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Restoring streams by feeding sediment from a single location is cost‐effective, allowing natural sediment distribution. Alternatively, placing sediment in predetermined patterns requires more planning but may provide controlled improvements to flow and habitat. However, the effectiveness of specific seeding patterns in achieving restoration goals remains unexamined. This study uses a flume model of a scaled fixed‐bed pool‐riffle channel and a 2D hydraulic model to investigate the impact of seeding patterns on sediment retention, hydraulic performance, and spawning habitat suitability within a restored pool‐riffle channel. We tested three seeding patterns—Head‐Seed (HS), Tail‐Seed (TS), and Full‐Seed (FS)—under flow conditions ranging from Qspawning to Q100. Results reveal that seeding patterns influence sediment retention in pool‐riffle sequences. While 95% of seeded sediment remained in the channel during Qspawning across all patterns, the FS pattern showed a greater sensitivity to increased flow, with a logarithmic decline in cover fraction and higher sediment export compared to HS and TS strategies. High shear stress zones, promoting full sediment mobility, appeared at the pool‐heads with steep bed slopes, while deposition occurred in low shear stress zones at pool‐tails. Minor changes in bed elevations from alluvial cover development did not alter shear stress distribution, highlighting the dominance of channel design over initial seeding conditions. Despite FS pattern provided more suitable spawning area (37%) compared to TS (22%) and HS (13%), its higher sediment export under elevated flows raises concerns about downstream sedimentation and long‐term habitat sustainability. This study emphasizes the importance of balancing short‐term habitat gains with long‐term stability.

Microplastics (MPs) have been detected ubiquitously in fluvial systems and advective transfer has been proposed as a potential mechanism for the transport of (sub‐) pore‐scale MPs from surface waters into streambed sediments. However, the influence of particle and sediment properties, as well as the hydrodynamic flow regime, on the infiltration behavior and mobility of MPs in streambed sediments remains unclear. In this study, we conducted a series of flume experiments to investigate the effect of particle size (1–10 μm), sediment type (fine and coarse sand), and flow regime (high/low flow) on particle infiltration dynamics in a rippled streambed. Quantification of particles in the flume compartments (surface flow, streambed interface, and in the streambed) was achieved using continuous fluorescence techniques. Results indicated that the maximum infiltration depth into the streambed decreased with increasing particle size (11, 10, and 7 cm for 1, 3, and 10 μm). The highest particle retardation was observed in the fine sediment experiment, where 22% of the particles were still in the streambed at the end of the experiment. Particle residence times were shortest under high flow conditions, suggesting that periods of increased discharge can effectively flush MPs from streambed sediments. This study provides novel insights into the complex dynamics of MP infiltration and retention in streambed sediments and contributes to a better understanding of MPs fate in fluvial ecosystems. Quantitative data from this study can improve existing modeling frameworks for MPs transport and assist in assessing the exposure risk of MPs ingestion by benthic organisms. Plain Language Summary Microplastics (MPs) (small plastic particles) are present in river systems worldwide. The processes that lead to their transport and retention in rivers are not fully understood. Scientists have proposed that the infiltration of surface water into the streambed can carry MPs with it. In this study, we conducted experiments in a controlled environment that resembles a stream and its streambed. We investigated how different sizes of plastic particles (1, 3, and 10 μm), the types of sediment (fine and coarse sand), and water flow rates (low and high) affect how far particles travel in a streambed. We found that the size of MPs played a significant role in their depth of infiltration. Larger particles did not infiltrate as deeply as smaller particles, and were also retained in the streambed. Fine sand trapped particles for a longer time than coarse sand, and 22% of the particles remained in the streambed until the end of the experiment. Faster flowing water quickly removed MPs from the streambed. Our research helps understand how MPs spread in river systems and how long they remain in the streambed. The data can be used to improve transport models and assess the risk MPs pose to aquatic organisms. Key Points (Sub‐) Pore‐scale microplastics were advectively transferred from the surface water into the streambed sediments in flume experiments Infiltration patterns depend on microplastic size, streambed sediment type and surface flow velocities Microplastic retention was observed for 10 μm beads, 1 μm beads were considerably retarded in fine sediments

