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 particle size of the dispersed mineral fraction on the settlement of marine and estuarine muds

Considering the sedimentation behaviour of suspensions, four states of different sediment-water mixtures are usually distinguished, these being (in increasing concentration) dilute suspensions, concentrated suspensions, fluid muds and mud deposits. As the concentration values delimiting these states vary considerably from one cohesive sediment to another, a procedure for their quantitative determination is proposed here. This paper deals mainly with the effects of hindered settling in interacting concentrated suspensions and of sedimentation in fluid muds. In contrast to the settling velocity of individual particles in a dilute suspension, which must be studied statistically as a stochastic variable denoted W, hindered settling and sedimentation velocities can be described by a scalar denoted V, as solids at any particular level of concentrated suspensions and fluid muds settle at the same velocity. On the basis of settlement tests carried out in this study and published data on organic-rich cohesive sediments, a concentration-dependent empirical law for a permeability coefficient (k) has been generated for cohesive sediments, using data from ten estuarine and nine marine environments, based on the one-dimensional Kynch theory of sedimentation. Based on the median diameters of the dispersed mineral fraction, the main provenances of the sediments are: fine to very fine clay from tropical marine/estuarine environments, medium clay to very fine silt from estuaries in western France, and fine to coarse silt from marine (harbour) environments in the Normandy region of France. A general trend for the influence of the grain size of the mineral fraction on the permeability coefficient has been established. It is demonstrated that the concentrations delimiting the different states of sediment-water mixtures can also be related to the grain size of the mineral fraction. Thus, hindered settling and sedimentation processes of muds, similar to the marine and estuarine cohesive sediments considered in this paper, can be studied as generic problems parameterized through a defined median diameter of the dispersed mineral fraction. Results for Loire estuary sediments are presented separately, based on specific tests to analyse the influence of experimental conditions on settlement. Moreover, the concentration values delimiting the different sediment-water mixture states have been largely established for this estuary.     

A simple method for calibrating optical backscatter sensors in high concentrations of non-cohesive sediments

A new method is introduced for calibrating optical backscatter sensors for suspended quartz sand concentrations of up to 200 kg m⁻³. Due to the high settling velocity of quartz sand in water, considerable difficulties have arisen in the past to maintain a spatially and temporally homogeneous suspension suitable for calibration. Traditional methods are clumsy and prone to errors. Here, the sediment is calibrated in glycerol, a clear fluid with a higher viscosity than water. The settling velocity is reduced by three orders of magnitude. An empirical relationship is obtained which is used to correct for any optical differences in response of the sensors in the two fluids. Any extra errors introduced by calibrating with a different fluid from that found in the field are outweighed by the simplicity and reliability of this method.     

Modelling of sand transport under wave-generated sheet flows with a RANS diffusion model

A 1DV-RANS diffusion model is used to study sand transport processes in oscillatory flat-bed/sheet flow conditions. The central aim is the verification of the model with laboratory data and to identify processes controlling the magnitude and direction (‘onshore’/‘offshore’) of the net time-averaged sand transport. The model is verified with a large series of measured net sand transport rates, as collected in different wave tunnels for a range of wave-current conditions and grain sizes. Although not all sheet flow details are represented in the 1DV-model, it is shown that the model is able to give a correct representation of the observed trends in the data with respect to the influence of the velocity, wave period and grain diameter. Also detailed mean sediment flux profiles in the sheet flow layer are well reproduced by the model, including the direction change from ‘onshore’ to ‘offshore’ due to a difference in grain size from 0.34 mm (medium sand) to 0.13 mm (fine sand). A model sensitivity study with a selected series of net transport data shows that the stirring height of the suspended sediment ε_s/w_s strongly controls the magnitude and direction of the net sediment transport. Inclusion of both hindered settling and density stratification appears to be necessary to correctly represent the sand fluxes for waves alone and for waves + a superimposed current. The best agreement with a large dataset of net transport measurements is obtained with the 1DV-RANS model in its original settings using a Prandtl–Schmidt number σ_ρ = 0.5.     

