Search Results (41)

Based on the numerical simulation software Delft3D v4.01.00, this study established a three-dimensional water and sediment transport model for the Yellow River subaqueous delta, and simulated the water and sediment diffusion as well as erosion/deposition processes in the study area in 2019. By comparing the water discharge, sediment discharge, and deposition volume of 2019 (a year with water and sediment regulation) and 2017 (a year without water and sediment regulation), the influence of water and sediment regulation on the Yellow River subaqueous delta was explored. The results showed that water and sediment regulation projects change the distribution and diffusion of suspended sediment. Suspended sediment concentration in nearshore areas showed a significant correlation with deposition depth, particularly in the estuary area. When the water and sediment regulation was interrupted in 2017, the overall performance of the study area showed erosion, while when the water and sediment regulation was implemented in 2019, the study area exhibited sedimentation. The implementation of the water and sediment regulation project can promote the sedimentation of the subaqueous delta of the Yellow River. Full article

Silty seabed sediments in the subaqueous delta of the Yellow River are heavily contaminated with petroleum-derived polycyclic aromatic hydrocarbons (PAHs). Storm-induced sediment resuspension and liquefaction are key mechanisms responsible for the remobilization of PAHs into the overlying water column. In this study, laboratory-scale wave flume experiments were conducted to simulate PAH release under three hydrodynamic scenarios: (i) static diffusion (Stage I), (ii) low-intensity wave action (5 cm wave height, Stage II), and (iii) high-intensity wave action (12 cm wave height, Stage III). Results revealed a strong positive correlation between suspended particulate matter (SPM) and PAH concentrations in the aqueous phase during sediment disturbance. In particular, sediment liquefaction significantly enhanced PAH release, with concentrations up to five times higher than those under static conditions. Furthermore, liquefaction facilitated vertical migration of PAHs within sediments, resulting in reductions in PAH levels below the original background concentrations. The release dynamics varied notably among PAH species: low-molecular-weight (2–3 ring) PAHs, with lower hydrophobicity, were primarily detected in the aqueous phase, while medium- and high-molecular-weight PAHs remained predominantly associated with sediment particles. These findings underscore the critical role of hydrodynamic disturbances—especially sediment liquefaction—in influencing PAH mobility and offer important implications for pollution risk assessment and coastal management in storm-impacted deltaic environments. Full article

Globally, sediment transport from rivers and the morphological evolution of deltas are strongly shaped by human activities. The Yellow River Delta is a typical representative of this. In this paper, Delft 3D v4.01.00 software was used to simulate the sediment diffusion in the subaqueous delta of the Yellow River in 2017 so as to explore the influence of the sediment and water transported by the Yellow River on the subaqueous delta without water and sediment regulation. The results reveal the occurrence of a low–high–low suspended sediment concentration distribution from the coastlines to the far shore. The main accumulation areas shifted from the coasts of Bohai Bay and Laizhou Bay in the dry season to the estuary in the wet season. The sediments entering the sea formed deposition zones along the coastline, and erosion zones were formed outside these deposition zones, with a maximum depth of about 5 m. In 2017, the impact of the sediment inflow into the Yellow River on its subaqueous delta generally resulted in the erosion being greater than the sedimentation, and the erosion/deposition volume in 2017 was −1.28 × 10⁸ m³, and the estimated critical value of the sediment inflow balance was 2.13 × 10⁸ tons. Full article

The present study focuses on the dynamics of trace metals (TM) in two European rivers, the Mosel and the Meuse. A deterministic description of hydro-sedimentary processes has been performed. The model used to describe pollutant transport and dilution at the watershed scale has been enhanced with the implementation of the MicMod sub-model. The objective of this study is to characterise the dynamics of TM in the water column and bed sediment. A multi-class grain size representation has been developed in MicMod. The dissolved and particulate TM phases have been calculated with specific partitioning coefficients associated with each suspended sediment (SS) class. The processes involved in TM fate have been calibrated in MicMod, including settling velocity, TM releases from the watershed (point and diffuse loads), etc. Following the calibration of the parameters involved in TM transport within the river ecosystem, the main goal is to describe TM dynamics using a pressure–impact relationship model. It was demonstrated that the description of at least one class of fine particles is necessary to obtain an adequate representation of TM concentrations. The focus of this study is low flow periods, which are characterised by the presence of fine particles. The objective is to gain a deeper understanding of the processes that control the transport of TM. This paper establishes consistent pressure–impact relationships between TM loads (urban, industrial, soils) from watersheds and concentrations in rivers. Full article

