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The present experimental research looks for ways to improve fine sediment transport measurements with ADV by further investigating the factors influencing the backscatter intensity of the acoustic wave. The optimal instrument settings are determined in order to maximally extend the region of increasing response to fine suspended sediments in the sampling volume and a spectral correction is used to obtain turbulent sediment concentration fluctuations.

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Sediment transport models are helpful tools to study the dispersion of contaminants attached to fine-grained sediments in the marine environment. Currently used 3D sediment transport models still fail to make good quantitative predictions. Several causes can be attributed to the inadequate description of physical processes which occur at the subgrid scale level. Specific attention is given to the modelling of turbulent mixing, particle turbulence interactions and near-bottom boundary layer processes. A procedure for the development of new subgrid scale closures for engineering models applied to problems at geophysical scales of coastal waters and estuaries, based on two-phase flow theory and data generated by Large Eddy Simulation and low-Reynolds RANS models, is described. This leads to a three-layer approach, comprising a supersaturated near-bed layer, a transition layer (under certain conditions) and the fully developed turbulent water column. ispartof: pages:1-8 ispartof: Qualité des Eaux Marines pages:1-8 ispartof: Colloque SHF Pollution Maritime location:Paris (France) date:22 Jan - 23 Jan 2007 status: published

Sediment transport due to fluid motion is a crucial process in many environmental and engineered systems. Therefore, understanding sediment transport is critical for predicting sediment movements and evaluating the short and/or long-term influence to the surface water systems. Despite the importance of sediment transport, the fundamental aspects involved are far from being completely understood. At the core of the problem is the complex interaction between a turbulent flow field and sediment particles. In this sense, for sediment transport modelling, how to account for as many turbulence generation or destruction mechanisms as possible is one of the keys to improving the accuracy of prediction. The most widely used and validated model for turbulence is two-equation k-ε model. However, the traditional k-ε model cannot provide accurate predictions when considering the presence of sediment in water due to missing the terms in the equations that can account for energy transfer between sediments and turbulence, as well as the terms for inter-particle interactions. Moreover, the standard k-ε model is only valid for fully developed turbulent flow, so it often has difficulties to deal with near-bottom layer (e.g. low-Reynolds effects and high sediment concentrations). Therefore, an alternative approach to overcome these difficulties is two-phase flow theory. In the first part of this study, a modified two-equation k-ε model with additional turbulence modulation terms, accounting for the influence of particles on the turbulent flow field, has been proposed. These extra terms are derived using a two-phase flow approach. In the numerical tests, the modified two-phase k-ε model reproduces the features of turbulence modulation observed in the experiments on sediment-laden flow. Another important aspect in sediment transport is the treatment of the near-bottom layer. It has been hypothesized for more than a decade that currently used sediment transport models for morphodynamic studies (e.g., harbour siltation, system response to dredging or structures) can be improved considerably in their predicting capacity when the near-bottom high-concentration effects in the last few centimetres above the bed can be accounted for with a physics-based model. Indeed, experimental observations and theoretical considerations (partially based on two-phase micro-scale models) show that the flow and suspension capacity of the water column are strongly affected by the processes of particle-fluid interactions in the benthic layer. Hence, the second part of this study focusses on the efficient modelling of near-bottom sediment transport in morphodynamic modelling codes through an innovative methodology. In this part, the performance of a few new physics-based process models has been investigated by implementation into a numerical model for the simulation of the flow and morphodynamics in the Western Scheldt estuary. In order to deal with the complexity within the research domain, and improve the prediction accuracy, a two-dimensional (2D) depth-averaged model has been set up as realistic as possible, i.e. including two-way hydrodynamic-sediment transport coupling, mixed sand–mud sediment transport (bedload transport as well as suspended load in the water column) and a dynamic non-uniform bed composition. A newly developed bottom friction law, based on a generalised mixing-length (GML) theory, is implemented, with which the new bed shear stress closure is constructed as the superposition of the turbulent and the laminar contribution. It allows the simulation of all turbulence conditions (fully developed turbulence, from hydraulic rough to hydraulic smooth, transient and laminar), and the drying and wetting of intertidal flats can now be modelled without specifying an inundation threshold. Erosion and deposition in these areas can now be estimated with much higher accuracy, as well as their contribution to the overall net fluxes. Furthermore, Krone’s deposition law has been adapted to sand–mud mixtures, and the critical stresses for deposition are computed from suspension capacity theory, instead of being tuned. The model has been calibrated and the results show considerable differences in sediment fluxes, compared to a traditional approach and the analysis reveals that the concentration effects play a very important role. The new bottom friction law with concentration effects can considerably alter the total sediment flux in the estuary, not only in terms of magnitude, but also in terms of erosion and deposition patterns. nrpages: 246 status: published

