useful in river management and restoration. [ABSTRACT FROM AUTHOR]

Agricultural headwater streams and ditches commonly host dense stands of aquatic vegetation that grow and decay over seasons and exert physical and biological controls on the transport of nutrients from cropland to larger rivers. This study examined changes in the transport of phosphorus (P) in an agricultural drainage ditch in the Maumee River Basin (Ohio, USA) by conducting constant rate injections of a novel tracer mixture (conservative salt [Cl as NaCl], dissolved P [KH2PO4], and a fluorescent fine particle) in spring, summer, and fall. We quantified transport behavior for solutes and particles using a traditional transient storage modeling framework consisting of mobile and immobile storage zones connected by a first‐order exchange rate constant. Transient storage was greatest during the spring, when thicker vegetation caused more pooling and flow stagnation, and decreased through fall, as vegetation thinned. Soluble P uptake lengths were 8.7 times longer in fall than spring, likely due to declines in biological uptake rates with colder temperatures and immobile zone storage with thinning vegetation. Particle capture lengths also decreased by a factor of 4.3 from fall to spring. With the increasing eutrophication of Lake Erie and waterbodies around the world that lie downstream from agricultural landscapes, it is beneficial to understand nutrient transport across watersheds, including small agricultural streams. This study highlights the physical and biological roles that aquatic vegetation plays in small agricultural streams by creating seasonally variable immobile zones that slow the flow of nutrients, providing surface area for biofilms, and capturing particles that bind nutrients. Plain Language Summary: In the Midwestern United States and many parts of the world, headwater streams in agricultural lands are often thickly vegetated with reed grasses or other water‐loving plants. An important question is how this vegetation slows down or holds back nutrients, particularly phosphorus (P), which reduces water quality in downstream water bodies by contributing to algal blooms. To explore this question, we introduced a novel combination of chloride, dissolved P, and brightly colored particles to the same headwater stream over three seasons when vegetation was in various stages of growth or decay. We found that vegetation was most effective at capturing fine particles in spring when stands were thickest. Similarly, rates of plant and microbial uptake of dissolved P were faster in spring, leading to stronger retention of dissolved P. This study shows that aquatic plants in headwater agricultural streams and ditches play an important role in the downstream movement of both dissolved and particulate forms of nutrients. They form a storage zone that expands and contracts over the year in terms of its size, biological demand for dissolved nutrients, and efficiency at capturing suspended particles. Key Points: Aquatic vegetation stands are the dominant storage zone for dissolved phosphorus and fine particles in an open‐canopy agricultural ditchSoluble phosphorus uptake velocity was 27 times greater in spring than fall, likely due to biofilm growth on dense vegetationParticle capture efficiencies were 3 times greater in spring than fall due to denser vegetation [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]

Vegetation present in the water streams, on the banks and in the floodplain areas largely affects the river hydraulics. Indeed, river vegetation significantly influences hydrodynamics, sediment transport, bedforms, and pollutant transport. Environmental management of rivers requires an understanding of the various processes and predictive capabilities of models. In the past, many studies were conducted, especially in laboratory settings, in order to quantify flow resistance due to vegetation. It is only recently that the effects of vegetation on sediment transport came to the attention of researchers. In particular, both suspended and bedload transport were considered. This paper reviews recent works conducted on the effect of vegetation on incipient sediment motion and bedload transport. With regard to the incipient sediment motion, methods based on critical velocity, turbulence, vegetation drag, and velocity in the bed roughness boundary layer have been discussed. For bedload transport, methods based on bed shear stress, turbulent kinetic energy, a revisiting of classical formulas for estimating bedload transport in non-vegetated channels, and estimation from erosion around a single vegetation stem are analyzed. Finally, indications on further research and new development are provided. Article Highlights: Incipient sediment motion in vegetated channels is different from that in bare channels New models for the estimation of sediment transport in vegetated channels consider turbulence effects The presence of vegetation in rivers contributes in promoting the stability of the streambed [ABSTRACT FROM AUTHOR]

