Abstract The presence of vegetation in aquatic environments alters hydrodynamics and sediment resuspension. A recent paradigm has suggested that turbulent kinetic energy (TKE) serves as a better predictor of sediment transport in aquatic canopies than bed shear stress. This observation has led to the development of formulations to predict TKE for vegetated flows in the laboratory. However, model validation from natural heterogeneous field environments is lacking. Here, we explore the application of laboratory‐based formulas in a real environment, characterized by multiple vegetation length scales. We measured turbulence within a sparse canopy of mangrove pneumatophores and saplings during an experimental period with negligible wind‐wave activity. The existing formulations for TKE performed well in the field, but only when using the measured values for horizontal eddy length scales. These length scales accounted for the generation of additional turbulence from the surrounding sapling canopy, leading to notably larger TKE values than in similar laboratory experiments.

Invasive species such as grass carp (Ctenopharyngodon idella) pose substantial ecological threats to North American freshwater ecosystems. Understanding their early life stage behavior is critical for management efforts. From spawning to hatching, invasive carp eggs must remain suspended in the water column while drifting downstream for the best chance of survival. This highly vulnerable life stage is a potential target for population control to reduce recruitment. However, studying egg transport and potential dispersal control techniques is challenging, because the availability of live eggs and time period for experimentation are extremely limited. Additionally, accurately replicating the physical characteristics and transport mechanisms of fish eggs using surrogates in laboratory and field studies is not trivial. This study presents a novel method to create fluorescein-dyed, preserved grass carp eggs as surrogates for live eggs in transport and dispersal control experiments. This technique enables year-round studies of grass carp egg transport, offering managers a reliable tool for developing and testing dispersal control and passive sampling methods for invasive carp eggs. In this study, we rehydrate and dye preserved grass carp eggs in varying concentrations of aqueous fluorescein for a range of rehydration times, evaluate dye retention and egg visibility under ultraviolet light (UV-A), and measure diameters and settling velocities for comparison with live eggs. Eggs rehydrated in 0.100 g per liter fluorescein for 30 min maintain adequate brightness for up to 40 min in mixed conditions and exhibit mean settling velocities and densities similar to live eggs, making them ideal for laboratory experiments using quantitative imaging techniques.

Coastal vegetation often grows in spatially distributed patches. However, the influence of individual vegetation patches on small-scale hydrodynamics has not been well characterized under natural conditions with tidally varying water depths. We present measurements from manipulative field experiments by creating artificial patches of vegetation mimics with different patch heights and densities. We found that increased vegetation density substantially reduces flow speeds and turbulent kinetic energy (TKE) within the patch. However, for submerged patches, an increase in vegetation height induced a contrasting effect, causing faster flows and greater TKE within the patch, despite an increase in the total vegetation frontal area exerting drag. Denser patches showed the same, although more pronounced, trend. Existing analytical models failed to accurately reproduce both the observed patterns and magnitudes in velocity and TKE as functions of vegetation height and density. In contrast, the corresponding numerical hydrodynamic simulations captured both patterns and magnitudes well. These results underscore the need to correctly account for such opposing effects of vegetation geometries on flow and turbulence, and hence on sediment transport when predicting geomorphic evolution in coastal vegetation habitats.

Recent evidence of invasive grass carp (Ctenopharyngodon idella) reproducing in tributaries to the Laurentian Great Lakes has highlighted the need for control efforts targeting multiple life stages. Initial attempts to control dispersal of downstream-drifting invasive carp ichthyoplankton (i.e., eggs and larvae) using an oblique bubble screen (OBS) revealed that nearly neutrally buoyant grass carp eggs and larvae enter helical-like motions driven by the OBS, preventing aggregation within a single capture location. To improve dispersal control methods for such early-life stage carp, we used a laboratory flume to investigate the efficacy of a streamwise-oriented bubble screen to facilitate their near-bank capture. Five early-life stages of grass carp were tested: live water-hardened eggs, pre-and post-gas bladder inflation larvae, dead larvae, and dead eggs (preserved in formalin solution and later rehydrated). A range of mean channel velocities (0.23, 0.45, and 0.75 m/s) was tested for all drifters. Capture percentages increased with increasing airflow. Preserved eggs, for instance, showed capture percentages up to 95 %, 87 %, and 69 % at low, medium, and high water velocities for the highest airflow rate, respectively, in contrast with the lower than 5 % capture measured for zero airflow cases. Symmetric secondary flow structures on either side of the bubble screen induced helical trajectories of drifters and facilitated their capture in net-arrays along each wall. Velocity data were used to estimate helical recirculation timescales, enabling calculation of optimal bubble diffuser and net-array lengths for desired capture rates. This study provides useful guidance for the design of effective systems to control dispersal of downstream-drifting ichthyoplankton of invasive carp in streams.

