Sediment Concentration Research Articles

Effects of microplastics on the rheological properties of sediment slurries in aquatic environments

Sediment slurries, characterized by their high concentrations of fine-grained cohesive sediment, are prevalent in various aquatic environments, including fluid mud, sediment gravity flows, and dredging slurries. Abundant microplastics have been detected in sediment slurries, which indicates that these slurries function as carriers for the transport of microplastics. However, there is a dearth of understanding on how sediment slurries transport microplastics. To ascertain the transport mechanisms, elucidating the effects of microplastics on the rheological properties of sediment slurries is a prerequisite because these properties govern the flow dynamics and mobility of such slurries. This study conducts experimental and theoretical investigations to examine, interpret, and quantify the effects of microplastics on the rheological properties of sediment slurries. Microplastics are shown to increase the yield stress and viscosity of sediment slurries via enhancing sediment aggregation. A new descriptor, specifically, the effective volume fraction, is proposed to characterize the effects of microplastics on sediment aggregation. Based on the newly-proposed descriptor, a new analytical model is proposed to predict the yield stress and viscosity of sediment slurries with microplastics. This study lays a foundation for further interpretating the flow dynamics and thus the transport processes of sediment slurries laden with microplastics.

Impacts of changing weather patterns on the dynamics of water pollutants in agricultural catchments: Insights from 11-year high temporal resolution data analysis

Widespread and long-term shifts in weather patterns are contributing to further degradation of surface water quality. This challenge caused by the increasing frequency of extreme weather events requires appropriate adaptation of current mitigation strategies. But to confirm the need to redesign such strategies, an understanding of the impacts of increasing weather extremes on pollutant losses in different catchment types is required. With this in view, this study investigated the impact of changing weather patterns on the inter-seasonal and inter-annual dynamics of nutrient losses in six agricultural catchments in Ireland over 11 years. The high temporal resolution data (10-min) from these intensively managed catchments represented different characteristics and management practices. Mann-Kendall Trend Analysis and Generalised Additive Models were used to study nutrient concentration trends, and to investigate the significance of water discharge, precipitation, potential evapotranspiration, soil moisture deficit, air temperature, and soil temperature on the losses of nutrients, respectively. The analysis of historical data revealed changes in the trends of daily average nitrate (NO3-N), phosphorus (P), and suspended sediment (SS) concentrations in association with significant increasing trends in air temperature, soil temperature, and precipitation across the same month over 11 years of monitoring. While discharge was significantly contributing to the concentrations of NO3-N, P, and SS across different catchments, air and soil temperature were significantly correlated to NO3-N losses, and precipitation was the major contributor to regulating P (total P and total reactive P) concentrations. In short, air temperature, soil temperature, soil moisture deficit, and precipitation were the main climatic drivers regulating the nutrient concentrations while the soil chemistry and drainage status were the non-climatically related drivers. The results revealed that the extent of the impact of climatic drivers depends on catchment characteristics. Therefore, expanding the application of this type of study would facilitate better understanding of current and future challenges to water management and provision of climate-resilient mitigation strategies for different catchment typologies.

Variability of Biodegradation Rates of Commercial Chemicals in Rivers in Different Regions of Europe.

Biodegradation is one of the most important processes influencing the fate of organic contaminants in the environment. Quantitative understanding of the spatial variability in environmental biodegradation is still largely uncharted territory. Here, we conducted modified OECD 309 tests to determine first-order biodegradation rate constants for 97 compounds in 18 freshwater river segments in five European countries: Sweden, Germany, Switzerland, Spain, and Greece. All but two of the compounds showed significant spatial variability in rate constants across European rivers (ANOVA, P < 0.05). The median standard deviation of the biodegradation rate constant between rivers was a factor of 3. The spatial variability was similar between pristine and contaminated river segments. The longitude, total organic carbon, and clay content of sediment were the three most significant explanatory variables for the spatial variability (redundancy analysis, P < 0.05). Similarities in the spatial pattern of biodegradation rates were observed for some groups of compounds sharing a given functional group. The pronounced spatial variability presents challenges for the use of biodegradation simulation tests to assess chemical persistence. To reflect the variability in the biodegradation rate, the modified OECD 309 test would have to be repeated with water and sediment from multiple sites.

