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The bottom boundary layer (BBL) dynamics play an important role in regulating the energy, momentum balance, and circulation in the shallow shelf areas. Unlike previous studies that disconnected BBL with background variable shelf circulation, we investigate the dynamic connection between the wind‐driven shelf circulation and BBL dynamics, and show the spatial characteristics of BBL dynamics in response to three‐dimensional (3D) heterogeneous transport over the highly variable shelf topography in the Northern South China Sea. Our process‐oriented modeling study demonstrates that the mixing dynamics and upslope buoyancy transport over varying shelf topography alter the spatial variability of BBL dynamics. Driven by southwesterly upwelling‐favorable winds, the along‐shelf current generated a frictional upslope Ekman transport. The along‐isobath pressure gradient force (PGFx∗ $PG{F}^{{x}^{\ast }}$) formed by the flow‐topography interactions over the meandering shelf induces the geostrophic cross‐isobath transport. The downwave (upwave) PGFx∗ $PG{F}^{{x}^{\ast }}$ enhances (offset) the frictional upslope transport over the east (west) of the shelf that has a concaving (uniform) bottom topography. Over the eastern shelf with concave isobaths, the intensified PGFx∗ $PG{F}^{{x}^{\ast }}$ and upslope cross‐isobath dense water transport strengthen stratification and weaken the effect of bottom stress‐induced mixing, limiting the development of the BBL. The antithesis occurs over the western shelf, where a small bottom stress controls the BBL. River discharge and the tidal current modulate the alongshore current, upslope transport, bottom stress intensity, and BBL development. We model the trajectory of seabed particles as they respond to the BBL dynamic regimes, and find that high (low) concentration, short (long) suspension time, and strong (weak) shoreward transport occurover the eastern (western) shelf, respectively. Plain Language Summary: This study investigated the dynamics impact of wind‐driven upslope transport on the bottom boundary layer (BBL). The BBL is a thin, turbulent layer of water above the seafloor that plays an important role in regulating the transport of sediment and other substances, as well as the energy and momentum balance in shallow shelf waters. We used a process‐orientated numerical model to investigate how wind‐driven currents interact with the shelf topography and what is the impact of the interaction on forming different BBL regimes in the northern South China Sea. We found that the current‐induced instability (determined by current intensity) and the upslope buoyancy transport‐induced stabilizing process (determined by current direction) jointly balance the BBL dynamics. Topographic characteristics over the southwestern and southeastern shelves off the Pearl River Estuary (PRE) regulated the intensities and directions of the bottom layer currents, controlled dynamics balance in the BBL and thus formed different BBL regimes. The BBL was further modulated by the river plume from the PRE and tidal currents over the shelf. The BBL dynamics is highly variable due to topographically regulated wind‐driven circulation and transport over the continental shelf. Key Points: The competition between flow‐induced shear mixing and upslope buoyancy transport governs the bottom boundary layer dynamics over a variable shelfThe mixing and buoyancy transport are highly regulated by variable flow‐topography interaction and modulated by plume and tidal forcingA varying bottom boundary layer shapes the characteristics of particle transport [ABSTRACT FROM AUTHOR]

