Search

This study focuses on a holistic modeling approach, in which water, fluid mud, and immobile mud are all calculated by only one set of equations. To integrate the immobile mud into this concept, a holistic transport equation including sediment transport and consolidation is developed. In some estuaries, extensive deepening and dredging resulted in tidal deformation and sediment import to such extent, that hyper‐turbid conditions developed. Recent measurements from the Ems estuary show that the locations of interfaces between water, fluid mud, and consolidated mud vary during a tidal cycle. Conditions are varying from fully mixed to stably stratified. As a suitable case study for the holistic model, a 1D vertical numerical simulation of the Ems has been set up, which is able to qualitatively reproduce the observed vertical velocity, concentration, and velocity shear profile. The simulation shows mud‐induced periodic stratification. Plain Language Summary: This study describes a 1D hydrodynamic‐numerical model, which is called holistic as water, fluid mud, and immobile bed are solved by only one set of equations. Many estuaries are deepened and straightened to enable passage for ever‐larger ships. In some estuaries, those artificial changes heavily influence the flow behavior and caused a shift to very high sediment concentrations resulting in thick layers of fluid mud. The presence of mud alters the flow‐ and turbulence behavior compared to pure water and subsequently must be included into the modeling equations. The model includes the immobile mud, where flow velocities and turbulence are zero. This is possible with a rearranged consolidation equation and the adaption of model parameters to be valid in cases of very high sediment concentration. Recent measurements from the Ems estuary show that the vertical concentration profile in the estuary strongly varies with time during one tidal cycle: There are phases of a stable fluid mud layer and phases of fully mixed conditions where no mud layer is present. As a possible application of the holistic model, a 1D vertical numerical simulation of the Ems estuary has been set up. The simulation results are in good agreement with field observations. Key Points: A holistic sediment transport equation including sediment transport and consolidation is developedThe rheological viscosity, the settling velocity, and a k‐ω turbulence model are adapted to be valid in the region of the immobile mudA 1D vertical simulation of the holistic model is set up to simulate the flow and sediment processes in hyper‐turbid estuaries [ABSTRACT FROM AUTHOR]

Zhang, H.; Yang, H., and Jeng, D.-S., 2015. Contaminant transport in capped deformable partially saturated sediments. Contaminated sediment tends to consolidate in response to the loading imposed by an overlying cap layer. The form of transient solute advection through sediments and caps is consequently changed. Therefore, the contaminant discharge into the overlying water is expected to increase and the effectiveness of the in situ capping system will be strongly affected. In this study, a one-dimensional sediment-cap model coupling consolidation and advection-dispersion equation was established. A simplified model was proposed through the sensitivity analysis, and then a parametric study was conducted. Numerical examples concluded that the absorption, degree of sediment saturation, cap thickness, and sediment depth significantly affect the contaminant discharge from the top of the cap. Therefore, these parameters should be considered carefully during cap design. [ABSTRACT FROM AUTHOR]

Sediment transport modelling in estuarine environments, characterised by cohesive and non-cohesive sediment mixtures, has to consider a time variation of erodibility due to consolidation. Generally, validated by settling column experiments, mud consolidation is now fairly well simulated; however, numerical models still have difficulty to simulate accurately the sedimentation and consolidation of mixed sediments for a wide range of initial conditions. This is partly due to the difficulty to formulate the contribution of sand in the hindered settling regime when segregation does not clearly occur. Based on extensive settling experiments with mud-sand mixtures, the objective of this study was to improve the numerical modelling of mixed-sediment consolidation by focusing on segregation processes. We used constitutive relationships following the fractal theory associated with a new segregation formulation based on the relative mud concentration. Using specific sets of parameters calibrated for each test-with different initial sediment concentration and sand content-the model achieved excellent prediction skills for simulating sediment height evolutions and concentration vertical profiles. It highlighted the model capacity to simulate properly the segregation occurrence for mud-sand mixtures characterised by a wide range of initial conditions. Nevertheless, calibration parameters varied significantly, as the fractal number ranged from 2.64 to 2.77. This study investigated the relevance of using a common set of parameters, which is generally required for 3D sediment transport modelling. Simulations were less accurate but remained satisfactory in an operational approach. Finally, a specific formulation for natural estuarine environments was proposed, simulating correctly the sedimentation-consolidation processes of mud-sand mixtures through 3D sediment transport modelling. [ABSTRACT FROM AUTHOR]

