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• Fast and batch of settling mud flocs measurement and 3D reconstruction. • Volumetric floc reconstruction improves the accuracy of irregular mud floc sizes. • Stable bimodal volumetric floc size distribution forms in equilibrium flocculation. • Settling velocity keeps stable and low variability between 1 and 2 mm·s−1 in >200 μm. • High Volumetric- but low Mass- Settling Flux in flocs>200 μm due to bio-components. Mud flocculation and settling play key role in understanding sediment transport cycle and affect water quality in estuaries and coastal seas. However, the morphological irregularity and structural instability of fragile mud flocs set huge obstacles for quantifying geometric property accurately and establishing reliable predicting tools in settling dynamics via previous observing strategies based on instant measured and 2-dimensional imagery floc parameterizations. Here we designed a multi-camera apparatus targeting capturing multiple angles of individual flocs, and developed a multi-view segmentation algorithm on floc images analysis. We finally accomplished batch of 3-dimensional reconstruction obtaining each settling floc's volumetric size in equilibrium flocculation. The results indicate a stable bimodal floc size distribution in equilibrium flocculation with a dominant peak of microflocs (<200 μm) and a secondary smaller peak of macroflocs (> 200 μm). The flocculi (<50 μm) shows more spherical outlines with dense structure while the larger-sized macroflocs (>200 μm) have high irregular morphologies with high porosity and visible biological debris attaching, and the microflocs (50–200 μm) tend to be irregular in shape and dense inside. The terminal settling velocity of mud flocs shows increasing with floc size in <200 μm but keeps stable around 1–2 mm s−1 after >200 μm due to the increase in size being compensated by the decrease of density according to the fractal theory on floc geometry. The higher organic matter content within larger porous flocs reduces the macroflocs effective density. These lead to high volumetric settling flux but low mass settling flux of macroflocs in natural water systems. This work provides new insight to reveal more accurate mud floc geometric parameterizations in volumetric aspect and reliable characterizations of equilibrium flocculation using a fast and sound batch of direct measuring approach. This may importantly improve the predictions of suspended mud dynamics in nature. [Display omitted] [ABSTRACT FROM AUTHOR]

Suspended sediment floc dynamics, particularly in the bottom boundary layer (BBL), has long been recognized as a complex and crucial factor influencing flow hydrodynamic and bed morphodynamics in tidal estuarine regions. Periodic salt-water intrusion in micro-tidal estuaries plays a crucial role due to its direct impacts on ecosystem, biological habitats, and human activities. To gain deeper understanding of the interactions between sediment flocculation and salt-water dynamics in estuaries, a continuous 115-h in-situ field survey onboard was conducted in a micro-tidal estuary, specifically the Huangmaohai estuary in China. This survey involved the measurements of suspended sediment concentrations (SSCs), floc properties, tidal current, turbulence and salinity in the BBL, utilizing state-of-art instrumentation such as the LISST-200X and ADV. Principal component analysis (PCA) was employed to examine the characteristics of sediment flocculation and analyze the influences of dynamic and structural factors on flocculation. The results reveal that, with the movement of the salt-wedge, two distinct stages of sediment flocculation process can be observed. Stage 1 occurs when the water column is vertically stratified, and a salt-wedge forms. Despite a reduction in SSC, this stage is characterized by a flocculation-dominated regime, in which large-sized, low density and low fractal dimension flocs settle to the bed. The presence of a stable salt-wedge enhances the flocculation by facilitating the formation of large-sized flocs with low density, low fractal dimension and high settling velocity while minimizing the disruptive effects of turbulence in the BBL. Statistical analysis reveals that saline water stratification contributes 19.76% to flocculation efficiency during this stage. Stage 2 occurs when the water column undergoes vertical mixing, causing the salt-wedge to dissipate. The regime transitions to a de-flocculation-dominated phase, leading to the resuspension of fine particles from the bed surface due to intense turbulence shear in the BBL. Consequently, flow shear becomes the key factor controlling factor for flocculation in the BBL with a contributing rate of 18.82%, statistically. A comprehensive conceptual model is presented to elucidate the intricate interactions between salt-wedge intrusion-retreat cycles and sediment flocculation within the BBL of a micro-tidal estuary. • Flocculation-dominated when the BBL within the salt-wedge. • De-flocculation-dominated when the BBL in the front of the salt-wedge. • Water stratification controls the flocculation in flocculation-dominated regime. • Flow shear controls the flocculation in de-flocculation-dominated regime. [ABSTRACT FROM AUTHOR]

