Scale problems in 3D sediment transport models and suggestions to overcome them (original) (raw)
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Improvements in 3D sediment transport modelling with application to water quality issues
2014
A new vertical scheme has been developed to represent suspended sediment transport processes in the TELEMAC-3D model. In comparison to the existing diffusion scheme, the newly developed advection/diffusion scheme is proved to be more robust. Three large scale test cases have been provided to assess the model accuracy in the presence of tidal flats and its ability to cope with distorted mesh elements. Finally, the 3D model has been applied to represent cohesive sediment transport processes and associated water quality issues in estuarine conditions. I.
Large-scale modelling of fine-grained sediment transport: Can we do any better?
XIIèmes Journées, Cherbourg, 2012
Numerical models have become a major tool for the study of sediment transport problems in coastal engineering. Reviewing the evolution of model progress, it is noticed that advancement has mainly been achieved in higher computational performance, allowing more detailed and even 3D simulation of large-scale problems at a reasonable cost. However, the prediction capacity remained disappointing and has hardly improved since the models are still based on the same basic process descriptions. Over the past 20 years many processes have been studied in more detail. Nevertheless, attempts to incorporate more complicated models for processes such as flocculation and erosion have not really brought the expected improvement. This paper presents an overview of the major shortcomings in presently used sediment transport software. Proposals to incorporate more physics, without increasing the computational cost too excessively, will be presented. New process models have been developed for flocculation and for particle-turbulence interaction in high-concentrated suspensions, and a new bed model, accounting for both consolidation and wave-induced fluidization, is under development. Furthermore, it is demonstrated that estuarine and coastal applications require the distinction between (at least) two floc populations of cohesive sediment and (at least) one sand fraction. Remaining problems can be related to the difficulty to account for the spatial and temporal variability of sediment properties, especially with regard to the bed, and to the effect of bio-engineering agents in the environment. This keeps large uncertainties in the model results which can hardly be reduced.
SEDDEER: A Sediment Transport Model for Water Quality Modeling
Transactions of the ASABE, 2012
In this study, a stand-alone sediment and contaminant model, SEDDEER (Sediment Deposition and Erosion), is presented. The model simulates one water box and underlying (multiple) sediment bed layers, and was developed for incorporation into water quality models. The processes incorporated in the model are: settling and deposition, resuspension, equilibrium adsorption/desorption, sediment-water interface diffusion, volatilization and decay, sediment bed, and consolidation. In contrast to other models, SEDDEER simulates multiple sediment classes, includes a flocculation model, and considers both cohesive and noncohesive sediments. Comparisons of SEDDEER-simulated results with analytical solutions, experimental data from other studies, and simulation results from another model, EFDC (Environmental Fluid Dynamic Code), were performed. The SEDDEER algorithm for flocculation settling was successfully verified for mass conservation of both sediment and contaminant. The test for estimating settling/deposition revealed that for shear stresses of 0.00 and 0.05 Pa, the coefficient of determination (R 2 ) ranged from 0.98 to 1.00. SEDDEER's contaminant transport algorithm was tested against reported measured data of dichlorodiphenyldichloroethylene (DDE). SEDDEER-calculated output for the water column and sediment bed fit the measured data with R 2 = 0.94 and R 2 = 0.74, respectively. Lindane (γhexachlorocyclohexane) field data and SEDDEER output for contaminant transport also agreed well (R 2 = 0.85).
Hydrology and Earth System Sciences
Hydromorphodynamic models are powerful tools for predicting the potential mobilization and transport of sediment in river ecosystems. Recent studies have shown that they are able to predict suspended sediment matter concentration in small river systems satisfactorily. However, hydrosedimentary modelling exercises often neglect suspended sediment properties (e.g. sediment densities and grain-size distribution), which are known to directly control sediment dynamics in the water column during flood events. The main objective of this study is to assess whether a better representation of such properties leads to an improved performance in the model. The modelling approach utilizes a fully coupled hydromorphodynamic model based on TELEMAC-3D (v7p1) and an enhanced version of the sediment transport module SISYPHE (based on v7p1), which allows for a refined sediment representation (i.e. 10-class sediment mixtures instead of 2-class mixtures and distributed sediment density instead of uniform). The proposed developments of the SISYPHE model enable us to evaluate and discuss the added value of sediment representation refinement for improving sediment transport and riverbed evolution predictions. To this end, we used several model setups to evaluate the influence of sediment grain-size distribution, sediment density, and suspended sediment concentration at the upstream boundary on model predictions. As a test case, we simulated a flood event in a small-scale river, the Orne river in northeastern France. Depending on the model setup , the results show substantial discrepancies in terms of simulated bathymetry evolutions. Moreover, the model based on an enhanced configuration of the sediment grain-size distribution (10 classes of particle sizes) and with distinct densities per class outperforms the standard SISYPHE configuration, with only two sediment grain-size classes, in terms of simulated suspended sediment concentration.
