In situ particle size spectra and density of particle aggregates in a dredging plume

An in situ laser particle sizer, the LISST-100, was used to describe the spatial variation of beam attenuation coefficient, in situ particle size spectra and aggregate densities in a dredging plume in the sound Øresund between Denmark and Sweden. The results proved that the above mentioned parameters varied significantly within the investigated length of the plume (approximately 2 km). It is shown how the small single primary particles aggregate and change the in situ particle size spectra into a slightly better sorted and coarser size distribution, and at the same time how the mean density of particles/aggregates decreases significantly. This shift in the state in which the particles exist in the water effectively changes the optical response of the mass of particles suspended in the water. It is shown that adequate correlations between mass concentrations and beam attenuation coefficients can only be obtained if parameters describing the in situ quality and state of the mass of particles, e.g. standard deviation of the size spectra and volume concentration, is included in the regression. From the spatial variation in mean density and in situ particle size, it was possible to calculate the spatial difference in settling velocity. It was found that the difference in settling velocity was only about a factor of 1.7, because increasing in situ particle size was counter-balanced by decreasing mean density. Furthermore, the time-scale of flocculation within the plume was found to be in the order of 50 min.     

Entropy analysis of in situ particle size spectra

Entropy analysis has been used to classify in situ particle (floc) size spectra of suspended particles into groups based on similar distribution characteristics. Results revealed that the in situ spectra sorted into groups that reflected different forcing conditions (e.g. variations in turbulence). The different forcing conditions were not necessarily reflected in other commonly used distribution measures such as median floc diameter. This suggests that entropy analysis may be an effective approach for investigating the effect of changes in forcing conditions on floc size. It is hypothesized that it may be possible to derive the average shape of floc size spectra from measurement of the forcing conditions alone and subsequently derive parameters such as floc fraction, floc density, floc settling velocity and the optical properties of the water column from the average spectra.     

On the variability of near-bed floc size due to complex interactions between turbulence,SSC, settling velocity,effective density and the fractal dimension of flocs

Interactions between turbulence, suspended sediment concentration (SSC), settling velocity, effective density, fractal dimension, and floc size were studied on the tide-dominated, muddy coastal shelf of the southwestern Yellow Sea, China. The measurements were carried out in July 2013 at two sites located in water depths of 21.2 and 22.1 m. Negative correlations were observed between shear rate, SSC, effective density, and mean floc size, which supports the results of previous numerical, experimental, and field studies. A significant positive correlation was observed between near-bed SSC and shear rate, an indication that SSC variations are controlled by turbulence and re-suspension. In addition, significant linear relationships were found between settling velocity and other parameters (floc size, turbulence, SSC, effective density, and fractal dimension) at the two sites, indicating that the controlling factors on settling velocity are spatially variable. Principal component analysis was applied to determine the relative importance of turbulence, flocculation ability, and SSC as controls on floc size in situ. The relative contributions of turbulence, flocculation ability, and SSC to floc size (at both sites) were ~33.0%, 30.3%, and 29.7%, respectively, this being a new field-based quantitative analysis of the controls on floc size. The findings demonstrate that, in nature, flocculation ability affects floc size to the same degree as turbulence and SSC. Therefore, predictions of floc size in coastal marine environments require constraints not only on turbulence and SSC, but also on flocculation ability.     

Dynamics of suspended particles in coastal waters (southern North Sea) during a spring bloom

Measurements of suspended particle concentration, size and settling velocity were made at a shallow site in the southern North Sea during a spring phytoplankton bloom. The site was characterised by strong differences in surface and near-bed residual flows; therefore particle processes in each layer are effectively decoupled as long as the water column is stratified. Four distinct energetic events during the observation period caused variation in the characteristics and behaviour of the particle population: (1) moderate spring tides with low wave activity; (2) strong winds, increased wave activity; (3) strong spring tides; (4) weak neap tides. During Event 1 weak tidal resuspension occurred, median particle diameter was relatively large, but median settling velocities of both chlorophyll and total SPM were low. During the higher energy Events 2 and 3 there was resuspension of relatively small, high-density particles producing high median total SPM settling velocities but low median particle diameter. In addition, a phytodetrital fluff layer, characterised by high chlorophyll settling velocity, was resuspended and dispersed during storm conditions (Event 2). During calm, weak neap tides (Event 4) there was negligible resuspension and enhanced particle settling and deposition, particularly in the phytodetritral component of the particle population, allowing rapid replenishment of the benthic fluff layer. This work indicates the relatively rapid rate at which fluff layers can be formed and dispersed, and highlights the need for high frequency measurements. The range of contrasting physical conditions over which the data-set was collected makes it an ideal candidate for parameterising and validating suspended sediment dynamics models.     