Fluxes,source and transport of organic matter in the western Sea of Okhotsk: Stable carbon isotopic ratios of n-alkanes and total organic carbon

Settling particles and surface sediments collected from the western region of the Sea of Okhotsk were analyzed for total organic carbon (TOC), long-chain n-alkanes and their stable carbon isotope ratio (δ¹³C) to investigate sources and transport of total and terrestrial organic matter in the western region of the sea. The δ¹³C measurements of TOC in time-series sediment traps indicate lateral transport of resuspended organic matter from the northwestern continental shelf to the area off Sakhalin via the dense shelf water (DSW) flow at intermediate depth. The n-alkanes in the surface sediments showed strong odd carbon number predominance with relatively lighter δ¹³C values (from −33‰ to −30‰). They fall within the typical values of C3-angiosperms, which is the main vegetation in east Russia, including the Amur River basin. On the other hand, the molecular distributions and δ¹³C values of n-alkanes in the settling particles clearly showed two different sources: terrestrial plant and petroleum in the Sea of Okhotsk. We reconstructed seasonal change in the fluxes of terrestrial n-alkanes in settling particles using the mixing model proposed by Lichtfouse and Eglinton [1995. ¹³C and ¹⁴C evidence of a soil by fossil fuel and reconstruction of the composition of the pollutant. Organic Geochemistry 23, 969–973]. Results of the terrestrial n-alkane fluxes indicate that there are two transport pathways of terrestrial plant n-alkanes to sediments off Sakhalin, the Sea of Okhotsk. One is lateral transport of resuspended particles with lithogenic material from the northwestern continental shelf by the DSW flow. Another is the vertical transport of terrestrial plant n-alkanes, which is independent of transport of lithogenic material. The latter may include dry/wet deposition of aerosol particles derived from terrestrial higher plants possibly associated with forest fires in Siberia.     

Fate and effects of a heavy fuel oil spill on a Georgia salt marsh

Addition of a heavy oil to a Spartina salt marsh in the autumn resulted in high concentrations of polycyclic aromatic hydrocarbons in sediment and benthic animals. The highest concentrations of phenanthrene, chrysene and fluoranthene in the sediment were 112, 105 and 75 ng/g sediment, respectively. These concentrations rapidly decreased during the 20 week period following the spill. The times for these hydrocarbons to decrease to 50% of their highest values, i.e. half-life, were approximately 100, 70 and 30 days in sediment, mussels and oysters, respectively. Benthic macrofauna species showed three responses to oil addition which included no change, an increase, or a decrease in the population. No changes were noted in populations of fiddler crabs (Uca pugnax), oysters (Crassostrea virginica), and mussels (Modiolus demissus). Mud snails (Nassarius obsoleta) increased in density after the spill due to immigration of adult snails from untreated areas to scavenge on animals killed by the oil. Many of the adult periwinkles (Littorina irrorata) were killed by the oil. In the spring, juvenile periwinkles recolonised to oiled areas as a result of larvae settling.     

长江口细颗粒泥沙絮凝沉降影响因素分析

根据静水和动水条件下的细颗粒泥沙沉降试验结果,应用灰色模型分析中的关联度分析理论,分析了影响细颗粒泥沙絮凝沉降的主要因素.根据关联度的大小指出,影响细颗粒泥沙絮凝沉降的主要因素依次为水温、沉降历时、盐度、粒度、含沙量和流速,其中盐度和粒度是阈值型影响因素,沉降历时、含沙量和流速是连续型影响因素,水温是具有阈值型和连续型双重特性的影响因素.只要阈值型影响因素达到或超过了阈值,细颗粒泥沙就发生絮凝作用,因素值的变化对沉降强度影响不大.连续型影响因素对细颗粒泥沙絮凝沉降的影响是连续的,它们不仅影响絮凝作用发生,而且影响絮凝沉降强度.     

Velocity and Sediment Concentration Profiles in Sediment-Laden Flows

A theoretical analysis of velocity profiles in sediment-laden flows is presented by means of Prandtl-Karman mixing length theorem. The study shows that the upward velocity of liquid-phase caused by settling sediment leads to the invalidity of the log-law and Rouse equation. The theoretical analysis takes into account the upward velocity and shows: 1) the mean velocity in sediment-laden flows follows the log-law, but the Karman constant reduces in the main flow region, 2) sediment concentration reduces the mixing length of fluid particles, 3) flow resistance reduces with the presence of sediment concentration, and 4) the sediment concentration profile deviates from the well know Rouse equation. The experimental data agree well with the equations derived on the basis of non-zero wall velocity. It is found that the wall-normal velocity should not be neglected for density gradient flows because it induces more than for pure water flows.     