The awareness of the transmission of the sticky sediments for the development and maintenance of reservoirs and water transfer network is very important. This research was carried out to recognize and understand the dynamic behavior of fine-sticky sediments to obtain the necessary information for the Karkheh dam reservoir management. Sediment samples were taken from the four different points located in the dam reservoir. Liquidity and plasticity behaviors and their indices of the samples that were combined together were determined by doing the Atterberg limits experiment. To investigate the initial erosion threshold shear stress, the impact of consolidation and sediment depth were examined by cylindrical settling columns. Using a circular flume in, Shahrekord University Lab, the concentration process, changes of eroded sediments, shear stress threshold of erosion, erosion rates, etc. in different consolidation periods (3, 14 and 30 days) were studied. The results showed that the concentration of eroded sediments is a function of time for the consolidation of reservoir sediment and bed shear stress and also observed that the duration of consolidation time is an effective factor on critical erosion shear stress. So, the threshold shear stress values for consolidation time of 3, 14 and 30 days were, 0.16, 0.22, 0.31 N/m2, respectively. The results of the erosion rate suggest an inverse relationship between this parameter and the life of the settled sediments based on the results the best flow shear stress for sediment removal by flashing from the Karkheh dam reservoir should be greater than 0.31 N/m2.

Several bridges failed because of scouring and erosion around the bridge elements. Hence, precise prediction of abutment scour is necessary for the safe design of bridges. In this research, experimental and computational investigations have been devoted based on 45 flume experiments carried out at the NIT Warangal, India. Three innovative ensemble-based data intelligence paradigms, namely categorical boosting (CatBoost) in conjunction with extra tree regression (ETR) and K-nearest neighbor (KNN), are used to accurately predict the scour depth around the bridge abutment. A total of 308 series of laboratory data (a wide range of existing abutment scour depth datasets (263 datasets) and 45 flume data) in various sediment and hydraulic conditions were used to develop the models. Four dimensionless variables were used to calculate scour depth: approach densimetric Froude number (Fd50), the upstream depth (y) to abutment transverse length ratio (y/L), the abutment transverse length to the sediment mean diameter (L/d50), and the mean velocity to the critical velocity ratio (V/Vcr). The Gradient boosting decision tree (GBDT) method selected features with higher importance. Based on the feature selection results, two combinations of input variables (comb1 (all variables as model input) and comb2 (all variables except Fd50)) were used. The CatBoost model with Comb1 data input (RMSE = 0.1784, R = 0.9685, MAPE = 10.4724) provided better accuracy when compared to other machine learning models.

The fate of nutrients and contaminants in fluvial ecosystems is strongly affected by the mixing dynamics between surface water and groundwater within the hyporheic zone, depending on the combination of the sediment's hydraulic heterogeneity and dune morphology. This study examines the effects of hydraulic conductivity stratification on steady‐state, two‐dimensional, hyporheic flows and solute residence time distribution. First, we derive an integral transform‐based semi‐analytical solution for the flow field, capable of accounting for the effects of any functional shape of the vertically varying hydraulic conductivity. The solution considers the uneven distribution of pressure at the water‐sediment interface (i.e., the pumping process) dictated by the presence of dune morphology. We then simulate solute transport using particle tracking. Our modeling framework is validated against numerical and tracer data from flume experiments and used to explore the implication of hydraulic conductivity stratification on the statistics and pdf of the residence time. Finally, reduced‐order models are used to enlighten the dependence of key residence time statistics on the parameters characterizing the hydraulic conductivity stratification. Key Points A new integral transform‐based semi‐analytical solution for hyporheic flows in stratified sediments is provided and tested against data The impact of hydraulic conductivity stratification on residence time distribution and its statistics is quantitatively analyzed ROMs are used to approximate key residence time statistics in the space of variability of parameters characterizing the conductivity profile

