Hindered settling velocity of cohesive/non-cohesive sediment mixtures

New methods are proposed for predicting the hindered settling conditions encountered by concentrated suspensions containing mixtures of sand particles and mud flocs. These methods, based on two-fraction formulations, are developed by consideration of the settling characteristics of monodisperse and polydisperse solid particle suspensions applied to cohesive/non-cohesive mixtures of mud flocs and sand particles. The behaviour of these predictive methods is evaluated over a wide range of mixture conditions and compared with existing formulations, with their parametric dependence on the relative volumetric concentrations and floc/particle sizes for the mud and sand constituents established. The results indicate that consideration of the full return flow effects generated by both fractions provides the best modelling framework for predicting the hindered settling conditions over a wide range of sand–mud mixtures.     

Experimental study on settling velocity of soil particles in dredged slurry

Studying sedimentation and consolidation of dredged slurry has significant implications to the design of storage yard and subsequent ground improvement. In this study, settling velocity of soil particles in dredged slurry during sedimentation and consolidation processes was investigated using an improved multilayer extraction sampling (MES) method. A series of sedimentation column tests were performed on dredged slurry with three different initial water contents. Distributions of volume of soil particles and density of dredged slurry were first obtained by the MES method, settling velocity of soil particles was then calculated by volume flux function approach. It was found that the density and velocity inflection points can be used to distinguish the settling zone and the consolidation zone. The experimental results reveal that the velocity of soil particles was quite low and monotonically decreased with sedimentation height at low initial water content throughout the whole test period, whereas it was increased at 0–1 hours and almost remained constant at 1–7 hours in the settling zone at high initial water content. The effects of initial water content on sedimentation and consolidation mode of dredged slurry and the settling velocity of soil particles were discussed. The relationship between settling velocity of soil particles and particle diameter was also studied. It is indicated that the measured velocity of soil particles was much lower than that calculated by the Stokes equation, and it was related to 0.4881–0.5906 order of particle diameter at 0–1 hours and 0.1117–0.1825 order of particle diameter at 1–7 hours for the test slurries.     

An improved “benchmark” method for estimating particle settling velocities from time-series sediment trap fluxes

A new method, based on fitting Fourier series to time-series (TS) data from sediment traps, was developed to estimate the settling velocities (SVs) of sinking particles in the open ocean. This new method was applied to data from MedFlux, as well as from the US JGOFS NABE, EqPac, and ASPS studies. Fluxes of mass and of four chemical tracers, as well as the molar ratios of the latter, were plotted on logarithmic scales; Fourier series were then fit to these data. In each case we determined the most likely settling velocity using a likelihood-based nonlinear fitting algorithm. Variation among estimates using single tracers was significantly less than variation using tracer ratios; we therefore concluded that estimates based on single tracers are to be preferred to estimates based on tracer ratios. Our results also showed no obvious differences among SVs estimated using different single tracers. The best estimate of settling velocity using single-tracer fluxes with good temporal resolution (i.e. for sites with cup rotation times 8.5 days) is 205 m/d, with standard deviation 74 m/d. For MedFlux data alone (which have a resolution of 4–6 days), the estimate is 220±65 m/d. This latter value is within 10% of the estimate of average settling velocity (242±31 m/d) made using MedFlux IRS traps in “settling velocity” mode.     

The variability of settling velocities of suspended fine-grained sediment in the Ems estuary

Settling velocities of suspended fine-grained sediment in estuaries vary over a range of several orders in magnitude. Variations in the suspended sediment concentration are often put forward as the principal cause. However, comparison of settling velocities from a number of estuaries shows that even in the case of the same suspended sediment concentration, large variations in the settling velocities can occur of up to two orders in magnitude. From measurements in the Ems estuary we found that even within a single estuary such large variations can occur. Field measurements and complementary laboratory experiments demonstrate that `other factors' can affect the settling velocity in the same order as the assumed effects of the suspended sediment concentration. To address these `other factors', which include physical–chemical and biological effects, the concept of `flocculation ability' is introduced, as a measure of the effectiveness of the collisions between suspended particles for floc growth. On the basis of the results from the Ems estuary, it is hypothesised that variations in the flocculation ability of the suspended fine-grained sediments are at the root of the large differences in settling properties of suspended fine-grained sediment in estuaries.     