Properties of coarse particles in suspended particulate matter of the North Yellow Sea during summer

Fine particles in seawater commonly form large porous aggregates. Aggregate density and settling velocity determine the behavior of this suspended particulate matter(SPM) within the water column.However, few studies of aggregate particles over a continental shelf have been undertaken. In our case study, properties of aggregate particles, including size and composition, over the continental shelf of the North Yellow Sea were investigated. During a scienti?c cruise in July 2016, in situ ef fective particle size distributions of SPM at 10 stations were measured, while temperature and turbidity measurements and samples of water were obtained from surface, middle, and bottom layers. Dispersed and inorganic particle size distributions were determined in the laboratory. The in situ SPM was divided into(1) small particles(<32 μm),(2) medium particles(32–256 μm) and(3) large particles(>256 μm). Large particles and medium particles dominated the total volume concentrations(VCs) of in situ SPM. After dispersion, the VCs of medium particles decreased to low values(<0.1 μL/L). The VCs of large particles in the surface and middle layers also decreased markedly, although they had higher peak values(0.1–1 μL/L). This suggests that almost all in situ medium particles and some large particles were aggregated, while other large particles were single particles. Correlation analysis showed that primary particles <32 μm in?uenced the formation of these aggregates. Microscopic examination revealed that these aggregates consisted of both organic and inorganic ?ne particles, while large particles were mucus-bound organic aggregates or individual plankton.The vertical distribution of coarser particles was clearly related to water strati?cation. Generally, medium aggregate particles were dominant in SPM of the bottom layer. A thermocline blocked resuspension of?ne material into upper layers, yielding low VCs of medium-sized aggregate particles in the surface layer.Abundant large biogenic particles were present in both surface and middle layers.     

Observation of the size,settling velocity and effective density of flocs,and their fractal dimensions

In situ instruments, particularly the instrument INSSEV (in situ settling velocity) have given new information on the sizes, settling velocities and effective densities of individual flocs within the spectrum of distribution. The low-density macroflocs (diameter >∼150 μm) contain a mixture of organic and inorganic constituents that become separated when the flocs are disrupted to form microflocs. Representation of the floc characteristics in terms of fractals reveals a range of fractal dimensions representing the distributions varying between 1 and 3, instead of the ideal value of 2. Measurements in estuarine turbidity maxima and on intertidal mudflats show that the fractal dimension is less than 2 in situations where turbulent shearing causes disruption of the flocs. At the same time increasing suspended sediment concentration tends to increase the fractal dimension. Measurements of size using an in situ Malvern sizer show that the floc size distribution is also affected by both turbulent energy dissipation and by concentration. Complementary laboratory studies suggest that, at a constant concentration, flocculation is enhanced by low shear, but that disruption occurs at higher shear. These experiments confirm the relationship between fractal dimension, shear stress and concentration.     

珠江口磨刀门泥沙絮凝特征

本文利用激光粒度仪实测得到珠江口磨刀门河口2013年夏季悬浮泥沙现场絮凝及絮凝体特征,同时对比悬沙分散粒径和含沙量,研究表明:悬沙分散粒径平均值为27.9μm,现场实测絮团粒径平均值为91.6μm,表明磨刀门口外的悬浮泥沙絮凝现象显著;实测絮团平均粒径变化范围为13.0~273.8μm,小潮期间絮团粒径平均值为131.5μm,大于大潮平均值76.9μm;絮凝体粒径在垂向上的变化表现为由表及底先变大再变小。絮团体积浓度、沉速与粒径的关系在不同情况下有差异,体积浓度和絮团粒径在表层和中层有明显正相关关系,絮团沉速在大潮时刻随着粒径的增大而增大。综合分析影响絮凝的因素,得知在珠江口盐度对于絮团大小影响不明显;而流速大小的差异是影响大小潮之间絮团大小不同的主要因素。研究结果有助于了解珠江口细颗粒泥沙输移特性和相关生物化学过程。     

