Suspended Cohesive Sediment Research Articles

Research on deposition and consolidation behaviour of cohesive sediment with settlement column experiment

The deposition and consolidation behaviour of cohesive sediment is studied with a newly developed integrated settlement column system. This system, which includes graduate (glass) and settlement column (PMMA), has been proved an effective way to study the settlement of sediment suspension with 20∼300 g/L initial concentration. Cohesive sediment deposition and consolidation behaviours are as follows: cohesive sediment settlement can be divided into different stages as the initial free settlement and the consolidation settlement with different sediment interface rate. Cohesive sediment settlement influence factors are mainly the initial concentration and the settlement height, from which, settlement column size effect is determined by three factors as the wall effect, convection effect as well as the flocculation effect. During the final weight consolidation stage, deposited sediment consolidation settlement develops much faster than the excessive pore-pressure dissipation, and the fitted nonlinear consolidation constitutive relationship also can be obtained through deposited sediment layer sampling method. Results from this study indicate that the newly developed integrated settlement column system can effectively simulate the cohesive sediment suspension deposition and consolidation, and the nonlinear consolidation characteristics also can be studied through further experiments.

Sedimentological and Rheological Properties of the Water–Solid Bed Interface in the Weser and Ems Estuaries, North Sea, Germany: Implications for Fluid Mud Classification

ABSTRACT Papenmeier, S.; Schrottke, K.; Bartholoma, A. and Flemming, B.W., 2013. Sedimentological and rheological properties of the water–solid bed interface in the Weser and Ems estuaries, North Sea, Germany: Implications for fluid mud classification. Fine, cohesive sediment suspensions are a common feature of estuarine environments. Generally, multilayer models are used to describe the vertical distribution of such sediments. Such conceptional models normally distinguish at least high suspended sediment concentrations (SSCs) as a topmost layer and a consolidated bed layer, often including an intermediate, fluid mud layer. Rheological, and in particular sedimentological properties are rarely included in these models. New data from two different estuaries provide new insights that can contribute toward the classification of near-bed cohesive sediments. The water–solid bed interfaces within the turbidity maximum zones of the Weser and Ems estuaries were sampled with 2–4-m-long cores. At 10-cm intervals, va...

Factors controlling floc settling velocity along a longitudinal estuarine transect