长江口悬沙沉速室内试验研究

在充分认识传统沉降筒缺陷的基础上提出了"大型可温控自动搅拌沉降试验筒"。通过室内系列试验发现:(1)含沙量对长江口细颗粒沉降速度影响最大;(2)温度上升,沉速增加,但不同阶段影响程度有所不同;(3)含沙量越高,盐度对沉速的影响越小,含沙量相同情况下,长江口北槽悬沙枯季水温下盐度对沉速的影响在1.8~5.7倍左右;洪季水温下盐度对沉速的影响在1.5~2.2倍左右。(4)枯季最佳絮凝盐度在7左右,最佳絮凝含沙量为7 kg/m3;洪季最佳絮凝盐度在10~12左右,最佳絮凝含沙量为4.5kg/m3。本研究成果可望加深我们对细颗粒泥沙动力过程相关机理的认识,同时可为相关港口、航道的淤积机理分析,数学模型、物理模型研究工作提供一定技术参考。     

The effect of suspended sediment concentration on the settling velocity of cohesive sediment in quiescent water

A laboratory study was carried to qualitatively investigate the effect of suspended sediment concentration C on the settling velocity w_s of cohesive sediment in quiescent water. A bay mud sample was mixed with water in a cylindrical container, and three optical back scatterance sensors were then used to measure suspended sediment concentrations of the mud–water mixture at three levels every 15 s for 5 h while sediments were settling in the quiescent water. Based on the measured sediment concentrations, the settling velocities at different concentrations were derived from the depth-integrated mass balance equation. This study has found that the settling velocity w_s is independent of C in the free settling regime of C<0.3 g/l, and then increases nonlinearly with C in the enhanced settling regime of 0.3<C<4.3 g/l, and finally decreases sharply with C in the hindered settling regime of C>4.3 g/l. The maximum settling velocity occurs at C≈4.3 g/l and is about nine times faster than the settling velocity in the free settling regime. A single empirical formula is also proposed to calculate the settling velocities at different sediment concentrations.     

The influence of the pycnocline on the oceanic settling of manganese nodule mining waste

The likelihood that manganese nodule mining discharge (essentially deep-seabed clays and some nodule fragments) would reside on the pycnocline for a long period of time was investigated by introducing mining waste particles into a two-layer laboratory settling column illuminated by laser. The experiments were repeated with polystyrene particles of uniform shape and size to further study the effect of a density interface on settling. The results indicate that mining particulates do not have sufficiently low density to accumulate on the pycnocline although a density interface can temporarily concentrate settling particles. A review of related studies of accumulation of inorganic particles on density interfaces suggests that the available evidence for pycnocline accumulation of inorganic particles is slight. A measurement of the wet density spectra of any oceanic discharge is necessary to accurately assess the possibility of particles rafting on a density surface.     

Lower Cd/P ratio in settling particles than in surface seawater in the Okinawa Trough

The characteristics of the ratios between cadmium (Cd) and phosphorus (P) in settling particles collected from the Okinawa Trough in the East China Sea were examined using a sediment trap, moored at a depth of 811 m for one year. The Cd/P ratios varied within a narrow range throughout the year, in spite of the large seasonal change in the total dry mass, Cd, and P fluxes. The average Cd/P ratio of settling particles was 0.062 (nmol/μmol), which was obviously lower than that of surface seawater around the study site (0.16). This lower ratio in the Okinawa Trough particles collected using the 811 m-moored trap certainly reflected the mixture of biologically produced organic matter around the study site and other components that were mainly transported as lateral flux from the shelf edge and slope area of the East China Sea.     