In this work, photothermal beam deflection spectrometry (BDS), combined with a passive sampling technique of diffusive gradients in thin film (DGT), is optimized to improve the method’s sensitivity. The limit of detection (LOD) is then reduced by a factor of 2 (to the value of 20 nM). The functionality of the technique is compared for Chelex-100 (Ch-100) and suspended particulate reagent–iminodiacetate resin (SPR-IDA), used as binding resins in passive samplers. The absorption capacity of SPR-IDA resin is found to be less than 1 μM and far below that one of Chelex-100 resin (around 6 μM). The BDS technique is applied for determination of iron redox species concentration in sediment porewater. It is found that Fe in sediment porewater occurs both in Fe²⁺ (0.073 μM) and Fe³⁺ (0.095 μM) forms. The validation of the presented method reveals that the BDS technique ensures good repeatability, reproducibility, and reliability. Full article

Port coasts are affected by waves and tidal currents, and sediment continues to silt up, leading to a reduction in the depth of water in the channel, blocking the channel and seriously affecting the safe operation of ports. The main cause of sediment deposition in ports is suspended sediment transport, and the characteristics of the vertical distribution of suspended sediment concentrations are the embodiment of the suspended sediment transport law. This paper is divided into three parts to study the vertical distribution characteristics of suspended sediment concentrations. Firstly, the shortcomings of the traditional diffusion model were analysed by using the finite mixing theory (FMT); secondly, the sediment mixing length coefficient κ_s model was introduced and combined with the sediment group settling velocity model to establish the vertical distribution model of suspended sediment concentrations under wave–current; finally, the effects of various factors on the vertical distribution of the suspended sediment concentration were investigated. The results show that the model in this paper has the characteristics of “low variance and low bias”, which solves the problem that κ_s is difficult to determine. When the model κ_s < κ_s′ (κ_s′ = 0.4), the concentration of suspended sediment predicted by κ_s′ is overestimated, and vice versa. As the sediment concentration increases, the interaction between particles increases and the vertical distribution of the suspended sediment concentration shows the pattern of “small top and large bottom”. The larger the particle size, the greater the sedimentation rate of the suspended sediment, and a large amount of sediment will be suspended near the bottom without mixing. The higher the wave height, the stronger the boundary layer turbulence and the movement of the water particles’ trajectory, and the smaller the difference in sediment concentration between the bottom and the sea surface. Full article

Sediment transport to the sea by rivers is crucial for the stability of estuaries and coasts. The Yangtze River, the largest river in China, like many large rivers worldwide, is experiencing a decrease in sediment load reaching the coast. However, the tidal flat around Qidong Cape, located at the entrance of the North Branch of the Yangtze Estuary, is undergoing extensive siltation. The source of this sediment is unclear. In this study, a sediment core was collected and the geochemical characteristics of rare earth elements (REE) were analyzed using inductively coupled plasma mass spectrometry (ICP-MS). The results indicate the following: (1) The average content of REE is 178.57 μg/g, and the average ratio between LREE and HREE is 8.66, which is comparable to sediments from the South Yellow Sea. The chondrite-normalized and UCC-normalized patterns resemble those of the Yangtze River and the South Yellow Sea, indicating a negative gradient, a weak Ce-negative anomaly, and a distinct Eu-negative anomaly. (2) The continental shelf deposits in eastern China are primarily derived from sediment flux delivered by rivers. The sediments in the South Yellow Sea mainly originate from the Yangtze River and the Yellow River, exhibiting characteristics of a mixed source due to long-term geological processes, namely geochemical processes. The REEs in the tidal flat around Qidong Cape inherit the source area’s characteristics and originate from the weathering of upper continental rock in mainland China. Moreover, the tidal flat around Qidong Cape is influenced by both runoff and tidal actions, leading to strong land–sea interactions and reducing the environment, explaining the Eu-negative anomaly. (3) Hydrodynamic forces in the North Branch of the Yangtze River have shifted from runoff to tidal dominance since the 1930s. However, marine hydrodynamics outside the estuary have remained unchanged. Consequently, the Subei coastal current plays a key role in sediment transport and diffusion. Sediments from the south wing of the Radiative Sand Ridge in the South Yellow Sea are transported southward by the Subei coastal current, and under tidal influence, suspended sediment is deposited in the tidal flat around Qidong Cape. Therefore, the sediment source has gradually shifted from the Yangtze River to the South Yellow Sea. Full article