A 147 km longitudinal transect of flocculated cohesive sediment properties in San Francisco Bay (SFB) was conducted on June 17th, 2008. Our aim was to determine the factors that control floc settling velocity along the longitudinal axis of the estuary. The INSSEV-LF video system was used to measure floc diameters and settling velocities at 30 stations at a distance of 0.7 m above the estuary bed. Floc sizes (D) ranged from 22 μm to 639 μm and settling velocities (Ws) ranged between 0.04 mm·s− 1 and 15.8 mm·s− 1 during the longitudinal transect. Nearbed turbulent shear stresses throughout the transect duration were within the 0.2–0.5 Pa range which typically stimulates flocculation growth. The individual D–Ws–floc density plots suggest the suspended sediments encountered throughout SFB were composed of both muddy cohesive sediment and mixed sediments flocs. Mass-weighted population mean settling velocity (Wsmass) ranged from 0.5 mm·s− 1 to 10 mm·s− 1. The macrofloc and microfloc (demarcation at 160 μm) sub-populations demonstrated parameterised settling velocities which spanned nearly double the range of the sample mean settling velocities (Wsmean). The macroflocs tended to dominate the suspended mass (up to 77% of the ambient suspended solid concentration; SSC) from San Pablo Bay to Carquinez Strait (the vicinity of the turbidity maximum zone). Microfloc mass was particularly significant (typically 60–100% of the SSC) in the northern section of South Bay and most of Central Bay. The transect took eleven hours to complete and was not fully synoptic. During slack tide, larger and faster settling flocs deposited, accounting for most of the longitudinal variability. The best single predictor of settling velocity was water velocity 39 min prior to sampling, not suspended-sediment concentration or salinity. Resuspension and settling lags are likely responsible for the lagged response of settling velocity to water velocity. The distribution of individual floc diameters and settling velocities indicates that floc density for a given floc diameter varies greatly. A small portion (a few percent) of suspended sediment mass in SFB is sand-sized and inclusion of sand in flocs appears likely. Fractal theory for cohesive sediment assumes that there is a single primary particle size that flocculates, which is not the case for these types of mixed sediment flocs. The wide variability in the physical, biological and chemical processes which contribute to flocculation within SFB means that spatial floc data is required in order to accurately represent the diverse floc dynamics present in the Bay system. The importance in determining accurate estimates of floc density has been highlighted by the SFB data, as these provide the basis for realistic distributions of floc dry mass and the mass settling flux across a floc population. However, although video floc sampling devices can produce the various floc property trends observed in SFB, good survey practice is still paramount. One can see that if the sampling coverage (i.e. data collection frequency) is poor, this could lead to potential mis-interpretations of the data and only limited conclusions may be drawn from such a restricted survey. For example, a limited survey (i.e. only 3 stations, compared to the 10 stations in the full survey) in South Bay produces an under-estimate in both the macrofloc SSCmacro distribution by a factor of four and the Wsmacro by a factor of two. To develop sediment transport numerical models for SFB, high quality floc size and settling data are needed to understand and simulate the depositional qualities of both suspended cohesive sediment and mixed sediments in San Francisco Bay. This study has shown that the most pragmatic solution is a physically-based approach, whereby the detailed flocs D vs. Ws spectra are parameterised in terms of their macrofloc and microfloc properties. This aids in model calibration, whilst retaining more of the dynamical aspects of the floc populations. All forms of flocculation are dynamically active processes, therefore it is important to also include both SSC and turbulence functions together with the floc data.

Effects of exopolymers on particle size distributions of suspended cohesive sediments

[1] The effects of exopolymers on the particle size distributions (PSDs) of suspended cohesive sediments were investigated in laboratory using four abundant clay minerals, kaolinite, illite, Na-montmorillonite, and Ca-montmorillonite, and two exopolymers, xanthan and guar, at six different exopolymer to clay ratios (E/C; i.e., 0, 1, 2, 5, 10, and 20 wt %) to represent the compositional variability of cohesive sediments in natural waters. Results show that the clay-exopolymer suspensions possess multimodal PSD. Statistical deconvolution of the PSD curves indicates that the suspensions consist of four discrete particle groups, primary particles, flocculi, microflocs, and macroflocs, all of which exhibit a unimodal lognormal distribution. Furthermore, such deconvolution quantifies the mean size and fraction of each particle group, leading to a more quantitative understanding of PSD kinetics of these sediments. Both clay surface charges and exopolymer polarity as well as the E/C affect the PSD kinetics. While neutral guar causes flocculation for all four clay minerals, anionic xanthan only induces flocculation for kaolinite with very low surface charges, but not for the other three clay minerals with relatively high charges. The fraction of each particle group also varies with the E/C, and such complex changes depend upon the interfacial interactions between clay particles and exopolymer molecules. For each exopolymer, critical E/C exists that can lead to a maximum or minimum fraction of microflocs or macroflocs. The role of exopolymer bridging, Coulomb force, and hydrogen bond in affecting the PSD kinetics of cohesive sediments is also discussed.

Characterization of particle size and settling velocity of cohesive sediments affected by a neutral exopolymer

