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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.

Hyporheic exchange leads to the transfer of gases, solutes, and fine particles across the sediment‐water interface, playing a critical role in biogeochemical cycles and pollutant transport in aquatic environments. While in‐channel vegetation has been recognized to enhance hyporheic exchange, the mechanisms remain poorly understood. Here, we investigated how an emergent vegetation canopy impacts hyporheic exchange using refractive index‐matched flume experiments and coupled numerical simulations. Our results show that at the same mean surface flow velocity, vegetation increases the hyporheic exchange velocity by four times compared to the non‐vegetated channel. However, the hyporheic exchange velocity does not increase further with increasing vegetation density. In addition, our results show that the hyporheic exchange velocity scales with the square root of sediment permeability. Our findings provide a predictive framework for hyporheic exchange in vegetated channels with varying vegetation densities and sediment permeabilities and could guide the future design of environmental management and restoration projects using vegetation.

Climatic warming is projected to change the duration and intensity of frozen periods in polar regions, impacting hydrology and riverbank erosion. Herein we present a series of 125 laboratory flume experiments conducted in a novel cryolab morphology facility using a small‐scale Friedkin channel. We assess the influence of discharge (flow velocity), water temperature, riverbank moisture content and temperature on riverbank erosion for varying air temperatures. The riverbank topography was quantified before and after each experiment and volumetric changes were calculated, using an array of images collected via a semi‐automatic camera and structure from motion method. Videos were used to determine bank edge retreat during the experiments. Surface flow velocities were measured using particle tracking velocimetry method. An infrared thermal camera aided understanding the temperature variations across the riverbank. A non‐linear relationship has been identified between volumetric erosion rate and air temperature, with the highest rates (at maximum up to 1.03 cm3/s) occurring at −5.2°C overnight air temperatures during highest tested discharge conditions. Erosion rates decrease when temperatures fall below or rise above −5.2°C, but increase again (at maximum up to 0.51 cm3/s) at +4.5°C. High moisture content slowed temperature propagation, caused by flowing water, through the riverbank. Erosion occurred as blocks in freezing conditions when the moisture content exceeded 18.9%, which further promoted thermo‐erosional niche development, a phenomenon observed also in polar/arctic river systems. The non‐linear dependency on air temperature highlights the importance of air temperature on erosion, with further implications for erosion with climate warming.

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.

The Western Himalayas in India have witnessed increased geohazards, notably debris flows, due to increased precipitation and subsequent rapid landslides. These flows threaten flat landscapes, particularly through the deposition fans they form. The increase in debris flow hazards makes it essential to understand the changes in runout deposits with varying water content and coarser particles to better capture solid–liquid interactions at a small scale. Additionally, there is a need for prediction models to analyze key features such as coarse-grained particles and water content in shaping deposits. This study offers an experimental exploration of debris flow deposition kinematics in the Western Indian Himalayas context. Utilizing reconstituted debris material from the region, experiments were conducted using a flume setup to simulate debris flow. Subsequent machine learning and Particle Image Velocimetry (PIV) provided insights into flow dynamics and helped analyze sediment accumulation patterns. Extreme gradient boosting (XGBoost) analysis revealed the significant role of stony particles in influencing mobility, with compositions between 8 and 12% showing pronounced effects of increasing deposit thickness and width. XGBoost demonstrated high predictive accuracy, with an impressive correlation between predicted and actual values for length (r 2  = 0.95), thickness (r 2  = 0.91), and width (r 2  = 0.94) of deposit fans. Water content was found to negatively impact the thickness of the deposits, with a greater reduction in thickness at higher water content. However, it positively influenced the overall mobility of the debris flow. The study underscores the importance of understanding debris flow mechanisms to mitigate the associated geohazard risks.

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