Extreme precipitation and flooding frequency associated with global climate change are expected to increase worldwide, with major consequences in floodplains and areas susceptible to flooding. The purpose of this review was to examine the effects of flooding events on changes in soil properties and their consequences on agricultural production. Flooding is caused by natural and anthropogenic factors, and their effects can be amplified by interactions between rainfall and catchments. Flooding impacts soil structure and aggregation by altering the resistance of soil to slaking, which occurs when aggregates are not strong enough to withstand internal stresses caused by rapid water uptake. The disruption of soil aggregates can enhance soil erosion and sediment transport during flooding events and contribute to the sedimentation of water bodies and the degradation of aquatic ecosystems. Total precipitation, flood discharge, and total water are the main factors controlling suspended mineral-associated organic matter, dissolved organic matter, and particulate organic matter loads. Studies conducted in paddy rice cultivation show that flooded and reduced conditions neutralize soil pH but changes in pH are reversible upon draining the soil. In flooded soil, changes in nitrogen cycling are linked to decreases in oxygen, the accumulation of ammonium, and the volatilization of ammonia. Ammonium is the primary form of dissolved inorganic nitrogen in sediment porewaters. In floodplains, nitrate removal can be enhanced by high denitrification when intermittent flooding provides the necessary anaerobic conditions. In flooded soils, the reductive dissolution of minerals can release phosphorus (P) into the soil solution. Phosphorus can be mobilized during flood events, leading to increased availability during the first weeks of waterlogging, but this availability generally decreases with time. Rainstorms can promote the subsurface transport of P-enriched soil particles, and colloidal P can account for up to 64% of total P in tile drainage water. Anaerobic microorganisms prevailing in flooded soil utilize alternate electron acceptors, such as nitrate, sulfate, and carbon dioxide, for energy production and organic matter decomposition. Anaerobic metabolism leads to the production of fermentation by-products, such as organic acids, methane, and hydrogen sulfide, influencing soil pH, redox potential, and nutrient availability. Soil enzyme activity and the presence of various microbial groups, including Gram+ and Gram− bacteria and mycorrhizal fungi, are affected by flooding. Waterlogging decreases the activity of β-glucosidase and acid phosphomonoesterase but increases N-acetyl-β-glucosaminidase in soil. Since these enzymes control the hydrolysis of cellulose, phosphomonoesters, and chitin, soil moisture content can impact the direction and magnitude of nutrient release and availability. The supply of oxygen to submerged plants is limited because its diffusion in water is extremely low, and this impacts mitochondrial respiration in flooded plant tissues. Fermentation is the only viable pathway for energy production in flooded plants, which, under prolonged waterlogging conditions, is inefficient and results in plant death. Seed germination is also impaired under flooding stress due to decreased sugar and phytohormone biosynthesis. The sensitivity of different crops to waterlogging varies significantly across growth stages. Mitigation and adaptation strategies, essential to the management of flooding impacts on agriculture, enhance resilience to climate change through improved drainage and water management practices, soil amendments and rehabilitation techniques, best management practices, such as zero tillage and cover crops, and the development of flood-tolerant crop varieties. Technological advances play a crucial role in assessing flooding dynamics and impacts on crop production in agricultural landscapes. This review embarks on a comprehensive journey through existing research to unravel the intricate interplay between flooding events, agricultural soil, crop production, and the environment. We also synthesize available knowledge to address critical gaps in understanding, identify methodological challenges, and propose future research directions. Full article

New Zealand’s agricultural sector faces the challenge of maintaining productivity while minimizing impacts on freshwaters. This study evaluates the cost-effectiveness of various green infrastructure systems designed to reduce diffuse agricultural sediment and nutrient loads. Utilizing a quantitative economic and contaminant reduction modeling approach, we analyze the impacts of five interceptive mitigation systems: riparian grass filter strips, constructed wetlands, woodchip bioreactors, filamentous algal nutrient scrubbers, and detainment bunds. Our approach incorporates Monte Carlo simulations to address uncertainties in costs and performance, integrating hydrological flow paths and contaminant transport dynamics. Mitigation systems are assessed individually and in combination, using a greedy cyclical coordinate descent algorithm to find the optimal combination and scale of a system for a particular landscape. Applying the model to a typical flat pastoral dairy farming landscape, no single system can effectively address all contaminants. However, strategic combinations can align with specific freshwater management goals. In our illustrative catchment, the mean cost to remove the full anthropogenic load is NZD 1195/ha for total nitrogen, NZD 168 for total phosphorus, and NZD 134 for suspended solids, but results will vary considerably for other landscapes. This study underscores the importance of tailored deployment of green infrastructure to enhance water quality and support sustainable agricultural practices. Full article

