CRISSP2D, a two-dimensional finite element model, was used to undertake a comprehensive hydrodynamic, thermodynamic, and dynamic ice study on the Red River near Netley Cut in order to determine the cut's effect on the local hydrodynamics and freeze-up processes. Open water hydrodynamic and thermodynamic models were developed, calibrated, and verified such that the measured data and simulation results were in acceptable agreement. These models were used as input to the dynamic ice model which was able to adequately predict ice thickness within the study area once the air-ice heat transfer coefficient was calibrated. The geometry of the dynamic ice model was subsequently altered to simulate the effects of sealing Netley Cut. The geometry change resulted in no noticeable difference in simulated ice thickness, but did affect the hydrodynamics within the study area. In particular, the water velocity in the Red River downstream of Netley Cut and water surface elevation upstream of Netley Cut both increased noticeably.

CRISSP2D, a two-dimensional finite element model, was used to undertake a comprehensive hydrodynamic, thermodynamic, and dynamic ice study on the Red River near Netley Cut in order to determine the cut's effect on the local hydrodynamics and freeze-up processes. Open water hydrodynamic and thermodynamic models were developed, calibrated, and verified such that the measured data and simulation results were in acceptable agreement. These models were used as input to the dynamic ice model which was able to adequately predict ice thickness within the study area once the air-ice heat transfer coefficient was calibrated. The geometry of the dynamic ice model was subsequently altered to simulate the effects of sealing Netley Cut. The geometry change resulted in no noticeable difference in simulated ice thickness, but did affect the hydrodynamics within the study area. In particular, the water velocity in the Red River downstream of Netley Cut and water surface elevation upstream of Netley Cut both increased noticeably.

This dissertation presents a detailed numerical study using fully three-dimensional Large Eddy Simulation (LES) simulations of the flow and mass exchange processes in straight channels containing one or multiple embayments with vertical spanwise walls on one side corresponding to the presence of groynes in river reaches. The main focus is on identifying, understanding and quantifying the role played by the coherent structures and large-scale motions in the momentum and mass exchange processes within the embayments and between the embayments and the main channel. Of the three configurations that are studied, the first two contain only one embayment, and the water depth in the embayment area is equal to the one in the main channel. The two groynes are fully emerged in the first configuration and fully submerged in the second. This allows the study of the intensification of the flow three-dimensionality and acceleration of the mass exchange processes between the embayment and the channel due to the top interface and associated detached shear layer that are present in the submerged case. In both cases, the mass exchange across the embayment-channel interface is highly non-uniform across the depth. The third configuration corresponds to a straight channel with multiple groynes and is identical to one of the cases studied experimentally in a previous scaled model study. The mean water depth in the embayment area is about half that in the main channel; the width and length of the embayments are large compared to the depth; and the width over length ratio is relatively large such that a one gyre circulation pattern is observed inside the embayments. The model is validated with the experimental data available at the free surface. Even for shallow emerged embayments, most of the contaminant leaves the embayment through the top layer of the channel-embayment interface. The present study shows that 3D LES simulations can clarify several important aspects of flow past groyne fields that are very difficult or expensive to quantify using experimental techniques.