The roots of some coastal and wetland trees grow peculiar vertical protrusions, the function of which remains unclear. Here, using computational simulations based on first-principles fluid and sedimentation dynamics, we argue that the protrusions work together to create an elevated patch of sediment downstream of the tree, thereby creating its own fertile flood-protected breeding grounds for the seedlings. In our simulations, we vary the vertical root diameter, root spacing and total root area and show that there is an optimal vertical root spacing that depends on root thickness. Next, we quantify and discuss the cooperative effects between adjacent vertical root patches. Lastly, by varying vertical root spacing of a patch of trees, we estimate a maximal vegetation density for which verticalroot production has a beneficial geomorphological response. Our hypothesis suggests that vertical roots, such as the 'knee roots' of baldcypress trees, have an important role in shaping riparian geomorphology and community structure. [ABSTRACT FROM AUTHOR]

Aquatic vegetation appears very often in rivers and floodplains, which significantly affects the flow structure. In this study, experiments have been conducted to investigate the effects of submerged vegetation arrangement patterns and density on flow structure. Deflected and non-bending vegetation is arranged in square and staggered configurations in the channel bed of a large-scale flume. Results showed that the staggered configuration leads to intensified streamwise velocity, turbulence kinetic energy (TKE), and Reynolds shear stress (RSS) compared to the square configuration. When vegetation density is low (λ = 0.04 and λ = 0.07), the produced wake in the rear of the vegetation is more expansive than that with high vegetation density (λ = 0.09 and λ = 0.17) because the velocity in the center of four vegetation elements is lower than that in the middle of two vegetation elements with low vegetation density. Results of TKE in the wake zone of the deflected vegetation indicate that the maximum root-mean-square velocity fluctuations of flow occur at the sheath section (z/H = 0.1) and the top of the vegetation (z/H = 0.4). In the wake zone behind the vegetation elements, the maximum value of the RSS occurred slightly above the interface between deflected vegetation and the non-vegetation layer, showing the Kelvin–Helmholtz instability that is associated with inflectional points of the longitudinal velocity. Within the range of vegetation density in this study (0.04 < λ ≈< 0.23), as the vegetation density increases, the negative and positive values of RSS throughout the flow depth increase. [ABSTRACT FROM AUTHOR]