大型桥梁钢沉井下沉过程局部冲刷研究

钢沉井下沉过程中局部冲刷的研究是同类桥墩设计和施工十分关心的重要课题,它对保证钢沉井安全有效施工、桥梁设计中施工期桥墩最大冲深、失稳和安全计算有重要参考价值。通过室内试验研究了钢沉井下沉过程中的局部冲刷机理和冲刷形态,探讨了桥墩下部钢沉井基础施工的相对高程对局部冲刷的变化规律,并将试验研究所获得的局部冲刷规律和影响因素,采用墩型系数方法引入局部冲刷计算中,给出了计算公式。研究成果补充了国家行业规范内容,对同类工程的设计、施工具有借鉴和指导意义。     

海州湾水流紊动强度和含沙量对沉降速度的影响研究

选取海州湾近岸潮流和含沙量实测资料,分析水体紊动强度与含沙量对近岸絮凝体沉降速度的影响,提出了新的沉降速度确定方法。研究表明：淤海州湾近岸泥沙沉降速率大部分在 0.05 ~ 2.50 mm/s 之间,潮周期内泥沙絮凝体的沉降速度 有明显变化。于含沙量较小时,泥沙絮凝体的沉降速度基本随含沙量的增加而增加;含沙量较大时,含沙量与沉降速度呈现出负相关,无论是大潮还是中潮,当含沙量达到 0.7 kg/m3左右时,絮团沉降速度最大,而随着含沙量的增大,絮团沉降速度开始减小。盂在涨落潮垂线平均流速最大时刻,紊动强度达到峰值,含沙量较低时,随着紊动强度增加,沉降速度也随之增加,大潮期间紊动强度对泥沙沉降速度的影响高于中潮。榆新的泥沙沉降速度计算公式不仅考虑了含沙量,还计入了紊动强度 G,大大提高了沉降速度计算值与实测值的相关性。     

The POC / Th ratio of settling particles isolated using split flow-thin cell fractionation (SPLITT)

The common assumption that the ratio between particulate organic carbon (POC) and particulate ²³⁴Th obtained from shallow sediment traps and filterable particles are representative of the ratio in the total particle settling flux should be treated with caution in view of well-known biases associated with tethered shallow sediment traps and the decoupling between size and settling velocity of many natural particle regimes. To make progress toward reliably constraining the POC / ²³⁴Th ratio on truly settling particles, we have tested here a settling collection technique designed to remove any hydrodynamic bias; split flow-thin cell fractionation (SPLITT). These first results from a North Sea fjord and an open Baltic Sea time-series station indicates that the POC / ²³⁴Th ratio on the more complete particle-settling spectrum, isolated with SPLITT, was higher than the POC / ²³⁴Th ratio obtained simultaneously from tethered shallow sediment traps in seven out of seven parallel deployments with an average factor of 210%. The POC / ²³⁴Th ratio from the SPLITT was either in the same range or higher than that obtained on filtered “bulk” particles. To explain this novel data we hypothesize that the slowest settling fraction is organic-matter rich and does not strongly complex ²³⁴Th (i.e., high POC / ²³⁴Th). We suggest that this ultra-slow sinking fraction is better collected by SPLITT than with tethered sediment traps because of minimized hydrodynamic bias.This was tested using the ratio of POC / Al as a tracer of detrital mineral-ballast influenced settling velocity. The higher POC / Al ratios in SPLITT samples relative to in traps is consistent with the hypothesis that SPLITT is better suited for collecting also the slow-settling component of sinking particles. This important slow-settling component appears to here consist primarily of non-APS/TEP components of plankton exudates or other less-strongly ²³⁴Th-complexing organic matter. Further applications of the SPLITT technique are likely to return increasingly new insights on the composition (including “truly settling” POC / ²³⁴Th) of the total spectrum of particles settling out of the upper ocean.     