Widely distributed in natural rivers and coasts, vegetation interacts with fluid flows and sediments in a variable and complicated manner. Such interactions make it difficult to predict associated drag forces during sediment transport. This paper investigates the drag coefficient for an emergent vegetated patch area under nonuniform flow and mobile bed conditions, based on an analytical model solving the momentum equation following our previous work (Zhang et al., 2020, https://doi.org/10.1029/2020WR027613). Emergent vegetation was modeled with rigid cylinders arranged in staggered arrays of different vegetation coverage ∅. Laboratory flume tests were conducted to measure variations in both the water and bed surfaces along a vegetated patch on a sand bed. Based on the experimental and theoretical analyses, a dimensionless drag model integrating both terms of flow properties and bed effects is proposed to predict the drag coefficient Cd over a mobile bed. The calculated values of Cd exhibit two different trends, that is, nonmonotonically or monotonically increasing along the streamwise direction, due to the combined effect of water surface gradient and bed slope. The morphodynamic response of the mobile bed to nonuniform flow manifests as an evolution in the bed slope within the vegetated patch. Ongoing scouring directs the flow's energy toward overcoming the rising Cd and bed slope, leading to a relatively stable stage with a low sediment transport rate. This study advances the existing understanding of the drag coefficient's role over a mobile bed within nonuniform flows. It also enhances the applicability of vegetation drag models in riverine restoration. Plain Language Summary The drag exerted by vegetation on a riverbed dictates the sediment transport rate with important implications for river morphological evolution. Predicting vegetation drag in nonuniform flow based on the bed characteristics of mobile sand bed conditions poses both theoretical and practical challenges. The implications of this endeavor include the formulation of predictive models for drag and a deeper understanding of the influence of gradually varied flow conditions in rivers. Through both experimental and theoretical investigations, this paper reveals that the drag coefficient exhibits varying patterns along the streamwise direction within the vegetated patch over a mobile sand bed. These patterns manifest in two distinct forms: a steady increase or a parabolic shape, wherein the coefficient initially rises before subsequently decreasing. This contrasts with prior studies on fixed beds, where the drag coefficient consistently follows a parabolic distribution in the streamwise direction. The discrepancy is attributed to the distinct physical contributions of pressure, advection, and bed friction to the drag coefficient. This study provides valuable insights into the importance of flow nonuniformity on vegetation drag, aiding in the prediction of backwater profiles in vegetated flows over a mobile bed. Furthermore, it facilitates modifications to sediment transport within vegetated patches. Key Points Vegetation drag in nonuniform flow over a mobile sand bed is explored using the momentum equation Drag coefficient in nonuniform flow over a mobile bed exhibits either a parabolic or a monotonic increase along the streamwise direction Water surface gradient and bed slope contribute to the flow nonuniformity, collectively influencing the variability of the drag coefficient

Tropical beaches provide coastal flood protection, income from tourism, and habitat for flagship species. They urgently need protection from erosion, which is being exacerbated by changing climate and coastal development. Traditional coastal engineering solutions are expensive, provide unstable temporary solutions, and often disrupt natural sediment transport. Instead, natural foreshore stabilization and nourishment may provide a sustainable and resilient long-term solution. Field flume and ecosystem process measurements, along with data from the literature, show that sediment stabilization by seagrass in combination with sediment-producing calcifying algae in the foreshore form an effective mechanism for maintaining tropical beaches worldwide. The long-term efficacy of this type of nature-based beach management is shown at a large scale by comparing vegetated and unvegetated coastal profiles. We argue that preserving and restoring vegetated beach foreshore ecosystems offers a viable, self-sustaining alternative to traditional engineering solutions, increasing the resilience of coastal areas to climate change.