Bed shear stress in coarse–bed rivers with vegetation patches is one of the challenging parameters in hydraulic engineering, mechanical engineering, fluvial morphology, and environmental studies. Based on this necessity, in this study, the values of bed shear stress in four reaches of rivers in Iran were estimated and compared using the methods of boundary layer characteristics, logarithmic law, and Darcy–Weisbach. Data collection in this study started in February 2021 and ended in April 2021. Estimation of flow resistance is a key factor in many numerical and physical models. In order to obtain a reasonable evaluation of this factor, it is necessary to measure and calculate the key variables of resistance to flow. Accordingly, the experimental design in this study includes surveying operations, velocity measurement, and sampling of bed sediments. The results show that due to bed forms, vegetation patches, and variations of flow depth and grain size in the river, the universal velocity distribution law (the log law) may not be suitable to estimate the shear velocity, which is a key parameter of flow resistance. This calls for more justifiable methods which are not affected by near–the–bed conditions. Accordingly, a three–parameter flow resistance model is presented, which shows an average error of 17%, indicating the accuracy of the model. The investigation of 71 measured velocity profiles shows the occurrence of the Dip phenomenon in the velocity profiles near the vegetation patches. However, by moving away from the vegetation patches, the effect of this phenomenon is decreased, and the profiles illustrate an S–shaped distribution. The results show that the relative differences between the logarithmic law and Darcy–Weisbach methods compared to the boundary layer characteristics method (BLCM) are equal to 87% and 39%, respectively, indicating a more reasonable agreement between the Darcy–Weisbach method and the boundary layer characteristics method. This is due to the application of key parameters of the boundary layer theory to calculate shear velocity by BLCM. However, to simplify data collection in the field, the application of the Darcy–Weisbach method is suggested. [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 hydrodynamics in a straight rectangular open channel containing novel lateral cavities constructed by an array of square emergent vegetation patches discontinuously distributed along the bank were explored numerically using three-dimensional large eddy simulations (LES). Five vegetation densities (Φ), ranging from 0.02 to 0.25, as well as the traditional lateral cavities created by impermeable solid media, were tested. The effects of the cavity aspect ratio (AR) were also examined. The LES results showed that the mean recirculation pattern inside the vegetation cavities and coherent structures in the horizontal shear layer were closely dependent on Φ and AR. When Φ ≥ 0.06, a main recirculation vortex that formed inside the vegetation cavities resembled that within solid media cavities, whereas the extent of the former increased upstream as Φ increased. Compared with the solid cases, the vegetation cavities exhibited a higher turbulent intensity within the shear layer and wider regions of enhanced turbulent kinetic energy, which decreased with increasing Φ. The penetration depth of the elevated turbulent kinetic energy into the cavities also decreased with increasing Φ, whereas a deeper penetration was expected at larger AR values. The interfacial turbulence was dominated by "cavities field"-scale coherent vortices at Φ ≤ 0.06, whereas "cavity element"-scale at Φ ≥ 0.15. When Φ = 0.1, the shear vortices of both scales contributed to the enhancement of the interfacial turbulence. The mean mass exchange showed a non-monotonic relationship with Φ and reached maximum values at Φ = 1. The total momentum transport efficiency decreased monotonically with increasing Φ. Despite the AR and Φ values, the turbulent motions dominated the momentum transport over most of the cavity length. [ABSTRACT FROM AUTHOR]

The drag coefficient of a one-line circular patch of emergent sparse vegetation in an open-channel flow or riverbeds with varying bed roughness sizes was experimentally investigated. The total roughness of the channel was accounted for using the rate of energy dissipation that incorporated resistance by bed particles and cylinder obstacles. A total of 48 flume experiments were conducted by varying the flow rate, bed roughness, and water depth. The analysis of flow resistance due to the one-line patch of sparse vegetation and the variation in bed roughness was achieved through dimensional analysis. The results show that the drag coefficient Cd is a function of stem Reynolds number, Rd, Froude number, Fr, and the ratio between stem diameter and flow depth (d/H). Experiments showed that the drag coefficient varied with the stem Reynolds number, Froude number, and bed roughness. The Cd generally increased with decreasing stem Reynolds number and Froude number, and this increase in drag coefficient appeared significant for a rough bed of large particles. Results showed that Reynold numbers for different bed roughness had a minimal influence on the drag coefficients. The Fr increases as bed materials are smoother. Statistical regression analysis was achieved to drive the relationship between drag coefficient versus Rd, Fr, and d/H with a coefficient of determination of more than 0.97. [ABSTRACT FROM AUTHOR]

With riverine flooding set to be more frequent in many parts of the world as a result of climate change, the interactions between fluvial morphodynamics and riparian vegetation may depend in part on the sequence of flood events. This paper describes a laboratory study of the geomorphic adjustment of a braided river to sequences of floods across five different strengths of braidplain vegetation. By using alfalfa as a proxy for braidplain vegetation, the differing plant life stages were used to represent the varying strengths of biogeomorphic feedbacks across the floods. Boundary conditions were constrained by sets of experimental runs with both equilibrium sediment loads and deficit loads. Changes in bed topography were monitored and assessed using a detailed digital elevation model, digital imagery and continuous monitoring of the transported sediment. Results demonstrate that in absence of plant colonization, vegetation placed the rivers in a non‐equilibrium condition, in which riparian vegetation encouraged the development of new channels, increased the system channel width and enhanced topographic irregularity, these effects being more noticeable during the low‐flow periods. The morphodynamics was found to be less sensitive to variations in flood discharges as the vegetation influence (strength) increased from minimum to maximum, until vegetation began to die back and the impacts of flood sequences became yet again evident. Although the overall sediment transport rate was reduced under full‐grown vegetation conditions, the presence of the mature plants across the braid bars resulted in the greatest channel scour depths. Results are considered in light of expected changes in flood frequency with climate and likely morphodynamic responses of river systems as a result. [ABSTRACT FROM AUTHOR]