Benthic storms are episodes of intensified near‐bottom currents capable of sediment resuspension in the deep ocean. They typically occur under regions of high surface eddy kinetic energy (EKE), such as the Gulf Stream. Although they have long been observed, the mechanism(s) responsible for their formation and their relationships with salient features of the deep ocean, such as bottom mixed layers (BMLs) and benthic nepheloid layers (BNLs), remain poorly understood. Here we conduct idealized experiments with a primitive‐equation model to explore the impacts of the unforced instability of a surface‐intensified jet on near‐bottom flows of a deep zonal channel. Vertical resolution is increased near the bottom to represent the bottom boundary layer. We find that the unstable near‐surface jet develops meanders and evolves into alternating, deep‐reaching cyclones and anticyclones. Simultaneously, EKE increases near the bottom due to the convergence of vertical eddy pressure fluxes, leading to near‐bottom currents comparable to those observed during benthic storms. These currents in turn form BMLs with thickness of O(100 m) from enhanced velocity shears and turbulence production near the bottom. The deep cyclonic eddies transport fluid particles both laterally and vertically, from near the bottom through the entire BML and may contribute to the formation of the lower part of BNLs. A sloping bottom reduces both the intensity of the near‐bottom currents and the extent of vertical transport. Overall, our study highlights a significant response of the abyssal environment to near‐surface current instability, with potential implications for sediment transport in the deep ocean. Plain Language Summary: Benthic storms are episodes of vigorous near‐bottom currents that are capable of sediment resuspension at abyssal depths in the ocean. They typically occur in regions characterized by strong and unstable currents near the surface, such as the Gulf Stream. In this study, we use an ocean circulation model to simulate the instability of a surface current like the Gulf Stream, and to explore the effects of this instability on the environment near the bottom in an idealized oceanic basin. We find that strong cyclonic and anticyclonic currents are simulated near the bottom in response to the instability of the surface current. The intensity of deep currents varies strongly with distance from the bottom, which creates turbulence and a well‐mixed layer adjacent to the deep seabed. In deep cyclones, particles from near the bottom can be transported upward through the entire well‐mixed layer. A steeper topographic slope reduces both the intensity of the bottom currents and the vertical displacement of particles. Overall, our study shows that the physical environment near the bottom may exhibit a strong response to the instability of surface currents, which may have a potentially significant impact on the transport of fine sediment from the seafloor. Key Points: Bottom currents simulated from the intrinsic instability of a near‐surface jet reach speeds similar to those observed during benthic stormsAbyssal eddy kinetic energy increases, producing deep eddies, strong local turbulence, and bottom mixed layers up to O(100 m) thickParticles near the bottom are advected both laterally and vertically in deep cyclones, with potential implications for sediment transport [ABSTRACT FROM AUTHOR]

New experiments in highly turbulent, steady, subcritical and uniform water open-channel flows have been carried out to measure the mean turbulent kinetic energy (TKE) budget of sediment-laden boundary layer flows with two sizes (dp =3 mm and 1 mm) of Plexiglas particles (relative density = 1.192). The experiments covered energetic sediment transport conditions (Shields number of 0.35 < θ < 1.2) ranging from non-capacity to full-capacity flows in bedload-to-suspension-dominated transport modes (suspension number of 0.5 < ws/u∗ < 1.3 where ws is the settling velocity and u∗ is the friction velocity) and for weakly to highly inertial, finite size turbulence-particle conditions (Stokes number of 0.110 where η is the Kolmogorov length scale). It was shown that the effects of sediments on the TKE budget are very pronounced in all large particle experiments for which a bedload layer of several grain diameter thickness is developed above the channel bed. When compared with the corresponding reference clear-water flows, the TKE shear-production rate for the 3 mm particle flows is strongly reduced in the wall region corresponding to the bedload layer. This turbulence damping is seen to increase with sediment load until full capacity for flows with constant Shields value, as well as with Shields number value. Inside this damped TKE shear-production zone, a distinct peak of maximal turbulence production appears to coincide with the upper edge of the bedload layer delimited by a sharp gradient in mean sediment concentration. This vertically upshifted peak of TKE production is accompanied by an enhanced net downward oriented TKE flux when compared with the reference clear-water flows. The downward diffused TKE is found to act in the bedload layer as a local energy source in reasonable balance with the sediment transport term. The mechanism behind this downward TKE transport was further analysed on the basis of coherent flow structure dynamics controlled by ejection- and sweep-type events. The agreement between the height of downward directed mean TKE flux and the height below which sweep-type events dominate the Reynolds shear-stress contribution over ejections, revealed the leading role played by sweeps in mean TKE transport. This agreement holds for all reference clear-water flows supporting the well-known wall-roughness-induced dominance of the sweep contribution in turbulent, rough clear-water boundary layer flows. Furthermore, for all 3 mm particle flows, the two referred to transition levels were significantly and similarly upshifted to the upper edge of the bedload layer. Only for these sediment-laden flows, the bedload layer thickness is seen to exceed the wall-roughness sublayer of the reference clear-water flows. This supports a strong analogy between wall-roughness effects in clear-water flows and bedload layer effects in sediment-laden flows, on the mean TKE budget induced by a similarly modified coherent flow structure dynamics. The bedload layer-controlled wall roughness is finally confirmed by the good prediction of the wall-roughness parameter ks of the logarithmic velocity distribution. An empirical formulation fitting the presented measurements is presented, valid over the range of Shields number values covered herein. [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]

