A travers cette thèse, nous avons cherché à développer de nouvelles méthodes de mesure, basées sur la rétrodiffusion acoustique, pour estimer la concentration massique des sédiments en suspension dans les rivières. Souvent, ces sédiments présentent une distribution granulométrique bimodale, i.e. constituée d’un mélange de sédiments fins et de sable. Le principal avantage des méthodes hydroacoustiques est leur capacité à fournir des mesures avec une résolution spatiale et temporelle bien meilleure que les techniques classiques type prélèvement. L’objectif est in fine d’améliorer l’estimation du flux sédimentaire dans les cours d’eau. Des mesures acoustiques multifréquences, associées à des prélèvements physiques, ont été réalisées sur le terrain et dans une cuve expérimentale au laboratoire. Des méthodes d’inversion du signal acoustique ont été testées et développées au laboratoire sur une suspension homogène de sédiments fins. Une nouvelle méthode en particulier, combinant analyse de la rétrodiffusion et de l’atténuation acoustique, a permis de retrouver la concentration massique des sédiments avec une précision de l’ordre de ± 20 %. En rivière, une méthode mixte alliant données de calibration et inversion du signal acoustique à deux fréquences a été développée, permettant, dans certaines conditions, d’estimer la concentration des sédiments fins et du sable sur l’ensemble de la section en travers d’un cours d’eau. Ce résultat confirme la capacité de la technologie hydroacoustique à fournir une information spatiale sur la suspension. Des écarts parfois importants ont été observés entre la réponse acoustique théorique, calculée à partir des données de concentration et de granulométrie, et les mesures acoustiques sur le terrain. Il semble que ces écarts soient dus à la présence d’autres corps diffusants dans les rivières, probablement des flocs et/ou des micro-bulles d’air. Ces travaux appellent au développement d’un cadre théorique plus performant et adapté aux suspensions rencontrées en rivière. / With this PhD, we have tried to develop new measurement methods, based on acoustic backscattering, to estimate the mass concentration of suspended sediments in rivers. These sediments often show a bimodal grain-size distribution, i.e. composed of a mixture of fine and sand particles. The main advantage of hydroacoustic methods is their ability to provide measurements with a much better spatial and temporal resolution than conventional sampling techniques. The ultimate goal is to improve the sediment load estimation in rivers. Multifrequency acoustic measurements, combined with physical sampling, were carried out in the field and in an experimental laboratory tank. Acoustic inversion methods were tested and developed on a homogeneous suspension of fine sediments in the laboratory. A new method was implemented, combining the analysis of acoustic backscatter and attenuation, and led to retrieve the sediment mass concentration with a precision in the order of ± 20%. In rivers, a semi-empirical method combining calibration data and acoustic inversion at two frequencies has been developed, allowing, under certain conditions, to estimate the concentration of fine and sand sediments throughout the entire river cross-section. This result confirms the ability of hydroacoustic technology to provide spatial information on the suspension. Significant differences were frequently observed between the theoretical acoustic response, computed from concentration and particle size data, and the acoustic measurements in rivers. It seems that these differences could be due to the presence of other scatterers in rivers, probably flocs and/or air micro-bubbles. This work calls for the development of a more efficient theoretical framework suitable for river suspensions.