Inundation events caused by hurricanes or tsunamis pose a substantial risk to the integrity of coastal infrastructure; however, their impact on the built environment can be greatly altered by natural and anthropogenic obstacles or disturbances to the flow, such as vegetation or neighboring structures. This paper investigates the impact of coastal vegetation on shoreline structural vulnerability due to wave runup loading. Using numerical simulation data, the load (base shear) and momentum flux are computed as a function of vegetation characteristics (length and density) for different excitation intensities (wave heights) at the location of a shoreline structure. This information is then used to estimate structural fragility. Motivated by recent reconnaissance data from Hurricane Matthew in 2016, emphasis is placed on the out-of-plane failure of infill masonry walls. Extension to a different structural typology, that of a reinforced concrete frame, is also discussed. Comparisons between different vegetation characteristics and the bare-earth case demonstrate the wave dissipation and reduction of structural fragility (and therefore ultimately vulnerability) achieved when vegetation is present for the investigated case-study structures. [ABSTRACT FROM AUTHOR]

Hydrochorous dispersal through agricultural channels plays a role in structuring plant communities across agricultural landscapes. To date, research on seed retention in vegetated areas has mainly focused on vegetation types with simple architecture (often cylinders), which consequently do not represent real vegetation features. Here, we test the hypothesis that vegetation cover estimated at the water surface best explains floating seed retention in open channels. We therefore proposed an experiment to measure seed retention in a controlled environment across a large range of hydraulic conditions and vegetation architecture types. We used three types of artificial plants with contrasting morphotypes, and real seeds of Rumex crispus. Vegetation metrics were calculated on the basis of 3D plant models. We also tested the additivity of seed retention as a function of the length of vegetated area crossed by the seeds. We developed a semi‐empirical formula for predicting seed retention. The main results of the experiment show that (i) the seed retention rate reacts differently to changes in density according to species, (ii) vegetation cover at the free water surface, potentially in contact with seeds, is a generic predictor of floating seed retention whatever the nature of the vegetated cover, and (iii) 95% of seed retention was reached for a large range of surface vegetation ratios and length of vegetation cover. The proposed formula could be used by stakeholders (farmers and ecologists) to estimate the amount of vegetation needed in a channel to limit or enhance seed dispersal. [ABSTRACT FROM AUTHOR]

Understanding scalar transport in solvents is important in chemical engineering, pollution control, and water remediation, where the longitudinal dispersion coefficient (LDC) is a key parameter for describing solute transport in fluids. For flow in classic conditions such as in a pipe or a regular open channel, formulas for LDC are derived from adopting the ideas of the advection–diffusion equation. However, when the flow encounters large-scale roughness, such as an open channel with vegetation, longitudinal dispersion becomes complicated. This paper aims to estimate LDC in an open channel that is partially covered with artificial vegetation. This kind of artificial vegetation is emergent, which comprises eight floats. A two-zone model is proposed to determine LDC in this channel condition. In validating our model, experiments are conducted with Rhodamine as a tracer, whose time-concentration curves are measured at two locations. The routing procedure is applied to obtain LDC from these curves. Results show that the measured LDCs are consistent with the predicted ones, thereby validating the accuracy and reliability of our proposed two-zone model. [ABSTRACT FROM AUTHOR]

