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Johan C. Winterwerp - One of the best experts on this subject based on the ideXlab platform.
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The hyperturbid state of the water column in estuaries and rivers: the importance of Hindered Settling
Ocean Dynamics, 2018Co-Authors: Yoeri M. Dijkstra, Henk M. Schuttelaars, Johan C. WinterwerpAbstract:Over the last few decades, some estuaries have undergone a transition to a hyperturbid state, characterised by suspended sediment concentrations of several grammes per litre averaged over the water column. To improve our understanding of this transition and of naturally hyperturbid estuaries, we systematically identify the processes allowing for high suspended sediment concentrations using a water column (1DV) model. Under a range of realistic forcing conditions, the state of the water column can be characterised by one of two equilibrium states. The first is an erosion-limited state, in which there still is sediment available for erosion at the bed. We find that this state only occurs with relatively low concentrations. The second is a supply-limited state, in which all erodable sediment is in suspension. The concentration in this state depends entirely on the amount of sediment in the system and can potentially be very high. We identify the conditions under which the state of the water column can jump from a low to a high concentration and identify hysteresis in the transition between the two states. The mechanism responsible for this hysteresis is Hindered Settling. It thus follows that hyperturbidity is only possible in a supply-limited state. From this observation we derive a necessary condition for an estuarine system to make the transition from low turbidity to hyperturbidity in a 1DV context. This is an important step towards understanding why some estuaries are hyperturbid and assessing the risk that particular estuaries may become hyperturbid in the future.
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Hindered Settling of Silt
Journal of Hydraulic Engineering, 2015Co-Authors: S. Te Slaa, D.s. Van Maren, Johan C. WinterwerpAbstract:AbstractSediment particles smaller than 64 μm, with a noncohesive base mineral, are referred to as silt. The Settling velocity of suspended sediment decreases with increasing concentration due to Hindered Settling effects. Compared with clay and sand, Hindered Settling of silt is poorly documented and understood. A formulation is proposed for the Hindered Settling of silt-water mixtures, derived from physical reasoning. The effect of silt on the mixture’s viscosity is addressed in particular. The formulation compares favorably with laboratory experiments on the Settling of silt-sized sediment. This study shows that effective Settling velocities can be derived from a single concentration time series measured within the Settling mixture, provided that the sediment is initially homogeneously distributed.
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On the Hindered Settling of silt-water mixtures
2012Co-Authors: S. Te Slaa, D.s. Van Maren, Johan C. WinterwerpAbstract:The sedimentation behavior of silt particles in the Hindered Settling regime has been considered. Therefore, laboratory experiments are carried in especially designed Settling columns. Silt-water mixtures are prepared and allowed to settle at various initial concentrations. During the Settling process, a continuous vertical concentration profile is measured. Settling velocities are obtained from these profiles and are compared with the widely used Richardson and Zaki (1954) expression. This comparison showed that the Richardson and Zaki expression underestimates the Settling velocities for the finest silts at high concentrations. For the coarsest silt the Richardson and Zaki expression performed will with the measured data. We anticipate that this is the effect of the particle size on the apparent viscosity of the Settling silt-water mixture.
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Chapter 18 On the Hindered Settling of highly concentrated mud-sand mixtures
Sediment and Ecohydraulics - INTERCOH 2005, 2008Co-Authors: Petra J.t. Dankers, Gilliane Sills, Johan C. WinterwerpAbstract:Abstract This paper deals with the Hindered Settling of highly concentrated mud-sand mixtures. The suspension's Settling characteristics and the Settling velocity of the mud and sand fraction are determined. X-ray concentration profiles show that the mud-sand mixtures settle with only one interface, while mud-only suspensions at similar initial concentrations develop two interfaces during Settling. This has a large influence on the resulting vertical concentration distribution. The disappearance of the lower interface is most likely caused by the disturbance created in the suspension by the Settling sand grains. The suspensions' Settling velocity is in the order of 0.02–0.3 mm/s, while the sand Settling velocity, measured with particle tracking velocimetry and particle image velocimetry techniques, is in the order of 1–3 mm/s. The sand is strongly Hindered by the presence of the highly concentrated mud suspension, and its Settling velocity is strongly decreased compared to the Stokes' Settling velocity of 10 mm/s. Once the suspension starts to gain strength, the sand can become arrested in the mud matrix and settle with the suspension's settlign velocity. However, we also observed that the sand grains follow the dewatering channels generated by water expelled during consolidation of the mud fraction. In that case, the sand particles settle at a larger velocity deeper into the suspension or bed. We anticipate that there exists a subtle interaction in the dewatering channels where the expelled pore water flow with the Settling sand grains and the sand pockets affect the overall Settling/consolidation behaviour of the mixture.