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2016-02-23T14:12:41Z No. of bitstreams: 2 Dissertação - Wanderlubio Barbosa Gentil - 2015.pdf: 4515031 bytes, checksum: 78402e1eba98343039253c31f031e84e (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2016-02-23T14:14:09Z (GMT) No. of bitstreams: 2 Dissertação - Wanderlubio Barbosa Gentil - 2015.pdf: 4515031 bytes, checksum: 78402e1eba98343039253c31f031e84e (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2016-02-23T14:14:09Z (GMT). No. of bitstreams: 2 Dissertação - Wanderlubio Barbosa Gentil - 2015.pdf: 4515031 bytes, checksum: 78402e1eba98343039253c31f031e84e (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2015-08-14 / The sedimentation in reservoir occurs due to the deposition of particles which are named sediments and are carried through the flow. It’s a process that will occur anyway, however it’s likely to be controlled. Whatever the purpose of the reservoir is, we can verify that the characteristics of the operation can be compromised over time, with its capacity of storage being partial or totally taken by sediments due to a series of variables. Due to these processes, the sedimentation in reservoirs is a very relevant subject given that Brazil is a country which has a strong dependency on the energy generated through hydropower and, therefore, has built several reservoirs. The construction of a dam can bring several socioeconomic and environmental impacts, among them the sedimentation, which appears as a problem that affects the volume and capacity of generating energy in reservoirs. The present research has as main objective the evaluation of some steps of the sedimentation process of Caçu’s Hydroelectric Power plant reservoir (Usina Hidrelétrica Caçu), where a study of silting behavior was developed relating the water speed to the suspended sediments and the bathymetry of the bottom. We were able to find that the places where there was the greatest sediments accumulation was in the section between the end of the zone classified as river and the beginning of the transition zone and that this accumulation occurred according to the speed alterations, when the water flux loses its strength and the sediments are deposited on the river bed. This study allowed us to verify, with the bathymetry, the changes in the silting and, with the mapping of the speed, the reservoir’s longitudinal compartmentalization as well as the most likely places of accumulation. / A sedimentação em reservatório acontece em função da deposição de partículas que são chamadas de sedimentos e que são carreadas por meio do escoamento. É um processo que vai ocorrer de qualquer forma; entretanto, é passível de ser controlado. Qualquer que seja a finalidade do reservatório verifica-se que as características de operação podem ser comprometidas ao longo do tempo, sendo sua capacidade de armazenamento parcial ou totalmente tomada por sedimentos em virtude de uma série de variáveis. Em função disso, a sedimentação em reservatórios é um assunto muito relevante, tendo em vista que o Brasil é um País que tem forte dependência na energia gerada por meio de hidroeletricidade e, portanto, com vários reservatórios construídos. A construção de uma barragem pode trazer vários impactos socioeconômicos e ambientais, dentre eles, a sedimentação, que aparece como um problema que afeta o volume e capacidade de geração de energia em reservatórios. A presente pesquisa tem como principal enfoque a avaliação de algumas etapas do processo de sedimentação do reservatório da Usina Hidrelétrica de Caçu, onde se desenvolveu um estudo do comportamento do assoreamento relacionando a velocidade da água aos sedimentos em suspensão e à batimetria do fundo. Constatou-se que os locais onde houve o maior acúmulo de sedimentos foi no trecho compreendido entre o final da zona classificada como rio e o início da zona de transição e que esse acúmulo ocorreu conforme as alterações de velocidade, quando o fluxo de água perde a força e os sedimentos são depositados no leito. O estudo permitiu constatar, com a batimetria, as alterações no assoreamento e, com o mapeamento da velocidade, a compartimentação longitudinal do reservatório, bem como os locais mais propensos ao acúmulo.

Thesis (Ph. D.)--Mississippi State University. Department of Computational Engineering. / Title from title screen. Includes bibliographical references.

published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

Particle-laden jets are an important type of multiphase flow which can be found in various natural and technical processes. This study focuses on the flow mechanisms in a horizontally discharging sediment-laden jet that is of particular interest in environmental science and engineering. Experimental techniques and mathematical models are developed to investigate horizontal sediment-laden jets, both for the buoyant and non-buoyant jet discharge cases. In the laboratory, the separation of images of the fluid and the particulate phases is achieved by harnessing light signals of visualization at different wavelengths. Whole field measurements of velocities of the two phases are made by the adoption of particle image velocimetry (PIV) algorithms. Numerical models are developed in two approaches with regard to the treatment of the particulate phase. In the Lagrangian approach, individual sediment particles are tracked while the flow field of the fluid phase is computed with large-eddy simulation (LES). This simulation successfully captures the transient nature of the particle-laden flow. In the Eulerian approach, a two-phase model is used to obtain steady flow simulations in a much shorter computation time. The experimental and numerical results for the horizontal momentum jets show that, at low initial particle concentrations, the sediment particles generally follow the jet flow but with some levels of deficit velocities. In the upper layer of the jet the particles do not follow the fluid flow as well as in its lower layer. More particles are observed in the lower layer than in the upper one. For the momentum-dominated zone of a horizontal buoyant jet, the flow exhibits similar behaviors as the horizontal particle-laden momentum jet, except that there are some slight modifications from the effects of buoyancy. In the bending zone of the buoyant jet, the effects of buoyancy become significant. Notably, the locations of maximum velocity magnitude and those of maximum turbulence intensity are well separated in this zone. A strong correlation of particle abundance and high turbulence intensity is observed in the lower outer jet layer in this bending zone. Significant modifications to the global behaviors of horizontal sediment jets are observed as the particle concentration increases to relatively high levels. The jet trajectories are brought downwards by the particle loads and the jet widths are also increased. For the flow regime being investigated, turbulence intensity in the fluid flow is found to be increased by the presence of sediment particles. The results suggest that turbulence helps suspend sediment particles in horizontally discharging jets. A Stokes number is proposed to represent the ability of particles to follow the fluid flow. It is defined as St=W_s/U_j , where ws is the particle settling velocity in still fluid and Uj is the jet exit velocity, which indirectly governs the turbulence characteristics of the jet flow. The advecting large eddies in a turbulent jet are found to play the role of organizing particles in patches. Interaction and coalescence between particle-concentrated eddies may result in the sudden drop of a group of particles, which contributes to sediments falling from a horizontal jet in the form of particle-rich “fingers”. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