Laboratory experiments were conducted in a unidirectional air-water flume with glass sidewalls to quantify drag forces acting on pelagic Sargassum spp. patches exposed to steady airflow and water current. Both live and surrogate patches, designed to replicate the morphology and buoyancy of natural sargassum, were tested over Reynolds numbers ranging from to for water and to for air. Drag forces were directly measured using a high-resolution force sensor, while flow velocities were characterized with ultrasonic anemometry and electromagnetic velocimetry for airflow and water current, respectively. The water-current-induced drag coefficient remained approximately constant across the tested range, whereas the airflow-induced drag coefficient exhibited an inverse relationship with the Reynolds number. Side-view imaging revealed elongation and deformation of the patches under increasing flow, emerging as a more reliable predictor of drag than the static frontal area in the experiments, while planform area that can be obtained from satellite may be considered as an alternative in natural marine environments. To address the underestimation of drag when using frontal area alone, a correction factor was introduced based on the ratio of the measured drag force to the estimated drag force. This formulation enables parameterization of drag using observable geometric features and flow conditions, with implications for modeling sargassum transport in coastal and oceanic environments.

Bubble curtains are widely used for different environmental applications relying on the generation of recirculation cells. Prior studies suggest that bubble curtains can modify the morphology of rivers and channels by keeping sediment in suspension, and as an alternative for sediment transport control. However, their use in coastal environments is still relatively unexplored. We investigate the interaction between the recirculation cell induced by a bubble curtain with incoming currents and waves. Laboratory experiments were carried out on a wave-current flume facility equipped with a bubble diffuser. Simulated cases considered different characteristics of the bubble curtain, water level, currents, and waves. Free-surface elevation and velocity were measured concurrently with high spatial resolution. Experimental observations show that the bubble curtain modifies wave- and current-induced velocity profiles. For unidirectional currents, as the flow approaches the curtain, the velocity magnitude decreases near the surface and increases near the bed due to the recirculation cell. In the presence of waves, the recirculation flow affects both the wave-induced velocity and asymmetry, both important parameters for sediment transport. Thus, synthetic particles were used as sediment proxy to investigate the role of the recirculation cell on the wave-induced near bed transport. Experimental results highlight the effects of both the bubble curtain recirculation cells and the structure of the diffuser itself in sediment deposition. These experiments reveal the potential of bubble curtains for modifying hydrodynamics and deposition patterns in coastal zones, and provide a novel data set which can be used for the calibration and validation of numerical models.

Ripples are ubiquitous in sandy beds and their geometry plays an important role in determining seabed roughness and intensifying near-bed turbulence. Ripple geometry in natural settings often deviates from equilibrium configurations. To understand how such nonequilibrium geometry and structures impact near-bottom hydrodynamics and hydraulic roughness, we performed laboratory experiments examining the effect of two different types of ripple configurations. We employed two distinct fixed 3D-printed ripple morphologies, uniform ripples and ripples with superimposed secondary crests, and replicated natural conditions by adhering sand grains, matching in size to the ripple scale, onto their surfaces. Our results show that the introduction of secondary crest disrupts the flow over not only the modified ripple but also over its neighbors. Secondary crests induce a thicker boundary layer than regular ripples. Velocities over the upstream side of secondary crest show substantial deviation from the regular ripple baseline case, while the downstream side experiences a lower effect. The shear velocity at the crest of ripples with a secondary feature is significantly higher, indicating an increased capacity for sediment transport and bedform evolution. The turbulent kinetic energy over ripples with secondary crests is twice as high as that over regular ripples. Our results further affirm that the hydraulic roughness is a function of not only the height and wavelength of the ripples, but also of specific structures and ripple geometry.