• Mineral flocs show unimodal eFSD and EPS-mineral flocs show bimodal eFSD. • EPS reduces the effective density in EPS-mineral flocs by 1∼2 magnitude. • Terminal settling is size-dominated in minerals but density-dominated by adding EPS. • EPS quantification of non-fractal flocs is necessity to model natural flocculation. Biophysical cohesive mud, consisting of clay minerals and extracellular polymeric substance (EPS), plays significant role in determining sediments, nutrients and pollutants transport in estuarine and coastal systems. Series of laboratory jar experiments have been conducted aiming at filling the gap of knowledge regarding how biological cohesive EPS affects equilibrium flocculation of EPS-mineral mixtures. Four types of common clay (chlorite, kaolinite, illite and montmorillonite) were chosen due to their abundance in estuarine mud and distinct crystal chemistry and structures. Turbulent shear throughout all the experimental runs were constantly provided at a mean shear parameter of G ≈ 15 s−1 being equivalent to high tidal influenced estuarine turbulent environment. The results reveal that adding EPS increases the equilibrium floc size evidently. The pure mineral flocs show unimodal equilibrium floc size distribution (eFSD) with single peak located at microfloc range (<200 μm) while the EPS-mineral flocs show bimodal eFSD with a secondary peak located in macroflocs range (>200 μm) mostly. Moreover, EPS largely reduces the effective density in EPS-mineral flocs by 1∼2 magnitude. Most importantly, the terminal settling velocity of flocs shows size-dominated in uniform mineral floc cases but density-dominated in EPS-mineral mixture floc cases especially in macroflocs. To model a full floc size or settling velocity distributions in natural environments, furtherly quantification of EPS functions within the large-sized non-fractal mixture floc individually becomes a necessity. [Display omitted] [ABSTRACT FROM AUTHOR]

Cohesive particles in aquatic systems can play an important role in determining the fate of spilled oil via the generation of Oil-Mineral Aggregates (OMAs). Series of laboratory experiments have been conducted aiming at filling the knowledge gap regarding how cohesive clay particles influence the accumulation of petroleum through forming different aggregate structures and their resulting settling velocity. OMAs have been successfully created in a stirring jar with artificial sea-water, crude oil and two types of most common cohesive minerals, Kaolinite and Bentonite clay. With the magnetic stirrer adjusted to 490 rpm to provide a high level homogeneous flow turbulence (Turbulence dissipation ε estimated to be about 0.02 m2⋅s−3), droplet OMAs and flake/solid OMAs were obtained in oil-Kaolinite sample and oil-Bentonite sample, respectively. Kaolinite clay with relatively low flocculation rate ( R f = 0.13 min−1) tends to physically attach around the surface of oil droplets. With the lower density of oil, these oil-Kaolinite droplet OMAs generally show lower settling velocity comparing to pure mineral Kaolinite flocs. Differently, Bentonite clay with higher flocculation rate ( R f = 0.66 min−1) produces more porous flocs that can absorb or be absorbed by the oil and form compact flake/solid OMAs with higher density and settling velocity than pure Bentonite flocs. In the mixed Kaolinite-Bentonite sample (1:1 in weight), oil is observed to preferably interacting with Bentonite and increase settling velocity especially in larger floc size classes. Image 1 • Microscopy analysis reveals distinct Oil-Mineral Aggregates (OMA) structures due to clay types. • LabSFLOC-2 further reveals the dependence of OMA settling velocity due to structures. • Distinct OMA structures & settling velocities are shown to be controlled by mineral stickiness. [ABSTRACT FROM AUTHOR]