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Hindered Settling of mud flocs: Theory and validation
Continental Shelf Research, 2007Co-Authors: P.j.t. Dankers, Johan C. WinterwerpAbstract:Abstract 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.
Lan Anh Van - One of the best experts on this subject based on the ideXlab platform.
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Hindered Settling of sand mud flocs mixtures from model formulation to numerical validation
Advances in Water Resources, 2013Co-Authors: Lan Anh Van, Damien Pham Van BangAbstract:Abstract Mixtures of non-cohesive and cohesive sediment are frequently encountered in natural environments such as estuaries. Depending on the concentration of both species, mixed sediment can record segregation effect or not, behaves like a non-cohesive sediment or like a cohesive sediment. The present study deals with the segregation effect between mud flocs and sand grains during Hindered Settling. Simulations of this process under various conditions of mixture are proposed by using two coupled mass conservation equations which are solved by a high order numerical model. Specific closure equations are proposed herein for the Hindered Settling of sand–mud mixed sediment. Comparisons between simulations and experiments are presented on vertical concentration profiles during the segregation process. Obviously, the model enables the description of Hindered Settling for pure non-cohesive (or pure cohesive) case and the segregation for bi-disperse suspensions.
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Hindered Settling of sand–mud flocs mixtures: From model formulation to numerical validation
Advances in Water Resources, 2013Co-Authors: Lan Anh Van, Damien Pham Van BangAbstract:Abstract Mixtures of non-cohesive and cohesive sediment are frequently encountered in natural environments such as estuaries. Depending on the concentration of both species, mixed sediment can record segregation effect or not, behaves like a non-cohesive sediment or like a cohesive sediment. The present study deals with the segregation effect between mud flocs and sand grains during Hindered Settling. Simulations of this process under various conditions of mixture are proposed by using two coupled mass conservation equations which are solved by a high order numerical model. Specific closure equations are proposed herein for the Hindered Settling of sand–mud mixed sediment. Comparisons between simulations and experiments are presented on vertical concentration profiles during the segregation process. Obviously, the model enables the description of Hindered Settling for pure non-cohesive (or pure cohesive) case and the segregation for bi-disperse suspensions.
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Hindered Settling of sand-mud flocs mixtures: From model formulation to numerical validation
Advances in Water Resources, 2013Co-Authors: Lan Anh Van, Damien Pham Van BangAbstract:Mixtures of non-cohesive and cohesive sediment are frequently encountered in natural environments such as estuaries. Depending on the concentration of both species, mixed sediment can record segregation effect or not, behaves like a non-cohesive sediment or like a cohesive sediment. The present study deals with the segregation effect between mud flocs and sand grains during Hindered Settling. Simulations of this process under various conditions of mixture are proposed by using two coupled mass conservation equations which are solved by a high order numerical model. Specific closure equations are proposed herein for the Hindered Settling of sand-mud mixed sediment. Comparisons between simulations and experiments are presented on vertical concentration profiles during the segregation process. Obviously, the model enables the description of Hindered Settling for pure non-cohesive (or pure cohesive) case and the segregation for bi-disperse suspensions. © 2012 Elsevier Ltd.
Damien Pham Van Bang - One of the best experts on this subject based on the ideXlab platform.