In natural waters, exopolymers or extracellular polymeric substances (EPS) exuded by microorganisms interact with clay particles, resulting in the flocculation of clays and hence alteration to the properties of suspended cohesive sediments. To investigate and further understand how neutral EPS affect cohesive sediment transport and the final sediment yield, an experimental study was conducted on laboratory-prepared clay and guar gum (used as an analog for neutral EPS) suspensions to characterize EPS-induced flocculation and the settling velocity of resultant flocs. Four different clays consisting of kaolinite, illite, Ca-montmorillonite, and Na-montmorillonite were studied to examine the influence of different layer charges on clay flocculation induced by neutral EPS. Floc size was determined by a laser particle size analyzer, and settling velocity estimated by analyzing the time-series floc settling images captured by an optical microscope. Results indicate that neutral EPS promote clay-EPS flocculation for all four clays with the particle/floc size significantly increased from ∼0.1-60 μm to as large as ∼600 μm. Clays' layer charge has a profound influence on the clay-EPS flocculation. With the same floc size, the settling velocity of clay-EPS flocs is typically smaller than that of pure clay flocs, which is attributed to the reduced density of flocs caused by the EPS. However, for flocs of the same composition (e.g. pure clay or hybrid clay-EPS mixture), the settling velocity increases with size. The fractal dimension of these clay-EPS flocs estimated from settling velocity ranges from 1.39 to 1.47, which are smaller than that of pure clay flocs, indicating that these flocs are less compacted than the pure clay flocs.

Erratum for “Physical and Numerical Modeling of the Entrainment by a High‐Concentration Mud Suspension” by A. W. Bruens, J. C. Winterwerp, and C. Kranenburg

This paper presents the results of a physical and numerical model study on the entrainment by a high-concentration, cohesive sediment suspension. This turbulent, near-bed suspension is referred to as a concentrated benthic suspension (CBS). In the current configuration, the CBS entrains low-concentrated overlying water. The results of the physical and numerical experiments are in line with the results of similar experiments, published in literature, in which, in contrast, the upper, turbulent layer entrains water and sediment from the lower layer. The entrainment rate for these experiments is almost inversely proportional to the bulk Richardson-number E∝Ri*-n, with n≈1, indicating a profound effect of the side wall of the experimental facility. We anticipate that the conditions studied in this paper are characteristic for the behavior of many high-concentration systems, such as the mouth of the Amazon River, and in the Loire Estuary and Ems River. A sequence of entrainment during high flow velocities, and settling/consolidation during low flow velocities, driven by tidally induced horizontal pressure gradients, keeps the sediment fluid and mobile; this process can be referred to as tidal pumping. The experimental results could be reproduced to a fair degree by a 1DV point model. This, however, does not necessarily imply that state-of-the-art numerical engineering models are readily applicable for simulations of the hydro-sedimentological conditions in complicated systems, such as the Amazon mouth and Loire Estuary. In these systems, one expects a gradual transition from a fully turbulent upper part of the CBS layer, to laminar conditions deeper down within this layer, with increasing values of viscosity and yield strength; such transition is not accounted for in state-of-the-art turbulence models. Yet, the current work provides a framework for a better understanding of these systems.

Physical and Numerical Modeling of the Entrainment by a High-Concentration Mud Suspension

AbstractThis paper presents the results of a physical and numerical model study on the entrainment by a high-concentration, cohesive sediment suspension. This turbulent, near-bed suspension is referred to as a concentrated benthic suspension (CBS). In the current configuration, the CBS entrains low-concentrated overlying water. The results of the physical and numerical experiments are in line with the results of similar experiments, published in literature, in which, in contrast, the upper, turbulent layer entrains water and sediment from the lower layer. The entrainment rate for these experiments is almost inversely proportional to the bulk Richardson-number E∝Ri*-n, with n≈1, indicating a profound effect of the side wall of the experimental facility. We anticipate that the conditions studied in this paper are characteristic for the behavior of many high-concentration systems, such as the mouth of the Amazon River, and in the Loire Estuary and Ems River. A sequence of entrainment during high flow velocit...