The prediction of sediment transport, related to different environmental and engineering problems, requires accurate mathematical models. Most available mathematical models for the concentrations of suspended sediments are based on the classical advection diffusion equation, which remains not efficient enough to describe the complete behavior related to sediment–water two-phase flows and the feedback between the turbulent unsteady flow and suspended sediments. The aim of this paper is to reinvestigate the kinetic model for turbulent two-phase flows, which accounts for both sediment–turbulence interactions and sediment–sediment collisions. The present study provides a detailed and rigorous derivation of the kinetic model equations, clarifications about the mathematical approach and more details about the main assumptions. An explicit link between the kinetic model and the classical advection diffusion equation is provided. Concentration profiles for suspended sediments in open channel flows show that the kinetic model is able to describe the near-bed behavior for coarse sediments. Full article

The Rouse formula and its variants have been widely used to describe the vertical distribution of the sediment concentration in sediment-laden flows in equilibrium. Han’s formula extends the Rouse formula to non-equilibrium regimes, where the diffusive flux is still assumed to be Fickian. The turbulent flow and suspension regimes downstream of a mega-reservoir, e.g., the Three Gorges Reservoir, usually exhibit fractal and unsaturated properties, respectively. To characterize the non-Fickian dynamics of suspended sediment and the non-equilibrium regime in natural dammed rivers, this study proposes a new formula for the concentration profile of unsaturated sediment based on the Hausdorff fractal derivative advection–dispersion equation. In addition, we find that the order of the Hausdorff fractal derivative is related to the sizes of the sediment and the degrees of non-equilibrium. Compared to Rouse and Han’s formulae, the new formula performs better in describing the sediment concentration profiles in the Jingjiang Reach, approximately 100 km below the Three Gorges Dam. Full article

Turbulence related to flow oscillations near the seabed, in the wave bottom boundary layer (WBBL), is the phenomenon responsible for the suspension and transport of sediments. The vertical distribution of turbulent eddy viscosity within the WBBL is a key parameter that determines the vertical distribution of suspended sediments. For practical coastal engineering applications, the most used method to parameterize turbulence consists in specifying the shape of the one-dimensional-vertical (1DV) profile of eddy viscosity. Different empirical models have been proposed for the vertical variation of eddy viscosity in the WBBL. In this study, we consider the exponential-type profile, which was validated and calibrated by direct numerical simulation (DNS) and experimental data for turbulent channel and open-channel flows, respectively. This model is generalized to the WBBL, and the period-averaged eddy viscosity is calibrated by a two-equation baseline (BSL) k-ω model for different conditions. This model, together with a β-function (where β is the inverse of the turbulent Schmidt number), is used in modeling suspended sediment concentration (SSC) profiles over wave ripples, where field and laboratory measurements of SSC show two kinds of concentration profiles depending on grain particles size. Our study shows that the convection–diffusion equation, for SSC in WBBLs over sand ripples with an upward convection term, reverts to the classical advection–diffusion equation (ADE) with an “apparent” sediment diffusivity ${\epsilon }_s^{*}=\alpha {\epsilon }_s$ related to the sediment diffusivity ${\epsilon }_s$ by an additional parameter $\alpha$ associated with the convective sediment entrainment process over sand ripples, which is defined by two equations. In the first, α depends on the relative importance of upward convection related to coherent vortex shedding and downward settling of sediments. When the convective transfer is very small, above low-steepness ripples, $\alpha \approx 1$. In the second, α depends on the relative importance of coherent vortex shedding and random turbulence. When random turbulence is more important than coherent vortex shedding, $\alpha \approx 1$, and “apparent” sediment diffusivity reverts to the classical sediment diffusivity ${\epsilon }_s^{*}\approx {\epsilon }_s$. Comparisons with experimental data show that the proposed method allows a good description of both SSC for fine and coarse sand and “apparent” sediment diffusivity ${\epsilon }_s^{*}$. Full article