Linking fish bioturbation to life history in a eutrophic wetland: An analysis of fish contributions to internal nutrient loading

Abstract Bioturbation (sediment disturbance by animal actions) effects on nutrient cycling and nutrient levels in surface waters are difficult to quantify, in part because the diversity and magnitude of species‐specific influences are poorly understood. These influences may have consequences for the management of the trophic state of freshwater ecosystems. Fish cause bioturbation in freshwater and marine ecosystems by digging in benthic sediments, manipulating periphyton mats while searching for prey and scraping hard substrates while feeding. We used experimental enclosures (2.25 m2) to quantify bioturbation‐mediated phosphorus (P) and nitrogen (N) regeneration from sediment by three species of fish that differ in interactions with the benthos (largemouth bass, Micropterus salmoides; tilapia, Oreochromis spp.; and sailfin catfish, Pterogoplichthys spp.) in shallow eutrophic wetlands in Southern Florida. Tilapia are omnivores that include detritus in their diet (winnowing or ingesting sediments) and dig nests in soft sediments year round, sailfin catfish actively burrow into substrate and consume detritus (digging and ingesting sediments), and largemouth bass are piscivores that do not routinely interact with the benthos when feeding but may dig nests in soft sediment in spawning season (January–April). We quantified the amount of suspended flocculent organic matter and changes in water column nutrients (total phosphorus [TP] and total nitrogen [TN]) in 2‐week trials for each species and estimated the portion of nutrient increases relative to fishless controls that could be attributed to bioturbation‐mediated internal nutrient loading through suspension of organic matter (as opposed to excretion or other sources of nutrient loading). Water column nutrient concentrations increased with increasing biomass for all species, but the bioturbation contribution differed by species. Largemouth bass increased water column nutrient concentrations (TP: 86% and TN: 5% relative to controls) but did not influence water column suspended particulate matter through bioturbation of sediment. Tilapia increased water column nutrients a modest amount (TP: 8%; TN: 15%), of which a small portion was attributed to bioturbation (c. 18% of TP). Sailfin catfish raised water column nutrients substantially (TP: 105%; TN: 46%) and up to 100% of the increased TP was attributed to bioturbation. Sailfin catfish also suppressed algal growth and TP accumulation on the sides of the enclosures and reduced nutrient concentrations of the flocculent sediments. Our results were consistent with our hypothesis that behaviour and foraging traits affect bioturbation contributions to nutrient loading. The results also demonstrated that species with similar net effects like largemouth bass and sailfin catfish, added nutrients via different mechanisms (i.e. excretion vs. bioturbation). Considering the feeding strategies and interactions with the substrate of common fish species may assist managers in meeting nutrient reduction goals for eutrophic wetlands and managed freshwater systems.

Space–Ground Remote Sensor Network for Monitoring Suspended Sediments in the Yellow River Basin

The Yellow River, China’s second-largest river, is renowned for its high sediment content. In response to the potential impacts of climate change on Yellow River water resources and water environmental management, an advanced monitoring and forecasting system for water and sediment throughout the entire Yellow River basin—from its source to the sea—is urgently needed. In this paper, based on the current status of water and sediment monitoring technologies, we proposed an integrated remote sensing monitoring network that combines satellite remote sensing, drone remote sensing, and ground-based wireless automatic monitoring networks, aiming to achieve the digital monitoring of water and sediment across the entire Yellow River basin, from its upper reaches to its estuary in the Bohai Sea. By utilizing ground-based in situ hyperspectral stations for sediment source areas in the upper reaches, such as the Qingshui River basin in Ningxia, and satellite remote sensing for midstream processes in the Xiaolangdi reservoir before the flood season in 2023, as well as downstream monitoring at the Yellow River estuary, this paper demonstrates the novelty and efficiency of the space–air–ground integrated remote sensing monitoring technology.