The pattern of cross-slope depositional fluxes

A simple model with horizontal and vertical diffusivities and settling velocity is used to calculate expected distributions of suspended particulate matter in a section across the continental shelf and slope. Dependencies on the shelf and slope profile, diffusivities, settling velocity, cross-slope advection and boundary sources/sinks are explored. It is found that the strongest factors are relative values of diffusivities and settling velocity, and the distribution of sources and sinks – including bottom deposition or resuspension. The latter is the principal means whereby an increased concentration near the bottom is likely, and is suggested as the usual reason for increased deposition recorded by sediment traps nearer the bottom. Observed thin, near-horizontal intermediate nepheloid layers put bounds on the vertical diffusivity and settling velocity, e.g. O(10^-4 m² s^-1, 10^-5 m s^-1) over Goban Spur in OMEX.     

Sediment distribution and transport across the continental shelf and slope under idealized wind forcing

Resuspension, transport, and deposition of sediments over the continental shelf and slope are complex processes and there is still a need to understand the underlying spatial and temporal dynamical scales. As a step towards this goal, a two-dimensional slice model (zero gradients in the alongshore direction) based on the primitive flow equations and a range of sediment classes has been developed. The circulation is forced from rest by upwelling or downwelling winds, which are spatially uniform. Results are presented for a range of wind speeds and sediment settling speeds. Upwelling flows carry fine sediments (low settling speeds) far offshore within the surface Ekman layer, and significant deposition eventually occurs beyond the shelf break. However, coarser sediments quickly settle out of the deeper onshore component of the circulation, which can lead to accumulation of bottom sediments within the coastal zone. Downwelling flows are more effective at transporting coarse sediments off the shelf. However, strong vertical mixing at the shelf break ensures that some material is also carried into the surface Ekman layer and returned onshore. The concentrations and settling fluxes of coarse sediments decrease offshore and increase with depth under both upwelling and downwelling conditions, consistent with trends observed in sediment trap data. However, finer sediments decrease with depth (upwelling) or reach a maximum around the depth of the shelf break (downwelling). It is shown that under uniform wind conditions, suspended sediment concentrations and settling fluxes decay offshore over a length scale of order τ_s/ρf|w_s|, where τ_s is the wind stress, ρ the water density, f the Coriolis parameter, and w_s is the sediment settling velocity. This scaling applies to both upwelling and downwelling conditions, provided offshore transport is dominated by wind-driven advection, rather than horizontal diffusion.     

Variability of wave-induced ripple migration in wave-flume experiments and its implications for sediment transport

A thorough discussion of results from laboratory experiments with regular waves sheds light on the gap that lies between the sediment transport associated with ripple migration and the performance of a standard bedload transport formula in terms of bed shear concept. It is found that the extent of deviations of the bedload transport formula by Ribberink (1998) from the measured rate of sediment transport associated with ripple migration becomes systematically apparent under conditions of increasing settling time factor Ω_s (= η/(w₀T); η is the ripple height, w₀ the settling velocity and T the wave period). Re-examination of previous two field studies demonstrates a further reinforcement for phase-lag argument addressed in this paper.     

Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen Gulf region,Canadian Arctic

To establish the relative importance of terrigenous and marine organic matter in the southern Beaufort Sea, we measured the concentrations and the stable isotopic compositions of organic carbon and total nitrogen in sediments and in settling particles intercepted by sediment traps. The organic carbon content of surface sediment in the Chukchi and southern Beaufort Seas ranged from 0.6 to 1.6% dry wt., without a clear geographical pattern. The C_ORG:N_TOT ratio ranged from 7.0 to 10.4 and did not vary significantly downcore at any one station. Values of δ¹³C_ORG and δ¹⁵N_TOT in the sediment samples were strongly correlated, with the highest values, indicative of a more marine contribution, in the Amundsen Gulf. In contrast, the organic matter content, elemental (C_ORG:N_TOT ratio) and isotopic (δ¹³C_ORG and δ¹⁵N_TOT) composition of the settling particles was different from and much more variable than in the bottom sediments. The isotopic signature of organic matter in the Beaufort Sea is well constrained by three distinct end-members: a labile marine component produced in situ by planktonic organisms, a refractory marine component, the end product of respiration and diagenesis, and a refractory terrigenous component. A three-component mixing model explains the scatter observed in the stable isotope signatures of the sediment trap samples and accommodates an apparent two-component mixing model of the organic matter in sediments. The suspended matter in the water column contains organic matter varying from essentially labile and marine to mostly refractory and terrigenous. As it settles through the water column, the labile marine organic matter is degraded, and its original stable isotope signature changes towards the signature of the marine refractory component. This process continues in the bottom sediment with the result that the sedimentary organic matter becomes dominated by the refractory terrigenous and marine components.     