Particle aggregation modifies sediment dynamics, which is a determining factor for morphodynamic and ecological processes in deltaic plains. Here, we investigated the link between intra-annual hydrodynamics variability and flocculation in the Grijalva-Usumacinta system. Monthly (2016–2017) and seasonal (2021–2022) river data was processed using analytical methods and the simplified sonar equation. Flocs were reformed and characterized in the laboratory, validating the in situ settling velocities (0.5–3.8 mm/s) and the existence of large low-density macro-flocs (>300 μm). We verified that flocculation prevailed, exhibiting seasonal patterns; (1) the highest aggregation rates matched the increase in total suspended solids at rising-flow (>100 mg/L), (2) periods of high-flow showed stable aggregation rates, and (3) an influence of marine conditions occurred at low-flow. Particulate phosphorous and organic fraction showed seasonal patterns linked to flocculation. Due to damming, the shear rates varied slightly (7–11 L/s) in the Grijalva, leading to high flocculation intensities affecting the diffusivity ratio. In the Usumacinta, aggregation was limited by shear rates that normally exceed 15 1/s. We found seasonal Rouse parameters representative of sediment dynamics. Full article

In environmental flows, field and laboratory measurements of suspended sediments show two kinds of concentration profiles. For coarse sediments, a near-bed upward convex profile is observed beneath the main upward concave profile. In this study, we consider two 1-DV models, namely, the classical advection–diffusion equation (ADE) based on the gradient diffusion model, and the kinetic model. Both need sediment diffusivity, which is related to the eddy viscosity, and an y-dependent β-function (i.e., the inverse of the turbulent Schmidt number). Our study shows that the kinetic model reverts to the classical ADE with an “apparent” settling velocity or sediment diffusivity. For the numerical resolution of the ADE, simple and accurate tools are provided for both the sediment diffusivity and hindered settling. The results for the concentration profiles show good agreement with the experimental data. An interpretation of the concentration profiles is provided by two “criteria” for shapes. The main for steady open-channel flows shows that the shape of the concentration profiles in the Cartesian coordinate depends on the vertical distribution of the derivative of $R$ (the ratio between the sediment diffusivity and the settling velocity of the sediments): dR/dy > −1 for the upward concave concentration profile while dR/dy < −1 for the near-bed upward convex profile. A generalization is proposed for oscillatory flows over sand ripples, where the time-averaged concentration profiles in the semi-log plots are interpreted by a relation between the second derivative of the logarithm of the concentration and the derivative of the product between the sediment diffusivity and an additional parameter related to the convective sediment entrainment process. Full article

Algae bloom event, an extreme ecological imbalance that the water environment experiences, changes the phosphorus (P) cycle in the aquatic environment, which makes the lake maintain a long-term eutrophication and frequent algae bloom state. This study compared P form characteristics and bacteria community structures in the aquatic environment of the cyanobacteria area and non-cyanobacteria area of Taihu Lake, aiming to clear the new P cycle pattern disturbed by algae bloom and decomposition processes. Compared with P forms in mediums of the middle of the lake and the east of the lake, there were higher concentration levels of total particulate P (TPP) in water, organic P (OP) in suspended particles, iron bound P (FeP) in sediments and phosphate (PO₄³⁻) in the pore water of Meiliang Bay, the cyanobacteria area. OP form was the dominant P fraction in suspended particles that occupied 69% in particulate total P, but OP proportion in sediments decreased to 26% of sediment total P, which indicated the strong occurrence of OP mineralization in sediments. The higher concentration and proportion of FeP in sediments of Meiliang Bay suggested the intensified effects of algae bloom and decomposition on sediment FeP accumulation. In Meiliang Bay, the positive correlation between Fe²⁺ and PO₄³⁻ in pore water and the higher diffusion fluxes of Fe²⁺, PO₄³⁻ from pore water to overlying water (0.45, 0.65 mg/m²·d) than that in the other lake areas also suggested the intensified effects of algae bloom and decomposition on FeP reductive dissolution in sediments accompanying sediment P remobilization. Moreover, there were higher concentrations of labile sulfide and high relative abundances of iron reducing bacteria (FRBs), sulfate reducing bacteria (SRBs) in sediments of Meiliang Bay. Results suggested that algae bloom event changed the natural P cycle in aquatic environment through intensifying the pathways of sediment OP mineralization, FeP accumulation and FeP reductive dissolution, which were mainly driven by the coupled factors of anoxic sediment condition, SRBs and FRBs activities. In addition, PO₄³⁻ diffusion from pore water to overlying water in the east of the lake may be prevented for its much higher Fe/P ratio (8.06) and Fe²⁺ concentrations in pore water, which may form a P-adsorbing barrier of iron oxides in the interface between pore water and overlying water. This study enhances the understanding of the vicious P cycle pattern in the aquatic environment driven by algae bloom and decomposition, which should be considered when conducting eutrophication prevention and control measures on lakes. Full article