Deglaciation in the subtropical Andes has led to a peak in sediment delivery

Glaciers are thinning and retreating as climate warms, thus eroding less of the Earth’s surface. However, other hydrological factors in glacierized catchments are likely producing a transient increase in sediment delivery, resulting in ‘peak sediment’. Estimating the trajectory of the peak sediment is ecologically and socially important but scientifically challenging because of the delayed and non-linear response of glacier sediment export to climate forcing. This study used time series of suspended sediment concentration starting in the 1960s from 11 Andean rivers at subtropical latitudes to analyse past changes in sediment export and infer its future behaviour. The recent decade has experienced anomalously high sediment concentration in most glacierized catchments, but the 1970s experienced even higher values. Decadal variations in the relationship between sediment concentration and ice melt suggest that the magnitude of the current decade was lower due to reduced glacial sediment rather than other factors. Combining this result with the fact that glacial runoff is decreasing, it is inferred that, for most of the glacierized catchments, the peak sediment generated by the anthropogenic deglaciation started two centuries ago has already passed its maximum.

Legacy and alternative plasticizers in sediment from artificial lakes and coastal waters near high-tech industrial complexes in Korea: Contamination, ecological risk, mass inventory, and dilution factor

Few studies have been conducted on the occurrence and distribution of alternative plasticizers (APs) in aquatic environments. Legacy plasticizers (LPs) and APs were measured in sediments collected from four artificial lakes and a bay surrounded by high-tech industrial complexes. Bis(2–ethylhexyl) phthalate (DEHP) and bis(2–ethylhexyl) terephthalate were major plasticizers. The concentrations of LPs and APs in sediment were similar, implying rapid adoption of APs. The highest AP concentrations were observed in sediment from a lake close to semi-conductor, liquid crystal display, and automobile manufactures, suggesting a preferential shift to APs. Contamination profiles of APs differed according to industrial type. The mass inventories of plasticizers in sediment from the lakes were 25 times higher than those from the bay. Hydrophobicity was a major factor determinant of dilution factors of plasticizers in sedimentary environments. The DEHP concentrations in lake sediments exceeded threshold values, indicating potential health risks to benthic organisms.

Wear fault diagnosis in hydro-turbine via the incorporation of the IWSO algorithm optimized CNN-LSTM neural network

When a hydropower unit operates in a sediment-laden river, the sediment accelerates hydro-turbine wear, leading to efficiency loss or even shutdown. Therefore, wear fault diagnosis is crucial for its safe and stable operation. A hydro-turbine wear fault diagnosis method based on improved WT (wavelet threshold algorithm) preprocessing combined with IWSO (improved white shark optimizer) optimized CNN-LSTM (convolutional neural network-long-short term memory) is proposed. The improved WT algorithm is utilized to denoise the preprocessing of the original signals. Chaotic mapping, bird flock search, and cosine elite variation strategies are introduced to enhance the WSO algorithm's robust performance, and the CNN-LSTM model's hyperparameters are optimized using the IWSO algorithm to improve the diagnostic performance. The experimental results show that the accuracy of the proposed method reaches 96.2%, which is 8.9% higher than that of the IWSO-CNN-LSTM model without denoising. The study also found that the diagnostic accuracy of hydro-turbine wear faults increased with increasing sediment concentration in the water. This study can supplement the existing hydro-turbine condition monitoring and fault diagnosis system. Meanwhile, diagnosing wear faults in hydro-turbines can improve power generation efficiency and quality and minimize resource consumption.