Observations and analyses of floc size and floc settling velocity in coastal salt marsh of Luoyuan Bay, Fujian Province, China

In coastal environments, fine-grain sediments often aggregate into large and porous flocs. ElectroMagnetic Current Meters (EMCM) and Laser In Situ Scattering and Transmissometry (LISST-ST) have been deployed within a Spartina alterniflora marsh of the Luoyuan Bay in Fujian Province, China, to measure the current velocity, the floc size and the settling velocity between 15 and 22 January 2008. During the observations, the near-bed water was collected in order to obtain the suspended sediment concentration (SSC) and constituent grain size. Data show that: (1) the nearbed current velocities vary from 0.1 to 5.6 cm/s in the central Spartina alterniflora marsh and 0.1–12.5 cm/s at the edge; (2) the SSCs vary from 47 to 188 mg/dm 3 . The mean grain size of constituent grains varies from 7.0 to 9.6 μm, and the mean floc sizes (MFS) vary from 30.4 to 69.4 μm. The relationship between the mean floc size and settling velocity can be described as: w s =ad b , in which w s is the floc settling velocity (mm/s), a and b are coefficients. The floc settling velocity varies from 0.17 to 0.32 mm/s, with a mean value of 0.26 mm/s, and the floc settling velocity during the flood tide is higher than that during the ebb tide. The current velocity and the SSC are the main factors controlling the flocculation processes and the floc settling velocity.     

A laboratory assessment of the relative importance of turbulence, particle composition, and concentration in limiting maximal floc size and settling behaviour

The fate of fine particulate material in aquatic environments is closely linked to aggregation and disaggregation processes. Understanding the mechanisms controlling these processes is fundamental to the development of predictive models of fate and effects for particulate discharges in the coastal zone from such sources as offshore hydrocarbon exploration and development. One of the variables required for the development of these models is maximal floc size. Using a non-invasive imaging technique, the significance of turbulence, composition, and concentration on maximal floc size in an inverting column flocculator was determined for materials commonly discharged during offshore hydrocarbon development. The settling velocity of the suspension was determined from volume concentrations of samples obtained by pipette during still water settling in a manner similar to that of Owen tubes. After 20 h, both maximal floc size and settling velocity showed a highly significant dependence on turbulence and type of material in suspension, but showed no effect from concentration.     

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.     

Anthropogenic Pb in settling particulate matter in the Northwestern Pacific examined using stable isotopes of Pb

We have investigated Pb concentrations and isotopic compositions in settling particles collected by sediment traps experiments over a period of two years, from May 2005 to April 2007, at two depths, 770 and 5100 m, at station KNOT in the Northwestern Pacific Ocean (44°N, 155°E). To the identify provenances of Pb, the samples were separated into two fractions by chemical leaching techniques, with the leachate expected to contain Pb of anthropogenic origin. Isotopic compositions of Pb and concentrations of Pb, Sc, Mn, La, Yb, and Th were measured by quadrupole ICP-MS. The isotope ratios of leachable Pb in settling particles were ²⁰⁷Pb/²⁰⁶Pb = 0.860 ± 0.001; ²⁰⁸Pb/²⁰⁶Pb = 2.116 ± 0.002 (mean ± 95% confidence intervals), which are similar to those of aerosols in China that are greatly affected by pollution from coal combustion. We estimated the mean contribution from anthropogenic Pb sources to the Pb in settling particles, using the conventional binary (anthropogenic and natural Pb) mixing equation for Pb isotopes, as 90% in the upper trap and 78% in the lower trap. Furthermore, we found a significant negative correlation between the isotope ratios of Pb and concentrations of leachable Mn, normalized to those of leachable Pb, suggesting that manganese oxides play an important role in transporting Pb from the upper layers of the ocean to the deeper layers. Our results support the speculation published in a previous study that Pb might be scavenged by Mn oxides in the Northwestern Pacific Ocean.     