A heuristic examination of cohesive sediment bed exchange in turbulent flows

Prediction of the concentration of suspended cohesive sediment in the marine environment is constrained by difficulties in interpreting experimental evidence on bed exchange, i.e. erosion and deposition of particles, which remains sparse in mechanistic details. In this paper, conditions under which bed exchange in turbulent flows collectively determines the concentration of suspended matter have been examined in the heuristic sense based on selective experimental data. It is argued that interpretation of such data can be significantly facilitated when multi-class representation of particle size, collisional interaction between suspended particles and probabilistic representations of the bed shear stress along with variables describing particle behavior (critical shear stress for deposition, bed floc shear strength) are taken into account. Aggregation—floc growth and breakup kinetics—brings about shifts in the suspended particle size distribution; bed exchange is accordingly modulated and this in turn determines concentration dynamics. Probabilistic representation of the governing variables broadens the suspended sediment size spectrum by increasing the possibilities of inter-particle interactions relative to the mean-value representation. Simple models of bed exchange, which essentially rely on single-size assumption and mean-value representation of variables, overlook the mechanistic basis underpinning particle dynamics.

Review of Cohesive Sediments in Open Channels: Erosion, Transport, and Applications by Emmanuel PartheniadesCohesive Sediments in Open Channels: Erosion, Transport, and ApplicationsButterworth-Heinemann$135.00

It is mainly because of their physicochemical properties that the development of our understanding of cohesive sediments has followed a course somewhat different from that of noncohesive sediments. A major and acknowledged role in the advance of cohesive sediments knowledge is played by the methods of physics as expressed through non-Newtonian fluid mechanics. Recent advances in understanding microscale dynamic behavior of cohesive sediments are also rooted in the physicochemical hydrodynamics and soil mechanics. The extraordinary growth of interest in cohesive sediments since the 1950s continues to provoke the publication of new engineering-oriented monographs into the early 21st century, notably, Whitehouse et al. 2000 , and Winterwerp and van Kesteren 2004 . Emmanuel Partheniades’s new book aims to provide a practical framework for theoretical, laboratory, and field studies of erosion and transport of cohesive sediments and the major environmental factors influencing those processes. This book reviews the current status of those research topics with particular emphasis on the work by the author and his associates and colleagues. Its scope is best seen in terms of the chapter headings: “The Mineralogy and the Physicochemical Properties of Cohesive Sediments,” “Forces between Clay Particles and the Process of Flocculation,” The Hydrodynamic Transport Processes of Cohesive Sediments and the Governing Equations,” “Rheologic Properties of Cohesive Sediment Suspensions,” “Erosion of Cohesive Soils,” “Deposition and Resuspension of Cohesive Soils,” and “Engineering Applications of Cohesive Sediment Dynamics.” Such topics would be of particular interest to civil and hydraulic engineers. Some of them could be of interest to hydrologists, geomorphologists, sedimentologists, and environmental scientists. The first chapter, “Introduction,” briefly highlights the importance and distinction of sediments in general, then provides an overview of the historical development of the science and engineering of cohesive sediment behavior, and finally states the overall objectives of this book and outlines its major contents. Chapter 2, “The Mineralogy and the Physicochemical Properties of Cohesive Sediments,” presents a detailed overview of the nature and mineralogy of clay particles within cohesive sediment suspensions and cohesive sediment deposits. This is a nice chapter summarizing and discussing the major fundamental physicochemical properties of cohesive sediments that are critical to their dynamic behavior. The main difference between this second chapter and similar chapters in other texts on cohesive sediments is in

The application of holography to the analysis of size and settling velocity of suspended cohesive sediments

Suspended cohesive sediments commonly occur in flocculated form. The flocculation process significantly increases particle size, settling velocity, and settling flux in comparison to unflocculated particles. To better understand suspended particles, a new instrument for in situ imaging using digital inline holography is presented and evaluated. With a resolution of 7.4 µm per pixel and a field of view of 7.4 × 7.4 mm, the instrument generates sharply focused images of particles from approximately 20 µm to 7 mm in diameter at up to 25 frames per second. A significant advantage of holography over current imaging systems is that in‐focus images are obtained over a substantial depth of field. Contained in small‐diameter cylindrical housings, the instrument presents minimal flow disruption and is easily deployable. Digitally recorded holograms are reconstructed and analyzed for number and size of particles in a fully automated manner. To assess the systems' potential, particle size distributions for two grades of quartz sand are compared with those from a laser diffraction particle sizer and are found to exhibit good similarity. To simultaneously estimate particle size and settling velocity, a modification to the system is demonstrated in which settling particles are automatically tracked and sized. The resulting relationships between settling velocity and effective density as functions of particle size are characteristic of suspended cohesive sediments and empirical power‐law descriptors of these relationships show excellent agreement with previously published studies.