Seafloor sediment characterization improves estimates of organic carbon standing stocks: an example from the Eastern Shore Islands, Nova Scotia, Canada

Abstract. Continental shelf sediments contain some of the largest stocks of organic carbon (OC) on Earth and play a vital role in influencing the global carbon cycle. Quantifying how much OC is stored in shelf sediments and determining its residence time is key to assessing how the ocean carbon cycle will be altered by climate change and possibly human activities. Spatial variations in terrestrial carbon stocks are well studied and mapped at high resolutions, but our knowledge of the distribution of marine OC in different seafloor settings is still very limited, particularly in dynamic and spatially variable shelf environments. This lack of knowledge reduces our ability to understand and predict how much and for how long the ocean sequesters CO2. In this study, we use high-resolution multibeam echosounder (MBES) data from the Eastern Shore Islands offshore Nova Scotia (Canada), combined with OC measurements from discrete samples, to assess the distribution of OC content in seafloor sediments. We derive four different spatial estimates of organic carbon stock: (i) OC density estimates scaled to the entire study region assuming a homogenous seafloor, (ii) interpolation of OC density estimates using empirical Bayesian kriging, (iii) OC density estimates scaled to areas of soft substrate estimated using a high-resolution classified substrate map, and (iv) empirical Bayesian regression kriging of OC density within areas of estimated soft sediment only. These four distinct spatial models yielded dramatically different estimates of standing stock of OC in our study area of 223 km2: 80 901, 58 406, 16 437 and 6475 t of OC, respectively. Our study demonstrates that high-resolution mapping is critically important for improved estimates of OC stocks on continental shelves and for the identification of carbon hotspots that need to be considered in seabed management and climate mitigation strategies.

Biogenic silica dynamics in coastal wetland sediments: A key driver of silicon and carbon biogeochemical cycling

AbstractBiogenic silica (biogenic Si) is a bioactive component crucial for the biogeochemical cycling of Si in terrestrial and aquatic ecosystems. Its formation and dissolution dynamics are intricately linked to carbon (C) cycling. However, knowledge about the source, composition, and factors controlling the distribution of biogenic Si in coastal wetland sedimentary environments is still limited. To address this lack of knowledge, we introduced a suite of geometric models for biogenic Si biovolume calculation and investigated biogenic Si assemblages, along with biogenic Si and biogenic Si‐occluded C contents, in sediments from representative coastal wetlands along the west coast of Bohai Bay. Our analysis showed that sedimentary biogenic Si predominantly derived from phytoliths (78.1% ± 5.8%), diatoms (18.2% ± 4.8%), and sponge spicules (3.7% ± 2.9%). Notably, phytolith assemblages were primarily composed of forms derived from the Poaceae family. The biogenic Si‐occluded C content (0.035–1.870 g kg−1) within these wetland sediments was consistent across both sites, accounting for 0.97–5.71% of the total organic C pool. Structural equation modeling indicated that total organic C, pH, and amorphous aluminum oxides either directly or indirectly influenced the biogenic Si content in coastal wetlands sediments. These results demonstrate the critical role that biogenic Si plays in the Si biogeochemical cycle and provide valuable information that advances our understanding of the biogeochemical interactions between Si and C in coastal wetland ecosystems.

Metal–organic framework-based SERS chips enable in situ and sensitive detection of dissolved hydrogen sulfide in natural water: Towards a bring-back-chip mode for field analysis

Hydrogen sulfide (H2S) in natural water plays an important role in carbon and sulfur cycles in biosphere. Current detection protocol is complicated, which need to “bring back water” to lab followed by gas chromatograph analysis. In situ, field detection is still challenging. Herein, a portable, sensitive surface enhanced Raman scattering (SERS) chip was proposed for in situ H2S sampling and SERS signal stabilizing, enabling a “bring back chip” manner for lab analysis. The SERS chip was composed of single core-shell gold nanorod-ZIF-8 framework (Au NR@ZIF-8) nanoparticle. Relying on headspace adsorption, evaporated H2S was enriched in the ZIF-8 shell and then reacted with Au NR, resulting in the weakening of the Au-Br bond Raman peak (175 cm−1) and the appearance of the Au-S bond Raman peak (273 cm−1). The SERS signal reached equilibrium in 10 min. The detection range of H2S was 0.1–2000 μg/L and limit of detection was 0.098 μg/L. SERS signal was not interfered by normal volatile gases. Moreover, SERS signal of a reacted chip was stable at an ambient condition, allowing for in situ sampling and bring-back detection. The applicability of the chip was verified by dynamic H2S monitoring during artificial black-odor water evolution, and in-field quantitative analysis of H2S content in river water and sediment. Finally, the chip was sealed in a waterproof breathable membrane device, which realized the detection of vertical profiles of H2S in the river. This work provided a promising tool for field analysis of H2S in natural environments.