Fluid retention and flow resistance due to natural vegetation remain poorly understood despite the importance of understanding these for flow routing and floodplain revegetation projects. Experiments were undertaken in a shallow earthen channel containing a natural cover of small trees, herbaceous plants, and leaf litter, which were sequentially removed and subjected to a range of flows. A dilution monitoring approach within the Aggregated Dead Zone framework was applied to a series of floodplain vegetated flows, yielding information on bulk flow parameters including tracer dispersion, fluid retention, and flow resistance at the reach scale. The primary response of flow to vegetation removal was a small increase in bulk velocity, with depth and wetted width decreasing only slightly. Reach mean travel time and the advective time delay decreased by about a factor of 2 with the removal of herbs, grass, and leaf litter, leaving only trees. Removing the trees, leaving a bare earthen channel, only slightly decreased travel times. Flow resistance and retention exhibited large values for low discharge and converged on a constant low value for relatively high discharges. It is concluded that flow resistance during low flow is higher than in a high flow with the same vegetation. Consequently, sparse vegetation has a prominent effect on hydraulic retention compared with an unvegetated channel at low discharges but this becomes negligible during high discharges as momentum increasingly dominates the flow. This outcome casts doubt on the efficacy of scrubby vegetation to impede higher‐velocity floodplain flows, showing need for field‐scale determination of integral floodplain resistance. [ABSTRACT FROM AUTHOR]

This paper investigates the influence of near-bank vegetation patches on the bed morphological adjustment in open channel flow systems. The 2D depth-averaged hydro-morphological model is adopted for this investigation, which is first validated by laboratory experimental data measured in an open channel with a single near-bank vegetation patch. The validated model is then applied for extensive numerical simulations, with the aim of conducting a systematic analysis of the influence of different geometric controlling parameters on the bed morphological evolution. The controlling parameters taken into account for numerical analysis include the angle of repose value (RAV) of sediment, vegetation density (VD), patch length (PL) and patch width (PW). The numerical results and analysis show that: (1) the RAV of sediment with slope-failure parametrization only influences the shape of the transverse bed topography in the junction region; (2) increase in VD, PL and PW that substantially enhances flow blockage effect encourages the growth of the pool adjacent to the patch in three dimensions; (3) increase in VD, PL and PW produces analogous retrogressive erosion (erosion toward the upstream) in the pool region, presumably due to the increase in flow resistance. Additional numerical experiments suggest that the staggered-order distribution of multiple patches might be an optimal choice for channel restoration and conservation since pools and riffles with larger scales can be produced. [ABSTRACT FROM AUTHOR]

Various vegetations are often grown on floodplains, and it has a significant influence on the movement of water flow and the protection of river slopes. In the experiments performed in this study, a cylindrical aluminum column with a diameter of 4 mm was selected to simulate natural vegetation and 7 classes of slopes (i = 0%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%, where i is the percentage of slope) and four categories of lodging angles (θ = 20°, 40°, 60°, and 80°) were assigned. The experimental results show that when i > 0%, the curves of Manning's roughness coefficient (n) and flow depth (h) converge, and the degree of convergence gradually increases with the slope. In addition, Manning's roughness coefficient increases with the increase in slope at shallow flow depths, and decreases with the increase in slope at deeper flow depths. Exploration of the relationship between slope and vegetation roughness not only provides a theoretical support for flood control, but also has practical significance for river ecological environment management. [ABSTRACT FROM AUTHOR]

In nature, aquatic vegetation is one of the important factors—along with hydraulic characteristics and geometric configuration—determining the dispersion of scalars. Previous research has studied the effects of vegetation on longitudinal dispersion, varying plant population with uniform arrangement. However, since vegetation grows more often in clumped and heterogeneous rather than uniform patterns, the present work investigates the effects of the vegetation arrangement on the flows characteristics such as longitudinal dispersion, mean velocities, and drag coefficients. Several types of vegetation arrangement are described with the isometric and allometric concept and quantified with the standardized Morisita index. Laboratory experiments were performed to investigate variations of hydraulic parameters and longitudinal dispersion coefficients according to vegetation arrangements, which were quantified with the standardized Morisita index. The hydraulic parameters of mean velocity, turbulent kinetic energy, and drag depend on the vegetation arrangements, and in particular, the longitudinal dispersion coefficient varies with these arrangements by a factor of 4 or more. Key Points: The standardized Morisita index can compare vegetation arrangements quantitatively and varies with dispersion parametersThe longitudinal dispersion gets larger as the patterns of vegetation become more allometric even with the same densityVegetation in 2‐D allometric arrangement generates velocity fluctuations in the lateral direction, increasing of longitudinal dispersion [ABSTRACT FROM AUTHOR]