Invasive carp have severely damaged aquatic ecosystems in the USA, particularly in the Mississippi River Basin. Behavioral deterrents have been developed in the last few decades to control population expansion into new ecosystems. However, none of these deterrents are capable of controlling early-life stage carp, which have limited or no mobility during their drifting stage in rivers. Capturing eggs and larvae in large numbers warrants new methods due to their distinct physical and biological properties as well as their lack of behavioral response. We tested a novel method to redirect downstream drifting eggs and larvae for their efficient removal in streams using an oblique bubble screen (OBS) deterrent. We investigated the effects of mean water velocity and airflow rate on redirection of eggs, pre-gas bladder inflation (GBI) larvae, near-GBI larvae, and dead larvae. Although similar OBS configurations had shown high efficacy redirecting plastic spheres (egg surrogates) in previous studies, they underperform redirecting live eggs and larvae. However, distinct patterns were identified for eggs, pre-GBI, near-GBI, and dead larvae. A detailed hydrodynamic analysis showed that eggs closely follow the larger scales of motions created by the OBS, and that larvae can actively respond to turbulence cues. This study yielded new insights into the movement of early-life stage grass carp in a turbulent flow with strong recirculation, and provided important data to improve the design of bubble screen dispersal barriers for invasive carp management and population control.

In aquatic environments, the presence of porous obstacles induces intricate flow dynamics as the flow passes through and around them. These flows exhibit large local vertical and lateral gradients, influencing the evolution of downstream flow structures across various scales. In this study, we investigated flow around five idealized porous obstacles with varying porosity and pore arrangements using Particle Image Velocimetry (PIV). By introducing a two-layer model and computing turbulent kinetic energy budgets, we quantified jet velocity and length to predict the development of downstream flow structures. Recirculation zones were observed downstream of obstacles with small pore sizes, while forward flow motions prevailed downstream with larger pore sizes due to increased jet velocity and length. To study the effect of multiple porous obstacles, we installed a second obstacle at various downstream distances, which showed minimal influence on jet length and velocity once the distance between obstacles exceeded the jet length determined from single obstacle analysis, particularly with obstacles featuring large pore sizes. Our study identifies the need to properly characterize in-stream obstacles based on both their porosity and their representative pore sizes, as the jets created through the obstacles significantly alter the expected flow structures from solid-obstacle predictions. Based on the insights from the hydrodynamic study and using the balance between resistance and driving force of sediment motions, we discuss ecological and geomorphic applications in the vicinity of porous obstacles, highlighting the potential locations for sediment erosion and deposition.

This study investigates the effect of submergence ratio on the transport of exogenous particles in streams, targeting particles with density and diameter representative of microplastic and eggs of invasive species, which are of major concern in management of aquatic environments. Transport of two types of surrogate particles, with mean diameters of 1 and 4.8 mm and specific gravities of 1.00 and 1.0025, respectively, was assessed through experiments in a laboratory flume. Submerged obstacles with simplified geometries were mounted on the bed of a flume to represent in-stream obstructions. Image processing techniques, Particle Image Velocimetry (PIV) and Lagrangian Particle Tracking, were used to obtain flow velocity fields and particle trajectories. Angular momentum theorem was used to quantify the emergence of coherent eddies, which increase particle entry and timespans between submerged obstacles. Two indices are introduced: particle entry ratio and timespan of particles, which depend on particle characteristics, submergence ratio, and gap length. The study provides insights into the fundamental physics of particle transport, offering practical implications for aquatic debris and invasive species management, including effective monitoring locations and trap designs.