Study on the influence parameters and anti‐abrasion performance of steam‐cured hydraulic concrete for prefabricated water conveyance channel

AbstractExcessive sediment concentration in water can cause damage to prefabricated concrete channels. This study evaluates the impact of sediment‐laden water flow on the mass loss of concrete after abrasion, incorporating scanning electron microscope‐energy dispersive spectrometer (SEM‐EDS) and X‐ray diffraction (XRD) analyses to assess the effects of four steam‐curing parameters—delay time, heating rate, constant temperature duration, and steam curing temperature—on the abrasion resistance of concrete used in enterprise prefabricated water conveyance channels. The results indicate that the abrasion resistance of concrete for prefabricated channels improves gradually with increases in delay time and constant temperature duration. When the delay time exceeds 3 h and the constant temperature time exceeds 4 h, the concrete's abrasion resistance can reach more than 5 h(g/cm2)−1. Rapid heating rates and excessively high steam curing temperatures adversely affect the concrete's abrasion resistance. However, when the heating rate is controlled within 20°C/h and the steam curing temperature does not exceed 70°C, the concrete's abrasion resistance can achieve more than 5 h(g/cm2)−1. In compliance with the standard DL/T5201‐2021 requirements, it is recommended that concrete with high demands for abrasion resistance should preferentially select steam‐curing parameters within these ranges.

Response of Runoff, Sediment Yield, and Runoff‐Related Dissolved Organic Carbon Loss to Variable Straw Mulching Rates on Sloping Lands of Regosols

ABSTRACTDissolved organic carbon (DOC), an organic carbon fraction with high activeness and mobility, migrated by runoff is a key part in carbon cycle. A rational straw mulching rate can be regulated to obtain maximum benefits while controlling runoff and sediment yield on sloping lands. However, little remains known about the optimal straw mulching rates required for effectively reducing the loss of DOC in runoff. Therefore, to overcome the existing limitations, this study investigated the effects of modified maize straw mulching rates on the loss of DOC during runoff, utilizing indoor rainfall simulation. Five mulching rates, including 0, 0.2, 0.4, 0.6, and 0.8 kg m−2 [control (CK) and treatments (T1, T2, T3, and T4), respectively], were tested in combination with three slope gradients (10°, 15°, and 20°) to evaluate how straw mulching rate influences runoff, sediment yield, and runoff‐related DOC loss under a heavy rainfall intensity of 90 mm h−1. Our results showed that various straw mulching rates did not significantly differ runoff rates; however, straw mulching significantly reduced sediment concentration and yield. Moreover, the reduction in sediment yield increased with an increase in mulching rate. Compared to the CK, T1 resulted in a 63% increase in DOC loss at a slope of 20°. Additionally, T2 caused an 8% and 7.2% increase in DOC loss at both 10° and 15° slopes. Conversely, T3 and T4 reduced DOC loss by 54.1%–80.8% and 51.1%–65.2%, respectively, across all slope gradients. These results suggested that mulching rates of 0.2–0.4 kg m−2 may potentially increase DOC loss in runoff on the sloping lands. Our results hold significant importance in optimizing the use of straw mulching for sustainable management practices in agricultural lands.

Machine‐Assisted Physical Closure for Coarse Suspended Sediments in Vegetated Turbulent Channel Flows

AbstractThe parameterization of suspended sediments in vegetated flows presents a significant challenge, yet it is crucial across various environmental and geophysical disciplines. This study focuses on modeling suspended sediment concentrations (SSC) in vegetated flows with a canopy density of avH ∈ [0.3, 1.0] by examining turbulent dispersive flux. While conventional studies disregard dispersive momentum flux for avH &gt; 0.1, our findings reveal significant dispersive sediment flux for large particles with a diameter‐to‐Kolmogorov length ratio when dp/η &gt; 0.1. Traditional Rouse alike approaches therefore must be revised to account for this effect. We introduce a hybrid methodology that combines physical modeling with machine learning to parameterize dispersive flux, guided by constraints from diffusive and settling fluxes, characterized using recent covariance and turbulent settling methods, respectively. The model predictions align well with reported SSC data, demonstrating the versatility of the model in parameterizing sediment‐vegetation interactions in turbulent flows.