黄河口潮滩泥沙絮凝研究

本文基于现场观测的絮团粒径、悬沙浓度及水动力数据,研究了黄河口南部潮滩泥沙絮凝特征。研究发现,黄河口潮滩絮团粒径在25.42～264.44μm之间,平均为95.20μm。水体紊动对黄河口潮滩絮凝的影响存在差异,紊动对絮凝促进作用的上限约为G_l=3.76 s^-1。紊动强度低于G_l时,紊动促进泥沙絮凝,絮团粒径随紊动加强而增大;反之水体紊动对絮凝主要起抑制作用,絮团粒径随紊动强度增大而减小。悬沙浓度对黄河口潮滩泥沙絮凝起抑制作用,同等紊动条件下高悬沙浓度对应的絮团粒径更小。黄河口潮滩絮团有效密度与粒径呈现负相关关系,沉速主要受粒径影响。本研究补充了对弱潮河口潮滩泥沙絮凝特性的认识。     

Measurements of suspended particulate matter with laser in-situ scattering and transmissometry in the Jiaozhou Bay in China

1 IntroductionKnowledge of suspended particle size and distri-bution is the key elements for better understandingthe sediment transport processes ( Wang et al.,2004), primary production (Ning et al., 2004),water quality controlling and pollution predictio…     

Settling velocity and porosity of large suspended particle

The settling velocity of large suspended particles which were formed in turbid water due to re-coagulation was measured in the laboratory, and the relation to the particle diameter which was taken as that of a sphere with the same volume, was obtained. This is, however, different from Stokes' law in regard to the relation to the particle diameter, though the Reynolds numbers are almost in the region of Stokes' law. This may be caused by the change of apparent density with the particle diameter. Stokes' law for a porous particle was applied to the settling velocity and the diameter found was compared with the real diameter to obtain an expression for the porosity. From the result, it is considered that the large suspended particles of “marine snow” have a porosity of above 90 %.     

Particle size of suspended matter in estuaries

Suspended matter usually flocculates into fragile flocs that break up during sampling and analysis. Coulter counter and pipette size analysis are therefore an indication of floc strength. Grain-size distributions (after removal of organic matter) give an indication of the relative amount of grains transported as flocs (as opposed to being transported as single particles).In situ size distributions of suspended matter (flocs) in the Scheldt and Rhine estuaries and the Gironde, as well as results from the Zaire River estuary, indicated that salt flocculation does not appear to be of importance for the formation ofin situ flocs. Also there is no clear relation with particle concentration. In estuaries probably no equilibrium between floc size and concentration is reached because of the rapid variations in turbulence and bottom shear. In the Zaire estuary a decrease in turbulence in the surface water results in flocs of the same size as differential settling over a depth of more than 100 meters.     

Resolving physically and biologically driven suspended particulate matter dynamics in a tidal basin with a distribution-based model

Understanding suspended particulate matter (SPM) dynamics in coastal waters is crucial to assess changes in coastal sediment budgets and biogeochemical fluxes. SPM dynamics are subject to various physical and biological factors and processes such as, e.g. tidal currents and aggregation which can be enhanced by extracellular polymeric substances (EPS) that are produced by algae and bacteria. It is still unclear how the different factors and processes interact and together determine SPM dynamics. To unravel the interacting processes and factors, we propose a new distribution-based modeling approach. Based on the derivation of explicit equations for size distribution characteristics of SPM such as the average radius, we derived a model of reduced complexity characterized by low initialization and parameterization effort as well as low computational cost. The proposed 0D model includes the processes of aggregation and fragmentation due to shear, aggregation due to differential settling, deposition, resuspension and tidal exchange, and describes the evolution of the SPM concentration in the water column linked by the settling velocity to the change of the mass average radius of the aggregate distribution. A systematic parameter variation for critical bottom shear stress of erosion, the size of resuspended aggregates, the fractal dimension, the collision efficiency, and the aggregate strength has been performed and compared to observations in the back-barrier basin of Spiekeroog Island in the German Wadden Sea. This analysis confirms the hypothesis that in winter biological influences on SPM dynamics are smaller compared to summer. This is mainly reflected by a significant shift in the various parameters. We hence conclude that biological control mechanisms have a much more quantitative relevance for SPM dynamics than currently represented by state-of-the-art SPM transport models.     

椒江河口洪季最大浑浊带核部的悬沙分选机制

以实测悬沙粒径、流速、含沙量资料为基础,通过悬沙不同粒级组分、中值粒径以及粒度参数的统计,分析椒江河口最大浑浊带核部悬沙粒径分布变化特性。研究表明,悬沙粒径分选主要受物质来源、潮流动力作用下底部部分物质再悬浮和絮凝沉降3个因素的影响。其中第二个因素起主导作用,大潮期比小潮期显著,第三个因素的作用主要发生在涨落憩前后低流速期,并在表层较近底层的水体明显。     

Velocity profiles in a salt marsh canopy

Flow velocity profiles, measured in aSpartina anglica canopy in a laboratory flume, change with the location of measurement and plant stem density. The shear velocity above the canopy is larger than that within the canopy. The reduction ofu _* within the canopy will favor the deposition of cohesive sediment. The reducedu _* and flow turbulence within the canopy can enhance particle flocculation and settling velocity. The canopy exerts a strong influence on the concentration, settling velocity of the flocs, and deposition rate of the suspended sediment through effects on bed shear stress and turbulence of flow within the canopy.     