Dimensioning and positioning structural beneficial management practices (BMPs) represent a “real life” challenge for soil conservation engineers, managers, planners and policy-makers. Different factors, such as trapping efficiency; implementation, management, and opportunity costs (resulting from cropland loss), and government policies and guidelines need to be weighed to meet this challenge. The trapping efficiency of structural BMPs may depend on many parameters, including: (1) characteristics of vegetated filters (VF) such as width and slope, vegetation height, vegetation density and species composition, (2) flow characteristics such as runoff velocity, discharge volume and water height, and (3) sediment characteristics such as particle size, aggregation and concentration. Government policies and guidelines may include dimension and location of VFs and/or a cropland percentage that needs to be converted into VF areas. The main objectives of this paper are to: (1) describe the development of the Vegetated Filter Dimensioning Model (VFDM), a mathematical model to determine the optimal dimensions of riparian vegetated filter strips (RVFSs) in agricultural watersheds, and (2) illustrate the potential use of the model on a pilot watershed, the Beaurivage watershed, in Quebec, Canada. The latter was done for the sole purpose of model testing with readily available input parameters and data. The model calculates the optimal width with respect to vegetation, topographical, hydrological and sedimentological characteristics. The results of this case study showed that the average recommended RVFS for the Beaurivage River watershed is about 3 m wide. Le dimensionnement et le positionnement de bandes riveraines (BR) représentent un défi important pour les ingénieurs, gestionnaires, planificateurs et les représentants politiques qui décident des régulations dans ce domaine. Plusieurs facteurs, tel que l’efficacité de captation des sédiments; l’implémentation, la gestion et les coûts d’opportunité (résultant de la perte de terres agricoles), et les politiques et régulations gouvernementales doivent être pris en compte pour relever ce défi. L’efficacité de captation dépend de plusieurs paramètres, incluant (1) les caractéristiques de la BR, tel que sa largeur et sa pente, la hauteur de la végétation, la densité de végétation et les espèces en présence; (2) les caractéristiques de l’écoulement, tel que la vitesse de ruissellement, le volume d’écoulement et la hauteur d’eau, et (3) les caractéristiques des sédiments, tel que la taille des particules, leur agrégation et leur concentration. Les politiques et régulations gouvernementales peuvent inclure la dimension et l’emplacement des BR et/ou un pourcentage de terres agricoles devant être converties en BR. Les objectifs principaux de cet article sont : (1) décrire le développement de VFDM (Vegetated Filter Dimensioning Model), un modèle mathématique pour déterminer les dimensions optimales des bandes riveraines dans des bassins versants agricoles, et (2) illustrer l’utilisation potentielle du modèle sur un bassin versant pilote, le bassin de la rivière Beaurivage, Québec, Canada. Ce dernier objectif a été fait dans le but de tester le modèle avec des paramètres et données d’entrées déjà disponibles. Le modèle calcule la largeur optimale en fonction des caractéristiques topographiques, hydrologiques, sédimentologiques et de la végétation. Les résultats de cette étude indiquent que les largeurs des bandes riveraines dans le bassin de la rivière Beaurivage devraient être de l’ordre de 3 m. [ABSTRACT FROM AUTHOR]