Using dredged sediments from Lake Taihu as a plant-growing substrate: Focusing on the impact of microcystins

Using dredged sediment as plant growth substrates is a promising way to deal with large amounts of excavated sediments. However, it is a big challenge to deal with various pollutants in sediments, among which microcystins (MCs) gained limited attention. In this study, sediments collected from Lake Taihu were mixed with agricultural soil at a 1:1 ratio to create various growing substrates for lettuce (Lactuca sativa L. var. ramosa Hort.). Results indicated that fresh weight and leaf area of lettuce increased in some sediment-amended treatments due to additional nutrients, but food quality was negatively affected by sediment amendment as suggested by the soluble sugar and Vitamin C levels. MCs were detected in all lettuce grown in sediment-amended substrates, particularly in treatments with sediments collected during the bloom. The highest MC contents were found in treatment amended with sediments collected from Meiliang Bay in August (88.6 μg kg−1 for MC-LR and 65.6 μg kg−1 for MC-RR). MC accumulation in lettuce and the associated human health risks were significant, especially in treatments with sediments from the bloom period. Ecological risk assessments revealed high RQ values, indicating potential harm to the soil ecosystem. This study underscores the importance of considering MC content in sediments when evaluating their use as growing substrates. The findings contribute to understanding the environmental and health implications of sediment reuse, offering insights for safer agricultural practices and sediment management.

A smart, multi-configuration, and low-cost system for water turbidity monitoring

Measuring the amount of suspended sediment in rivers is important for a better understanding of phenomena like soil erosion or floods. Since such phenomena are not spatially homogeneous, a distributed measurement approach is required to obtain fine-grained and realistic models. We designed and implemented a low-cost, flexible, and smart suspended sediment concentration monitor. The system only uses components that are easy to find and assemble. This, together with the reduced costs, enables the use of multiple instances for distributed monitoring. The system can operate according to different configurations, fixed or mobile, and using a variety of connection technologies, to possibly cover the various necessities that arise from the diverse installation sites. Each measuring unit automatically retrieves its operational parameters, easing the configuration of a possibly large distributed measurement system.

Source apportionment and ecological risk assessment of antibiotics in Dafeng River Basin using PMF and Monte-Carlo simulation.

Antibiotics, prevalent in aquatic ecosystems, pose a grave threat to human health and the ecological well-being. This paper performed a case study on Dafeng River Basin in southern China. Specifically, techniques including positive matrix factorization (PFM) and Monte-Carlo simulation were employed to comprehensively investigate the spatial variations, possible sources, and ecological risks of antibiotics in four groups: sulfonamides (SAs), macrolides (MLs), quinolones (QNs), and tetracyclines (TCs). The major findings were as follows: first, 43 and 39 antibiotics were detected in the surface water and sediments of the basin, respectively, where the respective total content were ND-490.08ng/L and ND-144.34μg/kg, and the QNs and TCs were the two dominating groups. Second, the highest antibiotic content in surface water (441.43ng/L) was observed in the midstream area, whereas the highest concentration in sediments (68.41μg/kg) was found in the upstream region. Third, the investigation identified five sources of antibiotics discharged to surface water: domestic sewage, agricultural drainage, livestock discharge, sewage treatment plants, and aquaculture; three sources were detected for antibiotics in sediments: aquaculture, sewage treatment plants, and livestock discharge. Fourth, QNs had a significantly higher ecological risk than the other three groups of antibiotics, and livestock discharge (31.4% contribution) and aquaculture (23.4% contribution) were the main sources of risks of antibiotic contamination in Dafeng River Basin. This study is expected to provide some reference for control and risk management of antibiotic pollution in Dafeng River Basin.