Sediment-laden turbulent buoyant jets are commonly encountered in the natural and man-made environments. Examples of sediment-laden buoyant jets include volcanic eruptions, deep ocean hydrothermal vents (“black smokers”), ocean dumping of dredged spoils and sludge, and submarine discharge of wastewater effluent. It is important to understand the fluid mechanics of sediment jets for environmental impact assessment, and yet there is currently no general model for predicting the mixing of sediment-laden jets. This study reports a theoretical and experimental investigation the sediment mixing, fall-out and deposition from sediment-laden buoyant jets. It is well known that turbulence generates fluctuations to the particle motion, modulating the particle settling velocity. A general three-dimensional (3D) stochastic particle tracking model is developed to predict the particle settling out and deposition from a sediment-laden jet. Particle velocity fluctuations are modelled by a Lagrangian velocity autocorrelation function that accounts for the loitering and trapping of sediment particles in turbulent eddies which results in the reduction of settling velocity. The model is validated against results of independent experimental studies. Consistent with basic experiments using grid-generated turbulence, the model predicts that the apparent settling velocity can be reduced by as much as 30% of the stillwater settling velocity. The mixing and deposition of sediment-laden horizontal momentum jets are studied using laboratory experiments and 3D computational fluid dynamics (CFD) modelling. It is shown that there is a significant settling velocity reduction up to about 25-35%, dependent on jet turbulent fluctuations and particle properties. The CFD approach necessitates an ad hoc adjustment/reduction on settling velocity and lacks generality. Using classical solutions of mean velocity, and turbulent fluctuation and dissipation rate profiles derived from CFD solutions, 3D particle tracking model predictions of sediment deposition and concentration profiles are in excellent agreement with measured data over a wide range of jet flow and particle properties. Unlike CFD calculations, the present method does not require any a priori adjustment of particle settling velocity. A general particle tracking model for predicting sediment fall-out and deposition from an arbitrarily inclined buoyant jets in stagnant ambient is successfully developed. The model incorporates the three flow regimes affecting the sediment dynamics in a buoyant jet, namely turbulent jet flow, jet entrainment-induced external flow and surface spreading current. The jet mean flow velocity is determined using a well-validated jet integral model. The external jet-induced irrotational flow field is computed by a distribution of point sinks along the jet trajectory. The surface spreading current is predicted using an integral model accounting for the interfacial shear. The model is validated against experimental data of sediment deposition from vertical and horizontal sediment-laden buoyant jets. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

BNFL Sellafield has been authorised to discharge radionuclides to the Irish Sea since 1952. In the aquatic environment the radionuclides are adsorbed by sediments and are thus redistributed by sediment transport mechanisms. This sediment is known to accumulate in the estuaries of the Irish Sea. BNFL Springfields is also licensed to discharge isotopically different radionuclides directly to the Ribble estuary. Thus there is a need to understand the sediment dynamics of the Ribble estuary in order to understand the fate of these radionuclides within the Ribble estuary. Estuaries are highly dynamic environments that are difficult to monitor using the conventional sampling techniques. However, remote sensing provides a potentially powerful tool for monitoring the hydrodynamics of the estuarine environment by providing data that are both spatially and temporally representative. This research develops a methodology for mapping suspended sediment concentration (SSC) in the Ribble estuary using airborne remote sensing. The first hypothesis, that there is a relationship between SSC and l37Cs concentration is proven in-situ (R2=O.94), thus remotely sensed SSC can act as a surrogate for \37Cs concentration. Initial in-situ characterisation of the suspended sediments was investigated to identify spatial and temporal variability in grain size distributions and reflectance characteristics for the Ribble estuary. Laboratory experiments were then perfonned to clearly define the SSC reflectance relationship, identify the optimum CASI wavelengths for quantifying SSC and to demonstrate the effects on reflectance of the environmental variables of salinity and clay content. Images were corrected for variation in solar elevation and angle to give a ground truth calibration for SSC, with an R2=O.76. The remaining scatter in this relationship was attributed to the differences in spatial and temporal representation between sampling techniques and remote sensing. The second hypothesis assumes that a series of images over a flood tide can be animated to provide infonnation on the hydrodynamic regime, erosion, and deposition. Spatial and temporal data demonstrated the complex controls on sediment transport. The data also showed the importance of microphytobenthos in the stabilisation of intertidal sediments, highlighting their importance in defining sources and sinks of radionuclides in intertidal areas. Water volume data from the VERSE model were combined with SSC from the imagery to calculate the total sediment in suspension for each flight line. This provided the figures used to detennine total erosion and deposition, which were then used to derive net suspended sediment and l37Cs influxes of 2.01xl06kg and 604MBq per flood tide.

Graduation date: 1999

Thesis (M.Sc.)--Memorial University of Newfoundland. / Typescript. Restricted until October 1994. Bibliography: l. 118-123. Also available online.

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.