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Hindered Settling of sand mud flocs mixtures from model formulation to numerical validation
Advances in Water Resources, 2013Co-Authors: Lan Anh Van, Damien Pham Van BangAbstract:Abstract Mixtures of non-cohesive and cohesive sediment are frequently encountered in natural environments such as estuaries. Depending on the concentration of both species, mixed sediment can record segregation effect or not, behaves like a non-cohesive sediment or like a cohesive sediment. The present study deals with the segregation effect between mud flocs and sand grains during Hindered Settling. Simulations of this process under various conditions of mixture are proposed by using two coupled mass conservation equations which are solved by a high order numerical model. Specific closure equations are proposed herein for the Hindered Settling of sand–mud mixed sediment. Comparisons between simulations and experiments are presented on vertical concentration profiles during the segregation process. Obviously, the model enables the description of Hindered Settling for pure non-cohesive (or pure cohesive) case and the segregation for bi-disperse suspensions.
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Hindered Settling of sand–mud flocs mixtures: From model formulation to numerical validation
Advances in Water Resources, 2013Co-Authors: Lan Anh Van, Damien Pham Van BangAbstract:Abstract Mixtures of non-cohesive and cohesive sediment are frequently encountered in natural environments such as estuaries. Depending on the concentration of both species, mixed sediment can record segregation effect or not, behaves like a non-cohesive sediment or like a cohesive sediment. The present study deals with the segregation effect between mud flocs and sand grains during Hindered Settling. Simulations of this process under various conditions of mixture are proposed by using two coupled mass conservation equations which are solved by a high order numerical model. Specific closure equations are proposed herein for the Hindered Settling of sand–mud mixed sediment. Comparisons between simulations and experiments are presented on vertical concentration profiles during the segregation process. Obviously, the model enables the description of Hindered Settling for pure non-cohesive (or pure cohesive) case and the segregation for bi-disperse suspensions.
Damien Pham Van Bang - One of the best experts on this subject based on the ideXlab platform.
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Hindered Settling of sand-mud flocs mixtures: From model formulation to numerical validation
Advances in Water Resources, 2013Co-Authors: Lan Anh Van, Damien Pham Van BangAbstract:Mixtures of non-cohesive and cohesive sediment are frequently encountered in natural environments such as estuaries. Depending on the concentration of both species, mixed sediment can record segregation effect or not, behaves like a non-cohesive sediment or like a cohesive sediment. The present study deals with the segregation effect between mud flocs and sand grains during Hindered Settling. Simulations of this process under various conditions of mixture are proposed by using two coupled mass conservation equations which are solved by a high order numerical model. Specific closure equations are proposed herein for the Hindered Settling of sand-mud mixed sediment. Comparisons between simulations and experiments are presented on vertical concentration profiles during the segregation process. Obviously, the model enables the description of Hindered Settling for pure non-cohesive (or pure cohesive) case and the segregation for bi-disperse suspensions. © 2012 Elsevier Ltd.
Huib E. Swart - One of the best experts on this subject based on the ideXlab platform.
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On the role of flocculation, Hindered Settling and sediment-induced damping of turbulence in trapping sediment in estuaries, with focus on the North Passage, Yangtze Estuary
2020Co-Authors: Chenjuan Jiang, Huib E. Swart, Jianan ZhouAbstract:<p>Many estuaries are characterized by one or more locations where the concentration of fine sediment attains a maximum. The locations and intensities of these estuarine turbidity maxima (ETM) are sensitive to river discharge, tides, depth and sediment properties. In this contribution, results are presented of a width-averaged process-based model that describes tides, residual currents and sediment transport in an estuarine channel. The aim is to quantify the sensitivity of location and intensity of ETM to 1) flocculation and Hindered Settling of fine sediment and 2) sediment-induced damping of turbulence. The model is applied to the North Passage of the Yangtze Estuary, which is a prototype estuary that undergoes strong variations in environmental conditions. The sediment Settling velocity is allowed to vary along the channel due to the effects of flocculation and Hindered Settling, by parametrizing Settling velocity as the function of the subtidal near-bed sediment concentration according to results obtained from laboratory experiments. Sediment-induced turbulence damping is taken into account by parametrizing eddy viscosity and eddy diffusivity coefficients as functions of bulk Richardson number.