We conceptualize and test a non-intrusive barrier, comprised of an oblique bubble screen (OBS) oriented at an angle to the mean flow, to prevent the downstream dispersal of invasive carp egg surrogates. Three surrogates of different densities and diameters were tested. Secondary flows created by the OBS were tuned to redirect surrogate eggs to facilitate their capture. Surface particle image velocimetry and acoustic Doppler velocimetry were used to characterize secondary flows. We assessed the influence of airflow rate, OBS angle, mean flow velocity, and surrogate density on particle redirection. In general, redirection efficiency improves by increasing the OBS angle with respect to the cross-section. At a mean flow velocity of 0.75 metres per second (m/s), the OBS system redirected up to 60% (%) of positively buoyant particles (specific gravity SG = 0.9, and diameter d = 7.09 millimetres [mm]) and 40% of semi-buoyant particles (SG = 1.001, d = 3.1 mm). Negatively buoyant particles (SG = 1.04, and d = 5.90 mm) were redirected by the physical structure of the diffuser rather than by OBS-induced flow. The study shows that an OBS system can be used to effectively redirect carp-egg surrogates over a wide range of particle sizes and densities, allowing for selective targeting of undesired particles in streams.

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Oyster populations within the Chesapeake Bay have been drastically reduced over the last century mainly due to unregulated human activities and diseases. Regulations and restoration efforts have focused on restoring oyster populations while also considering their ability to provide ecosystem services, such as coastal protection and water quality improvement, among others. To promote oyster growth and the settlement of new populations, a recent technique adopted along the east coast of the US is the use of oyster castles (OCs). OCs have proven effective in recruiting and retaining oysters and in promoting both vertical growth and horizontal expansion of oyster habitats. OCs are widely used in coastal protection as greener alternative to common engineering solutions. We quantified hydrodynamic differences that occur around these OCs during their early stage (i.e. castles without oysters), and with fully developed oysters covering the surface of the castles through a series of laboratory experiments. The experiments were conducted in a recirculating Odell-Kovasznay type channel at the Ecohydraulics and Ecomorphodynamics Laboratory (EEL) at the University of Illinois. OCs (both with and without oysters) were 3D printed at 1:7 scale to fit the canal, and Particle Image Velocimetry (PIV) was used for 2D flow characterization. Data showed noticeable differences in flow acceleration atop the castles when covered with oysters, as well as an increase in the generation and distribution of turbulent kinetic energy atop and around the oyster-covered castles. Magnitudes and spatial distribution of Reynolds stresses were also affected by the presence of oysters in both submerged and near-emergent conditions. Challenges associated with the estimation of the drag coefficient for both gray and oyster-covered OCs highlighted the need for more data besides the centerline 2D PIV output. Further research involving the whole three-dimensional structure of the flow, in both unidirectional and oscillatory conditions, will allow us to provide relevant guidelines on the design and use of oyster-populated breakwaters as a viable nature-based solution for coastal protection within low-energy environments.

Aquatic vegetation has the potential to increase suspended sediment capture while also increasing sediment resuspension and bedload transport. Suspended sediment can induce density stratification, which modulates the turbulence in the water column. We derive a Rouse-based formulation for suspended sediment concentration (SSC) including the effect of sediment-induced density stratification. We perform Large Eddy Simulations of vegetated and non-vegetated channels to explicitly highlight the effect of stratification on SSC profiles. We found that the impact of stratification is dominant in the near-bed region within the bottom boundary layer, affecting both sediment resuspension and bedload transport. Stratification reduces the likelihood of both dominant sweep and ejection events in the near the bed region which may affect sediment entrainment and bedload transport. Modifications to existing models of sediment entrainment and bedload transport are suggested to account for the effects of sediment induced stratification in vegetated and non-vegetated channels. One of the primary functions of rivers is to transport sediments due to the movement of water. Sediments come in different sizes- larger sediments are heavy and slowly move along the riverbed, while smaller lighter sediments are suspended in the water and move with the flow. Suspension of fine sediments in river is directly related to the amount of turbulence in the flow. Suspended sediments tend to settle under their weight, while the turbulence keeps them from settling. The balance of gravitational settling and turbulent mixing leads to an equilibrium suspended sediment concentration profile in the water column. This causes a variation in the density of the water-sediment mixture along the river's depth, which reduces the turbulence in the river. However, aquatic vegetation which is a natural part of the river increases turbulence in the flow. In this work, we study the effect of density stratification in channels with aquatic vegetation which has implications for sediment transport in natural waterways. We present a Rouse-type formulation including effects of sediment-induced stratification to predict suspended sediment concentration in vegetated channel flowsStratification effects are explicitly revealed using Large Eddy Simulations of vegetated and non-vegetated channelsStratification is relevant for near bed processes such as sediment entrainment and bedload transport