Non-equilibrium flocculation characteristics of fine-grained sediments in grid-generated turbulent flow

Results are presented from a series of settling column experiments investigating temporal variations in the flocculation characteristics of purely cohesive (kaolin clay) sediment suspensions and cohesive (kaolin) and non-cohesive (fine sand) sediment fraction mixtures. Experimental runs were conducted under controlled hydrodynamic conditions generated by a rigid array of in-phase oscillating grids. The results indicated that rapid initial floc aggregation occurred under low turbulent shear rates, with peak maximal and root-mean-square (r.m.s.) floc sizes (∼ 400 μm and ∼ 200 μm, respectively) attained after relatively short time periods, before reducing with time. By contrast, lower aggregation rates and smaller floc sizes were observed under higher shear conditions, with flocs retaining suspended in the settling column for longer time scales due to the increased turbulence. The mud input concentration displayed some correlation with maximal and r.m.s. floc sizes at higher shear rates but no correlation was apparent at low shear rates. This observed floc behaviour may be attributed to the differences in concentration gradients at high and low shear rates that affect both floc settling rate and time required for flocs to attain equilibrium size. The addition of the fine sand fraction to the kaolin clay suspension reduced both the initial floc formation (i.e. aggregation) rate and the maximal and r.m.s. floc sizes attained throughout the experiments. The reduction in maximal floc sizes appeared to be enhanced by an increase in the ratio of fine sand to kaolin clay content within the mixture.

Factors controlling the field settling velocity of cohesive sediment in estuaries

It has long been recognized that the suspended sediment concentration (SSC) is one of the major determinants for the flocculation of cohesive particles into sediment flocs in estuaries. It is furthermore well known that the turbulent shear of the water significantly influences the flocculation process and the equilibrium settling velocity of flocculated sediment in a turbulent flow. A vast number of authors have reported algorithms relating the median settling velocity ( W 50) to suspended sediment concentration. However, only a few studies have dealt with the impact of the turbulent shear (in this paper expressed as the root mean square [rms] velocity gradient, [ G]) in the water on the W 50 in situ. There is a strong need to establish algorithms based on in situ measurements describing the dual impact of both SSC and G on the flocculation process, and hence, W 50. The present paper addresses this topic. Field settling velocities of suspended cohesive sediment have been measured in micro-, meso-, and macro-tidal estuaries. Regression analyses between the W 50, SSC and G are presented. It is shown that by including both G and SSC in the regression analyses, a significant increase in the correlation of the description of W 50 and the controlling parameters from each area can be obtained. A generic algorithm describing the data from all the investigated areas is suggested. It works well within specific tidal areas but fails to give a generic description of the field settling velocity.

Using ADV backscatter strength for measuring suspended cohesive sediment concentration

Laboratory experiments were conducted at two institutes to reveal the relationship between acoustic backscatter strength and suspended sediment concentration (SSC). In total, three acoustic Doppler velocimeters (ADVs) with different frequencies (5, 10 and 16 MHz) were tested. Two different commercial clays and one natural sediment from Clay Bank site in the York River were checked for acoustic responses. The SSCs of selected sediments were artificially changed between a selected low and a high value in tap or de-ion water. Each ADV showed quite different backscatter responses depending on the sediment type and SSC. Not all devices had a good linear relationship between backscatter strength and SSC. Within a limited range of SSC, however, the backscatter strength can be well correlated with the SSC. Compared with optical backscattering sensor (OBS), the fluctuation of ADV backscatter signals was too noisy to be directly converted to the instantaneous changes of SSC due to high amplification ratio and small sampling volume. For the more accurate signal conversion for finding the fluctuation of SSC, the ensemble average should be applied to increase the signal-to-noise ratio. There are unexpected responses for the averaged backscatter wave strength: (1) high signals from small particles but low signals from large particles; and (2) two linear segments in calibration slope. These phenomena would be most likely caused by the different gain setting built in ADVs. The different acoustic responses to flocculation might also contribute somewhat if flocs are tightly packed. This study suggests that an ADV could be a useful instrument to estimate suspended cohesive sediment concentration and its fluctuation if the above concerns are clarified.