</p><p>In the flocculation (low concentration) regime, where the Settling velocity increases with sediment concentration, the rapid Settling of flocs induces larger landward sediment transport due to upstream flow in the lower layer of density-driven flow, leading to a landward shift and intensification of the ETM (with respect to the case of a constant Settling velocity). In the Hindered Settling (high concentration) regime, the Settling velocity decreases with bottom concentration. This induces a decrease in upstream sediment transport due to density-driven flow and an increase in seaward sediment transport due to river flow, leading to seaward migration and attenuation of the ETM. In both regimes, sediment-induced damping of turbulence results in stronger upstream flow in the bottom layer of density-driven flow and more vertically stratified sediment distribution, which significantly intensifies the landward sediment transport due to density driven flow, and hence causes a landward shift and intensification of the ETM.</p>
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Effects of bottom slope, flocculation and Hindered Settling on the coupled dynamics of currents and suspended sediment in highly turbid estuaries, a simple model
Ocean Dynamics, 2013Co-Authors: Jasper J. A. Donker, Huib E. SwartAbstract:This study aims at gaining basic understanding about two specific phenomena that are observed in the highly turbid estuaries tidal Ouse, Yangtze and Ems, i.e. (1) the accumulation of suspended matter in the deeper parts of the estuaries and (2) the relatively high values of turbidity near the surface in the area of the turbidity maximum. A semi-analytical model is analysed to verify the hypothesis that these phenomena result from bottom slope-induced turbidity currents and from Hindered Settling, respectively. The model governs the dynamics of residual flow, driven by fresh water discharge, salinity gradients and turbidity gradients. It further uses the condition of morphodynamic equilibrium (no divergence of net sediment transport) to compute the residual sediment concentration. New aspects are that depth variations on flow and mixing processes, as well as flocculation and Hindered Settling of sediment, are explicitly accounted for. Tides act as a source of mixing and erosion of sediment only, thus processes like tidal pumping are not considered. Model results show that the estuarine turbidity maximum (ETM) shifts in the down-slope direction, compared to the case of a constant depth. Slope-induced turbidity currents, which are directed down-slope near the bottom and up-slope near the surface, are responsible for this shift, thereby confirming the first part of the hypothesis above. The down-slope shift of the ETM is reduced by currents resulting from gradients in depth-dependent mixing, which counteract turbidity currents, but which are always weaker. Including flocculation and Hindered Settling yields increased surface sediment concentrations in the area of the turbidity maximum, compared to the situation of a constant Settling velocity, thereby supporting the second part of the hypothesis. Sensitivity experiments reveal that the conclusions are not sensitive to the values of the model parameters.
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Effects of bottom slope, flocculation and Hindered Settling on the coupled dynamics of currents and suspended sediment in highly turbid estuaries, a simple model
Ocean Dynamics, 2013Co-Authors: Jasper J. A. Donker, Huib E. SwartAbstract:This study aims at gaining basic understanding about two specific phenomena that are observed in the highly turbid estuaries tidal Ouse, Yangtze and Ems, i.e. (1) the accumulation of suspended matter in the deeper parts of the estuaries and (2) the relatively high values of turbidity near the surface in the area of the turbidity maximum. A semi-analytical model is analysed to verify the hypothesis that these phenomena result from bottom slope-induced turbidity currents and from Hindered Settling, respectively. The model governs the dynamics of residual flow, driven by fresh water discharge, salinity gradients and turbidity gradients. It further uses the condition of morphodynamic equilibrium (no divergence of net sediment transport) to compute the residual sediment concentration. New aspects are that depth variations on flow and mixing processes, as well as flocculation and Hindered Settling of sediment, are explicitly accounted for. Tides act as a source of mixing and erosion of sediment only, thus processes like tidal pumping are not considered. Model results show that the estuarine turbidity maximum (ETM) shifts in the down-slope direction, compared to the case of a constant depth. Slope-induced turbidity currents, which are directed down-slope near the bottom and up-slope near the surface, are responsible for this shift, thereby confirming the first part of the hypothesis above. The down-slope shift of the ETM is reduced by currents resulting from gradients in depth-dependent mixing, which counteract turbidity currents, but which are always weaker. Including flocculation and Hindered Settling yields increased surface sediment concentrations in the area of the turbidity maximum, compared to the situation of a constant Settling velocity, thereby supporting the second part of the hypothesis. Sensitivity experiments reveal that the conclusions are not sensitive to the values of the model parameters.