Flume experiments were conducted to comprehend the impact of different patterns of an emergent vegetation patch on the flow field and the scour process in natural rivers. Velocity measurements, flow visualization, and scour tests were undertaken around different vegetation patch patterns, which were simulated inspired by the expansion process of a typical instream vegetation. The patch expansion process was idealized with an initially circular patch of rigid emergent stems becoming elongated due to positive and negative feedbacks. The expansion of the vegetation patch was considered to occur in three stages, in which the density of the patch from the previous stage was increased while the patch was also elongated by connecting at its downstream side with another sparser vegetation patch. These stages were replicated succesively by increasing the density and elongating the patch. In this way, two processes (i.e., elongation and decrease in permeability), which usually have hydrodynamically opposite effects on flow fields, were simulated at the same obstruction. Despite generally elongated obstacles being streamlined bodies, the morphometric analysis obtained by laser scanner revealed that streamlined elongation of permeable patches amplifies global scour and enhances localization of the local scour hole. This situation implies that as the patch expands, in the wake region, the steady‐wake region becomes shorter, turbulence diminishes, lateral shear stress enhances, and deposition cannot occur far from the patch. Consequently, as the patch expands, the hydrodynamic consequences may restrict further patch expansion after a certain length/density. Plain Language Summary What effect does enlarging a single patch have on the local flow field and scour pattern? This research question was examined experimentally. Despite the fact that streamlined patches are hydrodynamically favorable formations, morphometric scour measurements show that the streamlined extension of permeable patches increases global scour and promotes local scour hole localization. As the patch develops, the steady‐wake zone of low velocity and suppressed turbulence that favors sediment entrapment decreases restricting its expansion. Key Points Experiments were conducted to see the impact of different patterns of an emergent vegetation patch on flow field and scour in rivers Streamlined bodies are hydrodynamically favorable bodies. Yet, tests showed that the elongation of patches increases scour and localization The steady‐wake zone becomes shorter as the patch elongated, hence restricting the patch's expansion

The reliable prediction of sediment transport capacity (Tc) is essential for soil erosion models. Although rock fragments are a common surface cover type, quantitative studies on their relationship with Tc are limited. Tc typically follows a power function with slope gradient (S) and flow discharge (q) under bare flumes, but varying exponents complicate practical application. This study aims to investigate the effect of rock fragment cover on Tc, explore the interactive effects of S, q, and cover on Tc, and ultimately develop a universal Tc prediction equation and assess its feasibility for different scenarios. Flume experiments on Tc with rock fragment cover have been conducted, and many existing Tc prediction equations have been reviewed. The results revealed that the effects of S and q on the relationship between rock fragment cover and Tc were minor and that the impact of rock fragment cover on the relationships of S and q with Tc was also not significant. Consequently, a new universal equation for Tc incorporating cover was developed. This equation featured fixed exponents of 1.66 for S and 1.22 for q and was applicable across various slope gradient, flow discharge, coverage and cover type conditions. Moreover, the impact of rock fragment cover on Tc reduction was significantly less than those of litter cover and stem basal cover (P < 0.05). Therefore, the role of rock fragments should be considered separately in soil erosion models. These findings could significantly advance the practical application of the Tc prediction equation. Key Points The effects of rock fragment cover, slope gradient and flow discharge on sediment transport capacity (Tc) were studied A new universal equation of Tc was developed, with fixed exponents of 1.66 for slope gradient and 1.22 for flow discharge The impact of rock fragment cover on Tc reduction was significantly less than those of litter cover and stem basal cover (P < 0.05)

Vegetation plays a crucial role in river hydrodynamic processes, and the accurate prediction of canopy drag force is essential for effective river management and ecosystem protection. The interactions within the vegetation canopies must be quantified to understand their impact on drag force. Through a series of flume experiments, we conducted an investigation into the canopy interaction mechanism of rigid emergent aquatic vegetation, particularly focusing on the blockage and sheltering effects. Our experimental design includes various combinations of lateral and longitudinal spacing, as well as special single‐row and single‐column arrangements. This allowed us to provide a more precise understanding of how lateral and longitudinal spacing affect the blockage and sheltering effects. Furthermore, we introduced a unified reference velocity that combines two effects, based on which we have established a widely applicable drag model that can predict drag under various density conditions. Lastly, we proposed a critical characteristic value for quantifying drag. This value is instrumental in revealing the ultimate performance of drag under different spacing arrangements. The findings provide a reliable approach for predicting drag in rigid emergent vegetation canopies, significantly enhancing our understanding of vegetation's influence on hydrodynamic processes and offering a practical tool for river management and ecosystem protection. Key Points Investigate the canopy drag under varied lateral and longitudinal spacing configurations Propose new reference velocities for accurately capturing the blockage and sheltering effects respectively Develop a unified drag prediction model and elucidate the drag performance under different densities