利用坐底观测估算济州岛西南海域再悬浮临界应力

底边界层中沉积物的再悬浮和沉降是控制陆架海悬浮沉积物的输运的关键过程。沉积物输运过程的数值^*模拟也依赖于沉积物侵蚀和沉降的关键参数的研究。本文根据济州岛西南泥质区的坐底观测估算了此处临界应力。通过底边界层声学仪器ADV和PC-ADP的流速和悬浮物浓度同步观测,基于湍生成与耗散平衡假设,使用惯性耗散法计算沉降速度。这种方法得到的沉降速度ws平均值为0.91 mm s^-1,标准差为0.20 mm s^-1,此结果远大于Soulbsy（1997）和LISST-ST现场观测粒径分析仪等经验方法的结果。这主要是由于两种方法的本质不同,惯性耗散法形象的刻画了底边界层的水动力,并且更加合理的现场估计沉降速度w_s,然而Soulsby的方法通常适用于静水环境。我们提出了一种估计临界应力的新方法,根据悬浮颗粒物浓度时空变化的统计分析（深度平均的悬浮颗粒物浓度对时间求导数）和对应的底应力估算侵蚀临界应力τ_ce和沉降临界应力τ_cd。侵蚀临界应力τ_ce和沉降临界应力τ_ce的变化范围为0.11-0.25 Pa,对应的中值分别为0.20 Pa和0.16 Pa,这也证实了侵蚀临界应力略大于沉降临界应力。除此之外,我们还使用了另一种方法估算临界应力,通过沉降速度间接估算的临界应力范围为0.06-0.17 Pa。     

Analysis of influencing factors on fine sediment flocculation in the Changjiang Estuary

Based on the test data in dynamic water and static water, the main factors, which influence the fine sediment flocculation, are analyzed with a gray model method of correlation theory. It is shown that the main influencing factors are water temperature, settling time, salinity, grain size, sediment concentration and current velocity according to the correlation coefficients. Among them, the salinity and the sediment grain size are critical type influencing factors (CrTIF); the settling time, the sediment concentration and the velocity are continuous type influencing factors (CoTIF); and the water temperature has the characteristics of both. When the critical values of CrTIF are reached or exceeded, the fine sediments will be flocculated, but values of CrTIF will not influence the settlement strength of floes. The influence of CoTIF is continuous. The values of the CoTIF will not only influence the occurrence of flocculation but also the settlement strength of the floes.     

A test of the ADV-based Reynolds flux method for in situ estimation of sediment settling velocity in a muddy estuary

Under conditions common in muddy coastal and estuarine environments, acoustic Doppler velocimeters (ADVs) can serve to estimate sediment settling velocity (w _s) by assuming a balance between upward turbulent Reynolds flux and downward gravitational settling. Advantages of this method include simple instrument deployment, lack of flow disturbance, and relative insensitivity to biofouling and water column stratification. Although this method is being used with increasing frequency in coastal and estuarine environments, to date it has received little direct ground truthing. This study compared in situ estimates of w _s inferred by a 5-MHz ADV to independent in situ observations from a high-definition video settling column over the course of a flood tide in the bottom boundary layer of the York River estuary, Virginia, USA. The ADV-based measurements were found to agree with those of the settling column when the current speed at about 40 cm above the bed was greater than about 20 cm/s. This corresponded to periods when the estimated magnitude of the settling term in the suspended sediment continuity equation was four or more times larger than the time rate of change of concentration. For ADV observations restricted to these conditions, ADV-based estimates of w _s (mean 0.48±0.04 mm/s) were highly consistent with those observed by the settling column (mean 0.45±0.02 mm/s). However, the ADV-based method for estimating w _s was sensitive to the prescribed concentration of the non-settling washload, C _wash. In an objective operational definition, C _wash can be set equal to the lowest suspended solids concentration observed around slack water.     