Contamination Assessment of Sediments and Bivalves in Estuaries of the Southern Iberian Peninsula

Human activities directly impact estuaries, where the biota is exposed to sediment contamination. A contamination assessment was carried out in several estuaries in the Southern Iberian Peninsula. Sediment samples were analyzed for the presence of metals/metalloids, and bioaccumulation tests were conducted with clams (Ruditapes philippinarum). Huelva Estuary had the highest contamination levels, while the inner bay of Cádiz and the outer stations of the estuaries from Guadiana, Guadalquivir, Palmones, and Guadarranque were the lesser contaminated. All sampling points (except Huelva) had low contamination levels of As and Cd, but they had high concentrations of Cu. The elements Pb, Cd, Zn, Cu, As, and Hg displayed correlations between the concentrations in sediments and the biota. High bioaccumulation of Zn, Cu, and As was observed in Huelva and Barbate. Important insights into the sediment contamination in Southern Iberian Peninsula estuaries suggest greater management and conservation efforts in these critical ecosystems.

Determination of Settling Velocity for Cohesive Sediments Using a Settling Column Experiment—Case Study: Ca Mau, Vietnam

The settling velocity (Ws) is a fundamental parameter in sediment dynamics, particularly in the coastal zones as estuarine and mangrove ecosystems, where various physical processes interact to influence sediment transport and deposition. Determining settling velocity is essential for understanding and predicting sediment transport, erosion–deposition processes in coastal environments. This study aims to determine the settling velocity and examines how sediment concentration affects settling velocity, using empirical models. The study applies these methods to compute the settling velocity of fine sediments in Ca Mau, Vietnam, which were collected in the Song Doc area (Western) in August 2014 and Rach Goc area (Eastern) in August 2015. The particle size analysis results indicate that the sediment samples from Western Ca Mau are coarser than those from Eastern Ca Mau, with sand ratios of 42.78% and 7.08% at the outer station, and 19.09% and 4.39% at the mangroves, respectively. The results also show that the average settling velocity of the sediment samples from Western Ca Mau is higher than that of the samples from Eastern Ca Mau. Specifically, the settling velocity in Western Ca Mau is 2.80 × 10−4 m s–1 at the outer station and 1.68 × 10−4 m s–1 at the mangroves. Meanwhile, in Eastern Ca Mau, the settling velocity of the sediment samples is 1.99 × 10−4 m s–1 at the outer station and 1.42 × 10−4 m s–1 at the mudflat-mangrove. Hence, it can be seen that the settling velocity of cohesive sediments corresponds well with the sediment’s particle size.

Numerical modeling of graded sediment transport with limited bed material supply – A case study in the Middle Yangtze River

Suspended sediment concentration in the Middle Yangtze River (MYR) has reduced sharply below the Three Gorges Dam since 2003, and led to significant channel degradation and riverbed coarsening. However, the complex interaction between riverbed coarsening and sediment transport is essential for channel evolution processes but is poorly understood. This paper presents a one-dimensional model for simulating graded sediment transport when the bed material supply is limited. The model takes account of the effect of bed material composition (BMC) and threshold probability on graded sediment transport, and thus improve the accuracy of recovery coefficient. The modeling results of graded sediment concentrations was calibrated and then satisfactorily verified using field observations in the JingJiang Reach from 2015 to 2016. The results showed bed material composition affect the simulated fine, medium, and coarse sediment concentrations, and the improvements of normalized errors are 17.6%, 45.9%, and 330.7%, respectively. With the BMC effect considered, the recovery factor of fine and medium were far less than coarse sediment in the MYR. In addition, the recovery ratios of medium and coarse sediment concentrations along the MYR were improved by 88.6%-265.4% and 75.1%-105.9% respectively, owing to the bed material supply after the TGD operation; however, there was no significant effect on fine sediment, with a low recovery ratio of 8.3%-8.6%. Therefore, the effect of bed material composition on graded sediment transport and channel evolution needs to be considered for higher simulation accuracy in the MYR.