A one-dimensional model has been developed to simulate mixed cohesive and non-cohesive sediment transport in open channels. Flocculation is considered by determining the floc settling velocity as function of sediment size, concentration, and bed shear stress. The critical shear stress of cohesive sediment erosion is related to the dry bed density that varies with time due to consolidation, and the critical shear stress of non-cohesive sediments is corrected when the cohesive fraction exceeds a certain limit such as 10%. Cohesive and non-cohesive sediment transport, bed change and bed-material sorting equations are solved in a coupled form. The model is tested using two field cases of mixed sediment transport in the Lower Fox River in USA and the Three Gorges Reservoir in China. The calculated results agree generally well with measured data. [ABSTRACT FROM PUBLISHER]

A one-dimensional vertical two-phase flow model for sedimentation–consolidation process is presented. The model is based on solving the continuity and momentum equations for both fluid and solid phases. In the non-cohesive case, the momentum transfer between the two phases is reduced to the drag force around a single particle modified to take the hindrance effects into account. In the cohesive case, Darcy–Gersevanov's law is used for the closure of the momentum transfer between the two phases and the concept of “effective stress” is introduced to take into account the bed structuring. These closure laws are validated against high-resolution experimental data in terms of settling curves and concentration profiles. The reliability of the model is illustrated from an analysis of the momentum balances at different stages during the process. Finally, the proposed closure laws and numerical algorithms are shown to be able to quantitatively reproduce sedimentation of non-cohesive and sedimentation–consolidation of cohesive sediments, including mud. [ABSTRACT FROM AUTHOR]

Abstract: Physical properties of the deposits in the DYNAS study area, the Mecklenburg Bay, were investigated using sediment echosounders and laboratory analysis were carried out on undisturbed short sediment cores. Wet bulk densities of about 1.2 g/cm3 for mud and up to 1.9 g/cm3 for silty sand were found in surface sediments of the Mecklenburg Bay. Sediment density–depth functions were approximated by logarithmic regression functions at different depth intervals. Sediment consolidation was studied by both (i) consolidation tests of sediment samples and (ii) from the void ratio–overburden pressure relation in natural sediments. Low shear strength values of 9–71 Pa were measured at the mud surface. Downcore, a depth gradient of about 14.5 Pa/cm was calculated. Sediments with high silt and sand contents are characterized by shear strength values of up to 3000 Pa. Published formulas derived from erosion studies were used to calculate the critical shear stress using wet bulk density and shear strength. The obtained results demonstrate clearly, that there is still a wide gap in knowledge about the relationships between erosion parameters and sediment physical properties. [Copyright &y& Elsevier]

Abstract: Erosion and deposition of bottom sediments reflect a continual, dynamic adjustment between the fluid forces applied to a sediment bed and the condition of the bed itself. Erosion of fine and mixed sediment beds depends on their composition, their vertical structure, their disturbance/recovery history, and the biota that inhabit them. This paper presents a new one-dimensional (1D), multi-layer sediment bed model for simulating erosion and deposition of fine and mixed sediments subject to consolidation, armoring, and bioturbation. The distinguishing characteristics of this model are a greatly simplified first-order relaxation treatment for consolidation, a mud erosion formulation that adapts to both Type I and II erosion behavior and is based directly on observations, a continuous deposition formulation for mud that can mimic exclusive erosion and deposition behavior, and straightforward inclusion of bioturbation effects. Very good agreement with two laboratory data sets on consolidation effects is achieved by adjusting only the first-order consolidation rate rc . Full model simulations of three idealized cases based on upper Chesapeake Bay, USA observations are presented. In the mud only case, fluid stresses match mud critical stresses at maximum erosion. A consolidation lag results in higher suspended sediment concentrations after erosional events. Erosion occurs only during accelerating currents and deposition does not occur until just before slack water. In the mixed mud and sand case without bioturbation, distinct layers of high and low sand content form and mud suspension is strongly limited by sand armoring. In the mixed mud and sand case with bioturbation, suspended mud concentrations are greater than or equal to either of the other cases. Low surface critical stresses are mixed down into the bed, constrained by the tendency to return towards equilibrium. Sand layers and the potential for armoring of the bed develop briefly, but mix rapidly. This model offers a relatively simple and robust tool for simulating the complex interactions that can affect muddy and mixed sediment bed erodibility. [Copyright &y& Elsevier]