Aquatic vegetation plays an important role in natural water environments by interacting with the flow and generating turbulence that affects the air-water and sediment-water interfacial transfer. Regular and staggered arrays are often set as simplified layouts for vegetation canopy to study both mean flow and turbulence statistics in vegetated flows, which creates uniform spacing between vegetation elements, resulting in preferential flow paths within the array. Such preferential paths can produce local high velocity and strong turbulence, which do not necessarily happen in natural environments where vegetation is randomly distributed. How the randomness of the canopy affects interfacial processes by altering spatial turbulence distribution, which can potentially lead to different turbulence feedback on the interfacial transfer process, remains an open question. This study conducted a series of laboratory experiments in a race-track flume using rigid cylinders as plant surrogates. Mean and turbulent flow statistics were characterized by horizontal- and vertical-sliced PIV. Based on the measured flow characteristics under different stem diameters and array configurations, we propose a method to quantify the randomness of the vegetation array and update a sediment-water-air interfacial gas transfer model with the randomness parameter to improve its accuracy. The updated model agrees well with the dissolved oxygen experimental data from our study and data from existing literature at various scales. The study provides critical insight into water quality management in vegetated channels with improved dissolved oxygen predictions considering vegetation layout as part of the interfacial transfer model. Aquatic vegetation plays an important role in natural water environments by interacting with the flow and generating turbulence, which affects the interfacial gas transfer across the air-water and sediment-water interfaces. Researchers often used regular and staggered arrays as simplified vegetation layouts in laboratory experiments to study the flow hydrodynamics of vegetated channels. However, such regular vegetation pattern creates uniform spacing between stem elements, resulting in preferential flow paths within the canopy, which is unrealistic in nature where plants are randomly distributed. To understand the discrepancies between the idealized and the actual field cases, laboratory experiments were conducted in a race-track flume using arrays of rigid cylinders as plant surrogates. We investigate the effects of randomly distributed vegetation on hydrodynamics and how spatial heterogeneity can alter turbulence feedback on interfacial transfer processes. A randomness index was proposed based on the measured flow characteristics under different stem diameters and array configurations to update previous interfacial transfer models. The study provides helpful insight into water quality management in vegetated channels, with improved dissolved oxygen predictions via a more accurate and universal interfacial transfer model with different vegetation distribution patterns. An index based on lateral flow variations is proposed to quantify the randomness in the distribution of vegetation elements in a canopy Layout of emergent vegetation does not affect surface gas transfer rates, as the average of the horizontal-shear turbulence remains the same Contributions from coherent structures by flow-stem-bed interaction need to be corrected by the randomness index for sediment-water transfer

Underwater obstacles are identified as local hotspots of various particulate matters in streams. As the transport of particles is dependent on surrounding flow, we expect that flow structures created in the vicinity of obstacles provide a favorable condition for high particle concentration. We quantify the particle behaviors at small to intermediate range of Stokes number as they move past two obstacles forming a gap of varying length, based on three indices: the ratio of particles entering the gap, the time span between entry and exit of particles, and the integral time scale of flow. We performed laboratory experiments in a recirculating racetrack flume using quantitative imaging, particle image velocimetry to obtain flow velocity fields, and particle tracking velocimetry to track full trajectory of particles. As the gap length increases, the interaction between inner and outer flows of the gap increases, which is followed by an increase of the ratio of particles entering the gap and a decrease of the time span of particles and the integral time scale. Noticeably, indices converged when the gap length reached a critical value and recirculating flow structures were fully developed in between the gap, which indicates that the transport of particles with small to intermediate Stokes number ( St = 0.1–0.5) is related to the large‐scale flow motions. Understanding these patterns allow us to select specific zones to sample and monitor both organic (seeds, eggs, and larvae of endemic aquatic species), and inorganic (sediment and microplastics) particles for better management of aquatic environments.