SEDVEL: An underwater balance for measuring in situ settling velocities and suspended cohesive sediment concentrations

An automated instrument (SEDVEL—Sedimentation Velocity) was developed to directly measure the mass-concentration and the mass-distribution of settling velocities of suspended particulate matter (SPM) in situ. This instrument consists of an underwater balance, which directly measures the variation in time of the immersed weight of particulate matter (PM) as it settles on a plate located at the bottom of a settling tube, under quiescent conditions. SEDVEL operates underwater and samples in situ for deployment periods of a few days. SEDVEL produced consistent and reproducible results when tested both in the laboratory and in the field under SPM dry-concentrations of 5 to 200 mg l − 1 . Errors in the estimates of maximum dry-concentrations from SEDVEL measurements were less than 30% in 69% of the cycles analysed in the laboratory. The corresponding figure for in situ measurements was 50%. These errors are likely related to uncertainties in the calculation of the dry-density of flocs and in the definition of the zero position (ZP) of the SEDVEL balance. The slow settling particles/flocs ( W s < 1 mm s − 1 ) represented 32–98% of the total mass of SPM at two sites of deployment in Cleveland Bay (Australia). Distinct settling behaviours were observed between the two sites and among different tidal stages associated with differences in the floc population; the aggregation of which probably varied as a function of the bottom grain size, shear stress, resuspension, advection and organic content of SPM.

Sediment TMDL Development for the Amite River

The Amite River is recognized as one of the 15 water bodies impaired by sediments in Louisiana, USA. Based on US EPA’s Protocol sediment TMDL (Total Maximum Daily Load) development is conducted for the Amite River and described in this paper. The TMDL development consists of four components: (1) development of a new model for cohesive sediment transport, (2) estimation of sediment loads (sources) due to watershed erosion, (3) river flow computation, and (4) determination of sediment TMDL for the Amite River. Using the mass conservation principle and Reynolds transport theorem a new 1-D model has been developed for computation of suspended cohesive sediment transport. Sediment erosion in the Amite River Basin is calculated by combining the USLE (Universal Soil Loss Equation) model with GIS and the digital elevation model of the Amite River Basin. Digital elevation data was imported into the GIS which generated inputs for USLE. The calculated average annual rate of soil erosion in the Amite River Basin is 13.368 tons per ha, producing a nonpoint sediment load of 103 mg/L to the Amite River. The flow computation is performed using the HEC-RAS software. The computed sediment concentration in the Amite River varies in the range of 3–114 mg/L and sediment TMDL is 281.219 tons/day. The reduction necessary to support beneficial uses of the river is 55% or 275.946 tons/day. Results indicate that the combined application of the new 1-D sediment transport model, GIS, USLE model, and HEC-RAS is an efficient and effective approach to sediment TMDL development.

Stochastic flocculation of cohesive sediment: Analysis of floc mobility within the floc size spectrum

A stochastic Lagrangian model is proposed to describe flocculation of suspended cohesive sediment flocs. This model takes into account the floc population in a parametric fashion, and predicts the time evolution of an individual floc by a sequence of stochastic aggregation and breakup events. Upon calibration on experimental data, the model was used to investigate floc mobility within the population size spectrum, bringing to light dynamical aspects not otherwise detectable with deterministic models. Our analysis showed that floc size fluctuations correlated with the population size distribution and breakup probability, and that size‐dependent burst events repeated intermittently in time. Further analysis showed the effect of the floc population distribution on individual floc size transition and equilibrium, and the effect of using either a constant or variable floc capacity dimension to describe floc geometry.