An overview of cohesive sediment transport processes is given in this paper, and a mass-balance equation that is commonly used to treat cohesive sediment transport is reviewed. The equation highlights transport parameters and processes that are important for modelling the transport of cohesive sediment. The flocculation mechanism that distinguishes cohesive sediment from its noncohesive counterpart is elaborated using a laboratory study that was carried out in a rotating circular flume using sediments from Hay River, Northwest Territories, Canada. A mathematical model of flocculation suitable for predicting flocculation of sediment in rotating circular flumes is reviewed. Other cohesive sediment transport processes such as erosion and deposition processes at the sediment-water interface, entrapment of fines in gravel beds, consolidation, fluid mud, and fluidization due to wave action are reviewed. Additional challenges and knowledge gaps that exist in the area of cohesive sediment transport are identified. [ABSTRACT FROM AUTHOR]

Abstract: The stabilising effects of natural benthic diatom and bacterial assemblages on cohesive sediments were compared with those caused by physico-chemical binding alone. Cohesive sediment beds were reconstructed in 4 annular laboratory miniflumes, using sediment collected at 5–6m water depth from a local fjord. The sediment was left to stabilise (consolidate) for 1, 2, 5 and 10 days, before being fully resuspended in a series of erosion experiments. The flumes were aerated and subjected to different light/dark conditions; antibiotics were used to isolate diatom from bacteria effects. During consolidation, a constant current velocity was maintained, at a speed well below erosion threshold. ‘Natural’ sediment regained 27% of its original stability after 1 day, and 85% after 5 days. Complete ‘natural’ sediment stability was regained within 10 days. Benthic diatoms (mainly Nitzchia sp., Gyrosigma sp. and Pennales sp.) were responsible for about 80% of the biostabilisation, whilst bacteria contributed ∼12%, indicating the importance of light as a controlling factor for surface sediment stability. Relative to physico-chemical binding, the increase in erosion threshold induced by benthic diatoms was 120%, and by bacteria 20%. Where the assemblage consisted of both diatoms and bacteria, the increase was 150%, indicating that natural bed values are not a simple summation of the two effects, i.e. diatoms plus bacteria. Using the results, a first generation empirical relationship describing the relative contributions of microbenthic and physico-chemical factors in the development of erosion threshold is presented. Empirical relationships containing biostabilisation coefficients are important for better computer model predictions of sediment transport. [Copyright &y& Elsevier]

Abstract: Mixed sediments are constituted of cohesive and non-cohesive materials with distinct behaviours that numerical models traditionally manage separately. This paper first introduces a rapid state of the Art in sediment transport modelling in order to point out the specific requirements for process-based models applied to mixed sediments. Based on a preliminary study by , which showed the validity of the advection approach to compute fine sand transport, a complete modelling strategy is described: it is applied to the suspended transport of sand and mud mixtures, and accounts for consolidation of mixed sediments. Special care is paid to the realistic representation of the structure and density of sand and mud mixtures, and to the segregation in consolidating sediment layers. The model state variables are the different classes of particles, generally classified according to their size, and grouped into categories that are either transported as bedload or in suspension. The bed is described as thin layers characterised by a distribution of these classes. The erosion law for fine sands and for sand and mud mixtures is a function of the excess shear stress calibrated against measurements in a small flume. The transition between cohesive and non-cohesive behaviours is parameterised through a critical mud fraction that depends on the sand grain size: the coarser the sand, the higher the mud content before the sediment becomes cohesive. The consolidation module is based on Gibson equation formulated for each class, and modified to account for segregation. Constitutive relationships are calibrated by means of laboratory settling tests. In the deposition module, new deposits may be managed in different ways (creation of a new layer or integration into the existing surficial layer) depending on the mud fraction and its relative concentration. When deposited material is mixed with the surficial sediment, pores between coarser particles are first filled up with finer particles before increasing the layer thickness. The new modelling frame has first been used to simulate laboratory settling tests with mixed sediments. When the initial mixture density is low, sand particles can settle through the mud and form a dense sandy layer on the bottom. In a second application, the model is used to describe sorting processes when tidal currents re-suspend a sand and mud mixture. A sand layer is then likely to form within the sediment, while the surficial layers are muddier. A dynamic bed armouring process is shown: although sand is easily resuspended, eroded grains in the sand layer settle rapidly, reducing the erosion of underlying sediment. Resulting suspended sediment concentration is strongly reduced, as well as sediment fluxes. The application demonstrates the model ability to simulate layering processes and time-variations of sediment erodibility. [Copyright &y& Elsevier]