Abstract: The formation of a broad tidal flat along the coast of the northern Jiangsu Province of China depends largely on the sediment supply from the Yellow and Yangtze Rivers although the relative contributions from each of these two large rivers remain uncertain. Knowledge of sediment sources to the tidal flat is critical for understanding the evolution of this muddy coast impacted by the large rivers. The present study focuses on tracking the sediment source of the tidal flat deposits based on sediment magnetic properties and geochemical analyses as well as statistical analysis. The study shows that sediments to the north of Yangkougang (site 9) have lower values of saturation isothermal remanence magnetization (SIRM), magnetic susceptibility (χ), demagnetization parameter S−100 ratios, lower Fe/Al values and higher Ca concentrations, while the opposite is true for samples south to Haozhigang (site 18). In addition, SIRM values of the <16μm and >63μm fractions generally display increasing trend from north to south. These results suggest that sediment sources rather than particle size variation are the dominant factors influencing the bulk magnetic properties. In light of the sediment composition comparison between the Yellow River and Yangtze River, factor analysis is used to identify sediment source of the tidal flat sediments. It is indicated that the Yellow River in its former course is the dominant supplier for sites to the north of Yangkougang (site 9), while the Yangtze River is the dominant supplier for sites to the south of Haozhigang (site 18). The coast between Yangkougang (site 9) and Haozhigang (site 18) is a transition zone influenced by both rivers. The inferred provenance contrasts are consistent with the pattern of coastal geomorphological evolution. Our data also suggest that sediments eroded from the former Yellow River delta have contributed to the evolution of the Yangtze Estuary in historical time. This study demonstrates that a combined magnetic and geochemical fingerprinting techniques in couple with statistical analysis is valuable for identifying sediment sources of tidal flats and deltas influenced by large rivers in the world. [Copyright &y& Elsevier]

Abstract: Loess deposits are widely distributed over the islands in the Bohai Gulf and neighboring regions of northern China. The coastal loess, located to the east of the Chinese Loess Plateau (CLP), is believed to have a local source from exposed marine sediments and a distant source from inner arid regions in Asia. In this study, magnetic, geochemical and diffuse reflectance spectroscopic analyses were conducted on the loess to explore its relationship with the loess on the CLP and its paleoclimatic implications. We find that the coastal loess shows similar geochemical composition, background susceptibility and magnetic enhancement to loess on the CLP. Magnetic minerals characteristic of pedogenesis are more important in the paleosol layer, with fine ferrimagnetic minerals abundant in paleosol layer SB1 and hematite in paleosol layer SB2. Geochemical evidence suggests that weathering intensity in paleosol SB2 is similar to or even stronger than in paleosol SB1. Pedogenic enhancement of magnetic susceptibility in the study area is not so strong, which may be due to enhanced hematite formation as opposed to ferrimagnetic mineral production and preservation. [Copyright &y& Elsevier]

The shape and size of sedimentary bedforms play a key role in the reconstruction of sedimentary processes in modern and ancient environments. Recent laboratory experiments have shown that bedforms in mixed sand–clay develop at a slower rate and often have smaller heights and wavelengths than equivalent bedforms in pure sand. This effect is generally attributed to cohesive forces that can be of physical origin, caused by electrostatic forces of attraction between clay minerals, and of biological origin, caused by 'sticky' extracellular polymeric substances (EPS) produced by micro‐organisms, such as microalgae (microphytobenthos) and bacteria. The present study demonstrates, for the first time, that these laboratory experiments are a suitable analogue for current ripples formed by tidal currents on a natural mixed sand–mud–EPS intertidal flat in a macrotidal estuary. Integrated hydrodynamic and bed morphological measurements, collected during a spring tide under weak wave conditions near Hilbre Island (Dee Estuary, north‐west England, UK), reveal a statistically significant decrease in current ripple wavelength for progressively higher bed mud and EPS contents, and a concurrent change from three‐dimensional linguoid to two‐dimensional straight‐crested ripple planform morphology. These results agree well with observations in laboratory flumes, but the rate of decrease of ripple wavelength as mud content increased was found to be substantially greater for the field than the laboratory. Since the formation of ripples under natural conditions is inherently more complex than in the laboratory, four additional factors that might affect current ripple development in estuaries, but which were not accounted for in laboratory experiments, were explored. These were current forcing, clay type, pore water salinity and bed EPS content. These data illustrate that clay type alone cannot explain the difference in the rate of decrease in ripple wavelength, because the bed clay contents were too low for clay type to have had a measurable effect on bedform development. Accounting for the difference in current forcing between the field and experiments, and therefore the relative stage of development with respect to equilibrium ripples, increases the difference between the ripple wavelengths. The presence of strongly cohesive EPS in the current ripples on the natural intertidal flat might explain the majority of the difference in the rate of decrease in ripple wavelength between the field and the laboratory. The effect of pore water salinity on the rate of bedform development cannot be quantified at present, but salinity is postulated herein to have had a smaller influence on the ripple wavelength than bed EPS content. The common presence of clay and EPS in many aqueous sedimentary environments implies that a re‐assessment of the role of current ripples and their primary current lamination in predicting and reconstructing flow regimes is necessary, and that models that are valid for pure sand are an inappropriate descriptor for more complex mixed sediment. This study proposes that this re‐assessment is necessary at all bed clay contents above 3%. [ABSTRACT FROM AUTHOR]