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

A Reynolds-averaged two-phase Eulerian model for sediment transport, SedFoam, is utilized in a two-dimensional domain for a given sediment grain size, flow period, and mobility number to study the asymmetric and skewed flow effects on the sediment transport over coarse-sand migrating ripples. First, the model is validated with a full-scale water tunnel experiment of orbital ripple driven by acceleration skewed (asymmetric) oscillatory flow with good agreement in the flow velocity, net sediment transport, and ripple migration rate. The model results showed that the asymmetric flow causes a net onshore sediment transport of both suspended and near-bed load (the conventional bed load and part of the near-bed suspended load, responsible for ripple migration). The suspended load transport is driven by the "positive phase-lag" effect, while the near-bed transport is due to the large erosion of the boundary layer on the stoss flank, sediment avalanching on the lee flank, and the returning flux induced by the stoss vortex. Together, these processes result in a net onshore transport rate. In contrast, for an energetic velocity skewed (skewed) flow, the net transport rate is offshore directed. This is due to a larger offshore-directed suspended load transport rate, resulting from the "negative phase-lag" effect, compared to the onshore-directed near-bed load transport rate. Compared to the asymmetric flow, the onshore near-bed load transport (and migration) rate is limited by the larger offshore directed flux associated with returning flow on the lee side, due to a stronger lee vortex generation during the onshore flow half-cycle. In the combined asymmetric-skewed case, the near-bed load and migration rate are higher than in the asymmetric flow case. Moreover, the offshore-directed suspended load is much smaller compared to the skewed flow case due to a competition between the negative (due to velocity skewness) and positive (due to acceleration skewness) phase-lag effects. As a result, the net transport rate is onshore directed but slightly smaller than the asymmetric flow case. • Vortex generation-ejection processes lead to different phase-lag effects for asymmetric and skewed flows. • The onshore ripple migration rate is limited (enhanced) by the lee (stoss) vortex induced returning flux for skewed (asymmetric) flows. • Combined asymmetric-skewed flow has the highest onshore migration rate (near-bed load). [ABSTRACT FROM AUTHOR]

Sediment motion on rippled bed under wave action plays an important role in coastal landforms, but the application of traditional numerical model is limited by large computational work or bottom sediment condition due to unsteady effects. A single-phase model is proposed to maintain low computation and high accuracy for the vortex structure development and suspended sediment motion on vortex rippled bed in wave-induced oscillatory flow. The 2D bottom sediment condition takes into account the acceleration effect and asymmetric boundary layer development under nonlinear waves as well as the sediment phase-lag. The particle inertia and wake flow around sediment particle are considered in sediment turbulence, and the SST k - ω turbulence model which is suitable for inverse pressure gradient flow on rippled bed is selected to close the Reynolds stress. The single-phase model well predicts the velocity on vortex ripples, periodic concentration at fixed points and vertical distribution of averaged sediment concentration. The results show qualitative formation, development and ejection of vortex, and demonstrate quantitative development of vortex center, radius and strength in symmetric and asymmetric flows. In addition, the model well performs the development of sediment cloud referring to vortex structure. • Efficient numerical model is developed for oscillatory sediment transport on vortex ripple and sheet flow. • Bottom sediment condition performs acceleration effect, asymmetric boundary layer and phase-lag. • Particle inertia and wake flow around particle are considered in sediment turbulence. • SST k - ω turbulence model is applied for inverse pressure gradient flow on vortex ripple. • Results show quantitative development of vortex structure and associated sediment cloud. [ABSTRACT FROM AUTHOR]