Investigating the effect of ballasting by CaCO3 in Emiliania huxleyi: I. Formation, settling velocities and physical properties of aggregates

To investigate the role of ballasting by biogenic minerals in the export of organic matter in the ocean, a laboratory experiment was conducted comparing aggregate formation and settling velocity of non-calcifying and calcifying strains of the coccolithophore Emiliania huxleyi. Experiments were conducted by making aggregates using a roller table and following aggregate properties during incubation for a period of 40 days. Size, shape, and settling velocities of aggregates were described by image analysis of video pictures recorded during the roller tank incubation. Our results show that biogenic calcite has a strong effect on the formation rate and abundance of aggregates and on aggregate properties such as size, excess density, porosity, and settling velocity. Aggregates of calcifying cells (AGG_CAL) formed faster, were smaller and had higher settling velocities, excess densities, and mass than those of non-calcifying cells (AGG_NCAL). AGG_CAL showed no loss during the duration of the experiment, whereas AGG_NCAL decreased in size after 1 month of incubation. Potential mechanisms that can explain the different patterns in aggregate formation are discussed. Comparison of settling velocities of AGG_CAL and AGG_NCAL with aggregates formed by diatoms furthermore indicated that the ballast effect of calcite is greater than that of opal. Together these results help to better understand why calcite is of major importance for organic matter fluxes to the deep ocean.     

伶仃洋河口泥沙絮凝特征及影响因素研究

泥沙絮凝对河口细颗粒泥沙运动过程起着极其重要的作用。本文通过LISST-100激光粒度仪等仪器实测伶仃洋河口2013年洪季悬浮泥沙絮凝体现场粒径及水动力、泥沙条件,结合实验室悬沙粒径分析,研究大小潮期间伶仃洋河口泥沙絮凝特征,探讨紊动剪切强度、含沙量、盐度分层及波浪等因素对伶仃洋河口泥沙絮凝的影响。结果表明:伶仃洋河口水体中现场粒径平均值为148.53 μm,大于实验室悬沙分散粒径36.74 μm,河口絮凝现象明显;沉速与有效密度、粒径呈正相关,絮团平均有效密度为153.49 kg/m3,平均沉速达1.13 mm/s;小潮时絮团平均粒径大于大潮,垂向上表底层絮团粒径小、中层大,中底层絮团沉速大于表层。伶仃洋河口水动力、泥沙条件是影响其泥沙絮凝的重要因素,低剪切强度（小于5 s-1）、低含沙量（小于50 mg/L）及高体积浓度有利于细颗粒泥沙之间的相互碰撞,促进絮凝作用;当剪切强度与颗粒间碰撞强度高于絮团所能承受的强度时,絮团易破碎分解成小絮团或更细的泥沙颗粒;伶仃洋河口盐度层化引起的泥沙捕获现象增大中层泥沙体积浓度,有利于中层絮凝体的发育;观测期相对较大的波浪增强水体紊动,增大了水体细颗粒泥沙的碰撞几率,表层絮团粒径随波高峰值的出现而增大。     

莱州湾口弱层结水体中沉积物再悬浮特征及其水平、沉降通量研究

近海悬浮物在海水中的运移受诸多因素影响,其中由于径流输入导致的水体层化是不可忽视的因素之一,研究层结水体中沉积物受潮流、波浪影响的再悬浮特征有重要意义.2005年5月15日在黄河口西侧18海里处的莱州湾口设立了一个周日连续观测站,试图揭示弱层结水体中悬浮物的再悬浮特征及其水平、沉降通量.利用ADCP回声强度反演得到了高...     

Suspended particles near the bottom in Osaka Bay

The vertical distributions of suspended particles in Osaka Bay were measured by using anin situ beam attenuation meter. The concentration of suspended particles near the bottom increases rapidly toward the bottom where size of sediment is in a range of silt. The settling velocity of suspended particles near the bottom was measured with the use of a settling tower in the laboratory. The settling velocity of the suspended particles with diameter from 10 to 100m is 2×10^–3cm s^–1 to 5×10^–2cm s^–1. The density of the particles ranges from 2.0 to 1.1 and decreases with increasing particle diameter.