Particulate organic carbon–Th relationships in particles separated by settling velocity in the northwest Mediterranean Sea

In order to better understand the relationship between the natural radionuclide ²³⁴Th and particulate organic carbon (POC), marine particles were collected in the northwestern Mediterranean Sea (spring/summer, 2003 and 2005) by sediment traps that separated them according to their in situ settling velocities. Particles also were collected in time-series sediment traps. Particles settling at rates of >100 m d⁻¹ carried 50% and 60% of the POC and ²³⁴Th fluxes, respectively, in both sampling years. The POC flux decreased with depth for all particle settling velocity intervals, with the greatest decrease (factor of 2.3) in the slowly settling intervals (0.68–49 m d⁻¹) over trap depths of 524–1918 m, likely due to dissolution and decomposition of material. In contrast the flux of ²³⁴Th associated with the slowly settling particles remained constant with depth, while ²³⁴Th fluxes on the rapidly settling particles increased. Taking into account decay of ²³⁴Th on the settling particles, the patterns of ²³⁴Th flux with depth suggest that either both slow and fast settling particles scavenge additional ²³⁴Th during their descent or there is significant exchange between the particle classes. The observed changes in POC and ²³⁴Th flux produce a general decrease in POC/²³⁴Th of the settling particles with depth. There is no consistent trend in POC/²³⁴Th with settling velocity, such as might be expected from surface area and volume considerations. Good correlations are observed between ²³⁴Th and POC, lithogenic material and CaCO₃ for all settling velocity intervals. Pseudo-K_ds calculated for ²³⁴Th in the shallow traps (2005) are ranked as lithogenic material opal <calcium carbonate <organic carbon. Organic carbon contributes 33% to the bulk K_d, and for lithogenic material, opal and CaCO₃, the fraction is 22% each. Decreases in POC/²³⁴Th with depth are accompanied by increases in the ratio of ²³⁴Th to lithogenic material and opal. No change in the relationship between ²³⁴Th and CaCO₃ was evident with depth. These patterns are consistent with loss of POC through decomposition, opal through dissolution and additional scavenging of ²³⁴Th onto lithogenic material as the particles sink.     

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.     

Organic biomarkers in the twilight zone—Time series and settling velocity sediment traps during MedFlux

The transfer of material through the twilight zone of the ocean is controlled by sinking particles that contain organic matter (OM) and mineral ballast. During the MedFlux field program in the northwestern Mediterranean Sea in 2003, sinking particulate matter was collected in time series (TS) and settling velocity (SV) traps and analyzed for amino acids, lipids, and pigments (along with ballast minerals) [Lee, C., Armstrong, R.A., Wakeham, S.G., Peterson, M.L., Miquel, J.C., Cochran, J.K., Fowler, S.W., Hirschberg, D., Beck, A. Xue, J., 2009b. Particulate matter fluxes in time series and settling velocity sediment traps in the northwestern Mediterranean Sea. Deep-Sea Research II, this volume [doi:10.1016/j.dsr2.2008.12.003]]. The goal was to identify how organic chemical compositions of sinking particles varied as a function of their in-situ settling velocity. The TS record was used to define the biogeochemical character and temporal pattern in flux during the period of SV trap deployment. Temporal variations in organic and mineral compositions are consistent with particle biogeochemistry being driven by the seasonal succession of phytoplankton. Spring diatom bloom conditions led to a high flux of rapidly sinking aggregates and zooplankton fecal matter; summer oligotrophy followed and was characterized by a higher proportion of slowly sinking phytoplankton cells. Bacterial degradation is particularly important during the low-flux summer period. Settling velocity traps show that a large proportion of particulate organic matter sinks at 200–500 m d⁻¹. Organic compositions of this fast-sinking material mirrors that of fecal pellets and aggregated material that sinks as the spring bloom terminates. More-slowly sinking OM bears a stronger signature of bacterial degradation than do the faster-sinking particles. The observation that compositions of SV-sorted fractions are different implies that the particle field is compositionally heterogeneous over a range of settling velocities. Thus physical and biological exchange between fast-sinking and slow-sinking particles as they pass down the water column must be incomplete.