The transport of particles in water is a mechanism observed from various scientific and engineering problems such as the removal of polluted particles, optimal sampling of organic matter, transport of fish eggs and plant seeds. Thus, it is necessary to understand the transport mechanism of particulate matter in water thoroughly to manage the aquatic ecosystem effectively. In this study, we analyze the transport of neutrally buoyant particles in a laboratory setting when obstacles are fixed at the bottom with different configurations, focusing on the spacing between neighboring obstacles at different flow depths and velocities. The spatial and temporal analysis of two-dimensional velocity fields yield valuable information on flow-structure-particle interactions and their response to changing hydrodynamic conditions. We conducted experiments on a closed-loop racetrack flume, using Particle Tracking Velocimetry (PTV) to identify plastic transport and their preferred locations relative to the obstruction, and Particle Image Velocimetry (PIV) to identify specific mean and turbulent conditions that determine particle retention or redirection. Based on the result, we computed two parameters, the dimensionless capture ratio and retention time of particles to quantify the effect of submerged obstructions on the transport of particles. Spectrum analysis and quadrant analysis are applied to show that large scale eddies appear at the threshold gap length and lead to high concentration of particles inside the gap between adjacent obstacles, creating a favorable condition to capture particles effectively.

A significant fraction (45%) of global environmental losses of plastics stems from mismanaged landfills and open dumps located mainly in the Asian region. These lost plastics cause substantial impacts on the ecosystems, human health and economy. To date, no large-scale quantification has ever been made to estimate urban, national, or global risks of plastics losses from landfills, covering all relevant pathways, like wind blowing, precipitation/ runoff, or flooding. Here, we have developed a novel framework to categorize urban centers into different risk levels based on a trivariate risk assessment model. We applied it to entire India, building on landfill data and modeling pertaining to 496 Indian urban centers with populations larger than 0.1 million inhabitants. Results show that similar to 11% of these cities present severe and very high risk levels, including largely populated metropolitan cities like Mumbai and Delhi. This nationwide assessment can help policy-makers identify and prioritize urban centers in dire need of actions to limit environmental losses of plastics. Owing to the importance of landfills as a plastics release source, such decision support offers strong opportunities to curb plastics pollution globally.

High-resolution large eddy simulations and complementary laboratory experiments using particle image velocimetry were performed to provide a detailed quantitative assessment of flow response to gaps in cylinder arrays. The base canopy consists of a dense array of emergent rigid cylinders placed in a regular staggered pattern. The gaps varied in length from Δg/d=4 to 24, in intervals of 4d, where d is the diameter of the cylinders. The analysis was performed under subcritical conditions with Froude numbers Fr∈[0.08,0.2] and bulk Reynolds numbers Re∈[0.8, 2]×104. Results show that the gaps affect the flow statistics at the upstream and downstream proximity of the canopy. The affected zone was Δx/d≈5 for the mean flow and Δx/d≈3 for the second-order statistics. Dimensionless time-averaged streamwise velocity within the gap exhibited minor variability with gap spacing; however, in-plane turbulent kinetic energy, k, showed a consistent decay rate when normalized with that at x/d≥1 from the beginning of the gap. The emergent canopy acts as a passive turbulence generator for the gap flow for practical purposes. The streamwise dependence of k follows an exponential trend within 1≤x/d≲2.5 and transitions to a power-law at x/d≥4. The substantially lower maximum values of k within the gap compared to k within the canopy evidence a limitation of gap measurements representative of canopy flow statistics. We present a base framework for estimating representative in-canopy statistics from measurements in the gap.