Many coastal and estuarine environments are dominated by mixtures of non-cohesive sand and cohesive mud. The migration rate of bedforms, such as ripples and dunes, in these environments is important in determining bed material transport rates to inform and assess numerical models of sediment transport and geomorphology. However, these models tend to ignore parameters describing the physical and biological cohesion (resulting from clay and extracellular polymeric substances, EPS) in natural mixed sediment, largely because of a scarcity of relevant laboratory and field data. To address this gap in knowledge, data were collected on intertidal flats over a spring-neap cycle to determine the bed material transport rates of bedforms in biologically-active mixed sand-mud. Bed cohesive composition changed from below 2 vol% up to 5.4 vol% cohesive clay, as the tide progressed from spring towards neap. The amount of EPS in the bed sediment was found to vary linearly with the clay content. Using multiple linear regression, the transport rate was found to depend on the Shields stress parameter and the bed cohesive clay content. The transport rates decreased with increasing cohesive clay and EPS content, when these contents were below 2.8 vol% and 0.05 wt%, respectively. Above these limits, bedform migration and bed material transport was not detectable by the instruments in the study area. These limits are consistent with recently conducted sand-clay and sand-EPS laboratory experiments on bedform development. This work has important implications for the circumstances under which existing sand-only bedform migration transport formulae may be applied in a mixed sand-clay environment, particularly as 2.8 vol% cohesive clay is well within the commonly adopted definition of “clean sand”. [ABSTRACT FROM AUTHOR]

• Median floc size is insufficient for calibrating PBE flocculation model. • A complete model calibration reveals a lower floc yield strength in larger flocs. • Incorporating a variable floc yield strength greatly improves the model prediction. Many aquatic environments contain cohesive sediments that flocculate and create flocs with a wide range of sizes. The Population Balance Equation (PBE) flocculation model is designed to predict the time-dependent floc size distribution and should be more complete than models based on median floc size. However, a PBE flocculation model includes many empirical parameters to represent important physical, chemical, and biological processes. We report a systematic investigation of key model parameters of the open-source PBE-based size class flocculation model FLOCMOD (Verney, Lafite, Claude Brun-Cottan and Le Hir, 2011) using the measured temporal floc size statistics reported by Keyvani and Strom (2014) at a constant turbulent shear rate S. Results show that the median floc size d 50 , in terms of both the equilibrium floc size and the initial floc growth, is insufficient to constrain the model parameters. A comprehensive error analysis shows that the model is capable of predicting three floc size statistics d 16 , d 50 and d 84 , which also reveals a clear trend that the best calibrated fragmentation rate (inverse of floc yield strength) is proportional to the floc size statistics considered. Motivated by this finding, the importance of floc yield strength is demonstrated in the predicted temporal evolution of floc size by modeling the floc yield strength as microflocs and macroflocs giving two corresponding fragmentation rates. The model shows a significantly improved agreement in matching the measured floc size statistics. [ABSTRACT FROM AUTHOR]