Um modelo morfodinâmico de transporte de sedimentos coesivos
2017
The dynamics of cohesive sediments in natural water bodies is of great importance for coastal engineering. Quite often, it is necessary to assess sedimentological processes in regions where cohesive sediments are prevalent, such as in many harbors. Such assessments poses challenges, especially in quantitative evaluations, as the physics involved in the transport, sedimentation and erosion of cohesive sediments is rather complex. For that, by all means, it is important to have analysis tools capable of computing cohesive sediments dynamics. This paper presents the development of a vertically averaged eulerian transport model for suspended cohesive sediments. Such model is currently available on SisBaHiA, which is a computational modeling system developed and maintained in COPPE/UFRJ. After describing the model implementation, we present a discussion on consistency and validation tests, for each implemented mechanism within the sedimentological process, with a focus in the erosion an...
Development of a Two-Dimensional Hybrid Sediment-Transport Model
Applied Sciences
This paper presents the development of a two-dimensional hydrodynamic sediment transport model using the finite volume method based on a collocated unstructured hybrid-mesh system consisting of triangular and quadrilateral cells. The model is a single-phase nonequilibrium sediment-transport model for nonuniform and noncohesive sediments in unsteady turbulent flows that considers multiple sediment-transport processes such as deposition, erosion, transport, and bed sorting. This model features a hybrid unstructured mesh system for easy mesh generation in complex domains. To avoid interpolation from vertices in conventional unstructured models, this model adopted a second-order accurate edge-gradient evaluation method to consider the mesh irregularities based on Taylor’s series expansion. In addition, the multipoint momentum interpolation corrections were integrated to avoid possible nonphysical oscillations during the wetting-and-drying process, common in unsteady sediment transport p...
Comparison of sediment transport formulae for the coastal environment
Coastal Engineering, 2003
Most existing sediment transport formulae to estimate transport rate in the coastal environment have a restricted range of applicability and are often used beyond this range. The aim of this paper is to investigate the limits of five of these formulae: the Bijker, Bailard, Van Rijn, Dibajnia and Watanabe, and Ribberink formulae. The sensitivity of these formulae to wave orbital velocity, wave period, wave asymmetry, sediment grain size, and steady current has been studied and tested against data for large velocities where significant errors can appear. The formulae behave in very different ways if one of the main parameters is slightly modified, particularly when fine sediments are present and phase-lag effect appears. But important discrepancies between formulae can also be observed for medium sand. At last, the wave-related sediment transport (due to wave asymmetry) has great importance for the morphodynamic and is only accounted for in the Bailard, Dibajnia and Watanabe, and Ribberink formulae. D
Analytical model for flux saturation in sediment transport
Physical Review E, 2014
The transport of sediment by a fluid along the surface is responsible for dune formation, dust entrainment and for a rich diversity of patterns on the bottom of oceans, rivers, and planetary surfaces. Most previous models of sediment transport have focused on the equilibrium (or saturated) particle flux. However, the morphodynamics of sediment landscapes emerging due to surface transport of sediment is controlled by situations out-of-equilibrium. In particular, it is controlled by the saturation length characterizing the distance it takes for the particle flux to reach a new equilibrium after a change in flow conditions. The saturation of mass density of particles entrained into transport and the relaxation of particle and fluid velocities constitute the main relevant relaxation mechanisms leading to saturation of the sediment flux. Here we present a theoretical model for sediment transport which, for the first time, accounts for both these relaxation mechanisms and for the different types of sediment entrainment prevailing under different environmental conditions. Our analytical treatment allows us to derive a closed expression for the saturation length of sediment flux, which is general and can thus be applied under different physical conditions.