Phragmites australis is a globally distributed wetland plant. At the mouth of the Mississippi River, P. australis on natural levees of the network of distributary channels appears to increase the flow in the deep draft navigation channel, which, in turn, may reduce the sedimentation and benefit the navigation dredging. For several years, P. australis has been dying in the Mississippi River's Bird's Foot Delta, which appears to be shortening the distributary channels and increasing the lateral flow from the remaining portions. A conceptual model based on D-FLOW FM was applied to calculate channel sedimentation in a series of idealized deltaic systems to predict the consequences of P. australis dieback and other factors that diminish the delta complexity, such as sea-level rise and subsidence, on sedimentation in the distributary channels. Channel complexity in each system, which was quantified with an index ranging from 0 to 10 that we developed. Model results indicate that sedimentation was insensitive to the channel complexity in simple deltas but was sensitive to the channel complexity in complex deltas, such as the current Mississippi River Delta with extensive P. australis. Channel sedimentation remains stable from 0 until the channel complexity index reaches 6. In more complex deltas, the sedimentation decreases rapidly as the channel complexity increases. The sedimentation is also affected by waves, river discharge, sediment concentration, grain sizes, and bed level. River managers in Louisiana may benefit from new models based on bathymetric data throughout the Bird's Foot Delta; data on the effects of the P. australis belowground biomass on bank erodibility across a range of current velocities; and data on the effects of P. australis stem density, diameter, and height on the lateral flow across a range of river stages and tidal stages to help them decide how much to respond to Phragmites dieback. Options include increased navigation dredging, increased restoration of the channel complexity via a thin layer of sediment deposition on natural levees and the planting of more salt-tolerant vegetation on natural levees. [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]

Flow resistance, velocity distribution, and turbulence intensity are significantly influenced by aquatic vegetations (AV) in riparian zones. Understanding the hydraulics of flow with planted floodplains is of great significance for determining the velocity distribution profile and supporting the fluvial processes management. However, the traditional flume experiment method is inefficient. Therefore, the multigroup simultaneous flume test method was carried out to describe the flow patterns affected by emerged rigid (reed and wooden stick) and submerged flexible vegetations (grass and chlorella). The Acoustic Doppler Velocimeter (ADV) was utilized to measure the velocity at one point for different experimental conditions. The results showed that hydraulic features were influenced by different types of vegetation. Furthermore, the relative depth (z/h) was a determining factor of those variations. In addition, the time-averaged velocity distributions of planted floodplains are not logarithmic. Instead, they represented "s-shape" profiles. In detail, for the vegetated floodplains, reed and wood followed an s-shape profile, but for grass and chlorella, they followed reverse s-shape profile. For all cases, turbulence is not isotropic and the change law of turbulence intensity is different in different sections. The flow resistance, turbulence intensities, and Reynold stresses influenced by different types of vegetation were also analyzed. [ABSTRACT FROM AUTHOR]

Vegetation on the banks and flooding areas of watercourses significantly affects energy losses. To take the latter into account, computational models make use of resistance coefficients based on the evaluation of bed and walls roughness besides the resistance to flow offered by vegetation. This paper, after summarizing the classical approaches based on descriptions and pictures, considers the recent advancements related to the analytical methods relative both to rigid and flexible vegetation. In particular, emergent rigid vegetation is first analyzed by focusing on the methods for determining the drag coefficient, then submerged rigid vegetation is analyzed, highlighting briefly the principles on which the different models are based and recalling the comparisons made in the literature. Then, the models used in the case of both emergent and submerged rigid vegetation are highlighted. As to flexible vegetation, the paper reminds first the flow conditions that cause the vegetation to lay on the channel bed, and then the classical resistance laws that were developed for the design of irrigation canals. The most recent developments in the case of submerged and emergent flexible vegetation are then presented. Since turbulence studies should be considered as the basis of flow resistance, even though the path toward practical use is still long, the new developments in the field of 3D numerical methods are briefly reviewed, presently used to assess the characteristics of turbulence and the transport of sediments and pollutants. The use of remote sensing to map riparian vegetation and estimating biomechanical parameters is briefly analyzed. Finally, some applications are presented, aimed at highlighting, in real cases, the influence exerted by vegetation on water depth and maintenance interventions. [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]