The overall objective is to improve the one-dimensional numerical prediction of the fine and non-cohesive bed material load in alluvial rivers, especially during high intensity episodes during which sediment beds are strongly remobilized. For this reason, we attempt to reduce the major inaccuracy sources coming from the alluvial resistance and bed material load relations needed to close the mathematical system. Through a shared parameter called the control factor m, the interactions occurring in alluvial rivers are incorporated more deeply into the mathematical model and more particularly into the closure laws: bed material load (SVRD, Suction-Vortex Resuspension Dynamics) and the energy slope (Verbanck et al. 2007). The control factor m is assumedly related to the Rossiter resonance modes of the separated flow downstream the bed form crest. To further improve the representation of the flow-sediment-morphology interactions, a fully coupled model approach has been naturally chosen. In this work the terminology fully coupled means that the three equations forming the system are solved synchronously and that the terms often neglected by more traditional decoupled models are kept. The feasibility of the new closure methodology has been drawn up by reproducing numerically the silt-flushing experiment conducted by the Yellow River Conservancy Commission (Y.R.C.C.) in the Lower Yellow River (LYR) in Northern China. The objective of the silt flushing experiment is to reverse the aggradation trend of the Lower Yellow River which, in the last decades, has become a perched river. The numerical simulation specifically reproduces the silt-flush effects in a reach of LYR located in the meandering part of the river. This reach (around 100 km) is delimited by Aishan and Luokou hydrometric stations.Since the SVRD formulation has been developed from flume observations, the law has first been confronted to river datasets. The confrontation has revealed that the SVRD law becomes less suitable for fine sediment fluxes (ratio of water depth over median particle size > 5000). Therefore, a modified equation SVRD-2 has been built to enlarge the validity range.The suitability of the SVRD-2 equation to predict fine sediment fluxes has been tested on data available from several hydrometric stations located in the meandering reach of the LYR: historical observations and measures collected during the flushes. The SVRD-2 has also been compared with relations specifically calibrated for this configuration. The comparison has pointed out that the performance of the two formulas is similar, which is encouraging for the SVRD-2 approach as it has not been calibrated on those data. The closed equation system has been written on its quasi-linear form and is solved by a Finite Volume Method combined with a linearized Riemann algorithm. The numerical model has been checked up on two test cases: deposition upstream of a dam and the aggradation experiment conducted by Soni 1975. As it is not yet possible to predict dynamically the value of the control factor m, a possible solution would be to extract its value from the measured data at the inlet cross section. Unfortunately, the necessary data are not measured locally. Moreover, a uniform value of the control factor m may not suffice to reproduce the flow along the whole reach. Therefore, it has been proposed to work temporarily in the reverse way. From the comparison between the numerical results and the experimental data, a time evolution of the control factor m has effectively been extracted and it has been shown that it varies along the reach. At Aishan, the evolution of the control factor m corresponds to the evolution expected from the data analysis previously conducted on other data sets: the value of the control factor m decreases during the flush as it tries to reach the optimal value m=1. The time evolution at Luokou behaves differently to the one at Aishan, but remains in agreement with m evolution patterns observed historically for the river section flowing round Jinan City walls. For Luokou, the highlighted differences may come from three dimensional effects coming from the meander bend upstream the station.Generally, the results obtained for the hydraulics, the sediment transport and bed adaptation are encouraging but still need improvements and additional feeding from the experimental data. The results for the concentration and therefore the bed elevation are very sensitive to the value of the control factor m as it influences most of the terms of the bed material load equation (SVRD-2). The major remaining difficulties are, firstly, to deal with the rapid transients for which the model is less suitable and, secondly, to improve the prediction of the value of control factor m. Before paying more attention into the transients, enhancements concerning the flow along the reach (initial condition and discharge rates during the first days of the flush) must be conducted in priority. Indeed as the prediction of the bed or the cross section evolutions depend directly on the quality of the prediction of the sediment concentration and the hydraulics, one should first improve these aspects. To perform this study, more information about the water levels or sediment concentrations is necessary at some intermediate stations. One solution is to lengthen the studied reach, upstream to Sunkou and downstream to Lijin, totaling a river length of 456 Km.A more entire signal of the energy slopes and the associated bed configurations at different stations would enlighten how the control factor m evolves along the reach during the silt-flush events., Doctorat en Sciences de l'ingénieur, info:eu-repo/semantics/nonPublished