A sheetflow sediment transport model for skewed-asymmetric waves combined with strong opposite currents

Prototype scale physical model tests were conducted to investigate the sheetflow sediment transport of uniform sand under different skewed-asymmetric oscillatory flows with and without the presence of relatively strong currents in the opposite direction against wave propagation. Experiments show that in most cases with fine sands, the “cancelling effect” which balances the on-/off-shore net transport under pure asymmetric/skewed oscillatory flows and results a moderate net transport was developed for combined skewed-asymmetric shaped oscillations. However, under certain conditions (T > 5 s) with coarse sands, the onshore sediment transport was enhanced for combined skewed-asymmetric flows. Additionally, the new experimental data under collinear oscillatory flows and strong currents show that offshore net transport rates increase with decreasing velocity skewness and acceleration skewness. Sediment movement behaviors were investigated through analysis of experimental data obtained from the image analysis technique and attempts were made to estimate and formulate the sheetflow layer thickness. Accordingly, sediment transport under oscillatory sheetflow conditions was studied and successfully explained by comparing the bed shear stress and the phase lag parameter at each half cycle. Consequently, these parameters were incorporated in an improved Dibajinia and Watanabe's type sediment transport model. The formula is calibrated against a comprehensive experimental data (331 in total). Good agreement obtained between predictions and measurements shows that the new formula is fulfilled for practical purposes.     

Studies on the Application of Steady State Assumption in 1DV Model for the Estimation of Settling Velocity of Suspended Sediments in a Shallow Estuary

Settling velocity is one of the important parameters in sediment transport modeling of estuaries. The methods adopted for its determination vary from theoretical equations to experimental methods. The theoretical equation generally adopted in the 1DV model include assumptions in order to simplify the solution. It is generally assumed that either the condition is steady or the vertical diffusion is negligible. This study evaluated the relative importance of the two assumptions made for the estimation of settling velocity. Two approaches were adopted: unsteady and negligible vertical diffusion (NS-NVD) and steady with vertical diffusion (S-VD) to estimate the settling velocity. The Muthupet Estuary in the Coromandal coast of India was selected for the study. The S-VD approach estimated settling velocity fairly well at the two locations with appreciable vertical diffusion. The NS-NVD approach was observed to be superior for estimating settling velocity at shallow reaches of the estuary having low flow velocity. The calculated settling velocity was further applied in 1DV model to predict the suspended sediment concentration. The S-VD approach predicted suspended sediment concentration at those locations with appreciable vertical diffusion with an R² value of 0.82 against 0.67 for the NS-NVD approach. At the other shallow reach of the estuary with low flow velocity, the NS-NVD approach gave an R² value of 0.822 against 0.71 for the S-VD approach. The vertical diffusion was observed to play a secondary role at those locations which are shallow with a water depth of 0.6 m and with a low flow velocity of the order of 0.01 m/s. The study demonstrated that localized hydrodynamic conditions influence the method adopted for the estimation of settling velocity.     

Sedimentological parameters and erosion behaviour of submarine coastal sediments in the south-western Baltic Sea

The aim of this study was to evaluate the erodibility of submarine coastal sediments for the purpose of modelling sediment dynamics in Mecklenburg Bay, south-western Baltic Sea. Erosion thresholds derived from experiments with a device microcosm on cores of fine sand (n=5, mean grain size=132 µm) and mud (n=5, medium silt size, mean=21 µm), collected at different times of the year, were compared to theoretical critical shear stress velocities based on grain-size measurements. For this purpose, a sedimentological map of natural surface sediments was constructed for the study area. Calculated values for critical shear stress velocities (u* _{cr-Hjulström} ) are 1.2 cm s^?1 for fine sand, and 3.75 cm s^?1 for cohesive mud. At the mud station, erosion experiments showed an initial transport of the fluffy surface layer (u* _cr-initial ) at a mean critical shear stress velocity of 0.39 cm s^?1. Initial rolling transport at the fine sand station for single sand grains was recorded at values of 0.5 cm s^?1. At higher shear stress velocities, the two sediment types showed diverging erosion behaviour. Measurable erosion (ε>5.0×10^?6 kg m^?2 s^?1) of fine sand starts at a mean critical shear stress velocity (u* _cr-erosion ) of 1.15 cm s^?1 whereas fluffy surface material on mud cores was eroded at mean u* _cr-erosion of 0.62 cm s^?1. This indicates that measured erosion thresholds at the fine sand site fit well to calculated critical shear stress velocities whereas calculated erosion thresholds for cohesive mud are roughly 6 times higher than measured values. As erosion behaviour at the mud station was dominated by fluffy surface material, the comparability of measured and calculated threshold values may be reduced. The underlying silt-sized sediment itself was stable due to cohesive effects. This behaviour has to be taken into consideration by using sediment types instead of mean grain sizes for mapping and modelling sediment dynamics. A comparison of the near-bottom hydrodynamic conditions in the study area and experimentally derived critical shear stress velocities suggests that particle transport is controlled by storm events whereas under calm conditions shear stress velocities do not exceed the critical values.