Variable fractal dimension: A major control for floc structure and flocculation kinematics of suspended cohesive sediment

While the fractal dimension of suspended flocs of cohesive sediment is known to vary with the shear rate, electrochemical properties of the sediment and environment, geometrical restructuring, and presence of organic matter, experimental data presented in this work suggest changes in fractal dimension also during floc genesis at constant sedimentological and hydraulic conditions. A power law function is proposed to describe these changes in floc fractal dimension during floc growth and is used to analyze its impact on floc structural parameters, settling velocity, and kinematics of aggregation and breakup. An analysis of this model for the fractal dimension highlights changes of approximately a factor of 2 or more in floc porosity and aggregation and breakup frequencies and of approximately 1 order of magnitude in floc excess density and settling velocity compared to values estimated with constant fractal dimension. The results from this model compare well with prior experimental data collected in situ (Khelifa and Hill, 2006; Manning and Dyer, 1999).

Effect of variable fractal dimension on the floc size distribution of suspended cohesive sediment

Flocculation of suspended cohesive sediment, well-known to impact the floc size distribution and vertical fluxes, and cause morphodynamic changes of marine and riverine environments, is modelled by means of a population balance equation that implements a novel description of floc geometry: the capacity dimension of fractal flocs, normally assumed constant over the population, has recently been argued to change during flocculation. Our experiments have shown that a power-law function of the dimensionless floc size can conveniently be used to describe these changes. This description of floc capacity dimension is used to explore in detail the extent to which the geometrical properties of flocs affect aggregation and breakup processes, and contribute to shaping their size distribution. A comparison of experimental floc size distributions from settling column test with computed distributions for two hypotheses of floc capacity dimension (i.e., constant and variable) and two hypotheses of flocculation reactions (i.e., semi-stochastic and deterministic) are shown. This suggests that the use of variable rather than constant floc capacity dimension, and the use of semi-stochastic and asymmetric reactions rather than deterministic and symmetric, result in better predictions of the floc size distribution in the environmental conditions herein analysed.

Hindered settling of mud flocs: Theory and validation

This paper deals with settling of highly concentrated cohesive sediment suspensions. We evaluate a new hindered settling formula. New settling experiments on these highly concentrated suspensions are described to test this formula. For the analysis of the experiments both an analytical and a numerical method are used. Kynch's analytical theory, based on the method of characteristics is used to study the type of settling. Furthermore, a 1DV-point model is used for analysis of the settling process. We have implemented the new hindered settling formula in this model, which is tested against experimental data. It is concluded that the data are described fairly well. The analysis with the theory of Kynch [1952. A theory of sedimentation. Transactions of the Faraday Scociety 48, 166–176] and the model show that highly concentrated suspensions can settle with either one interface or with two interfaces, depending on the initial concentration and the shape of the settling flux function.

An evaluation of bed shear stress under turbid flows

This paper investigates the effect of various turbidity levels on the drag coefficient and bed shear stress in decelerating flows over a smooth bed. Turbid flows were simulated in a laboratory annular flume (Lab Carousel) using clay suspensions of different known mass concentrations. Flow velocity measurements were taken in the turbulent boundary layer and the values of bed drag coefficient and shear stress were calculated using the method of flow deceleration. Results showed that the bed drag coefficient responds significantly to an increase in the suspended sediment concentration (up to 20 g\L) showing a decrease of up to 50% in drag due to turbulence suppression. However, subsequent increases in mass concentration resulted in an increase in the drag coefficient reaching the clear water value at approximately 200 g\L because of the influence of viscosity. Thus the application of the law of the wall in flows of high concentrations of suspended cohesive sediments up to 200 g\L results in overestimation of the bed shear stress.