Ice regime plays a significant role in River hydraulics and morphology in Northern hemisphere countries such as Canada. The formation, propagation and recession of ice cover introduce a dynamic boundary layer to the top of the stream. Ice cover affects the water velocity magnitude and distribution, water level and consequently conveyance capacity. A stable ice cover also tends to reduce bed shear and associated sediment transport, but bank scour and ice jamming events can increase sediment entrainment. These effects are even more intense during the ice cover break-up period when extreme conditions such as ice-jamming and release and mechanical ice cover break-up can locally accelerate the flow, and ice can mechanically scour the river bed and banks. The presence of ice has some important implications for hydro-electrical power generation operations too. The ice cover changes the channel conveyance capacity (and therefore increases the flood risk), may increase sediment transport and causes scouring, and is likely to block water intakes and turbines. The rate of water release should, therefore, be adjusted in the presence of the ice cover to avoid unwanted consequences on the dam structure and equipments as well as on the downstream channel and the environment. Even though the influence of ice cover on rivers is widely recognized, large gaps still exist in our understanding of ice cover processes in rivers. Two main reasons for such a shortage are the difficulty and danger involved in collecting hydraulic and sediment transport data under ice cover, especially during the unstable periods of freeze-up and break-up. In the absence of sufficient data, the applicability of available formulae and theories on hydraulic processes in ice-covered rivers cannot be extensively tested and improved. The purpose of this research mainly is a) to perform a continuous, in-situ monitoring of water velocity profiles, sediment loads and ice-cover condition during several years through winter field campaigns at a section of the Lower Nelson River, Manitoba, Canada.The Lower Nelson River is a regulated river (Manitoba Hydro). It receives augmented flow from the Churchill River Diversion, and is subject to operation of many hydro-electricity facilities, one of which is currently under construction, while others are planned to be constructed in the future. Due to the geographical location of the study reach, it is covered by ice and experiences severe ice condition for several months during the year. b) Analysis of the collected data in order to study the impact of ice cover on the hydraulic properties and sediment conveyance capacity at the study reach and c) using the insight gained from the field data analysis to improve a river ice simulation model to apply in the study of Lower Nelson River ice regime. The selection of the Lower Nelson river is motivated by intention of Manitoba Hydro (MH) ,as the industrial partner in this research, to study the winter flow regime at the Lower Nelson River. Manitoba Hydro operates several dams on the Lower Nelson River and is considering more hydropower developments in the future. This study is composed of six steps as are described in the following main steps. Step 1: Selection of potential study sites and data collection techniques: The particular study site for this research is located immediately upstream of Jackfish Island, between Limestone generating station and Gillam Island in Lower Nelson River, Manitoba, Canada. River width at the study site location is about 1km. Water depth at the deployment site varies between 10-12 meters depending on both the time of year and the time of day due to hydropeaking fluctuations. Given the low accessibility to the field during winter time and considering the type of the required data, acoustic techniques were selected as the main approach for the field measurements. Two types of acoustic instruments, Acoustic Doppler Current Profiler (ADCP) and Shallow Water Ice Profiling Sonar (SWIPS) are selected for field investigations in this study. Both of them were planned to be deployed in the river for an extended period of time in order to record necessary data during the ice cover and open water periods. Step 2: Data acquisition. After the site selection and defining the appropriate techniques, data acquisition has been started through a series of annual field measurement campaigns starting from winter 2012. Measured data mainly consist of water velocity and sediment suspension during various ice cover stages, including river ice break-up. The velocity profiles are analyzed to determine dynamic changes in boundary shear stress and hydraulic resistance and stresses in the flow during the both open water and ice cover periods. Step 3: Data analysis and development/testing of roughness and sediment transport formulas. Several aspects of river-ice interactions are covered in the recorded data including ice cover condition and cover thickness variation, river hydraulic characteristics such as depth and velocity and finally information about the concentration of suspended particles. These data are analyzed to define the behavior of the ice cover and river during different ice stages. Ice effect on river conveyance capacity is also evaluated . The accuracy of common assumptions in composite roughness calculations in rivers is estimated and a new approach is developed and validated using the field observations and measurements. Ice cover influence on suspended sediment concentration is also studied as the other part of this research. Considering the type of the river sediment load (mostly bed load) available methods for sediment transport simulation are studied and applied for estimation of the sediment transport under ice cover condition. According to the results, the most suitable methods were planned to be a part of the river ice numerical simulation model, developed in this study. Turbulent characteristics in ice covered flows are also studied through two years of data recordings. Acoustic Doppler Current Profiler employed in this study is programmed for appropriate recording of the water velocity for this purpose. Results are analyzed and turbulent structures in the river are studied in this research as well. Step 4: Testing of Hatch-MH’s river ice simulation model. A numerical model has been selected in order to simulate the river ice process at the study site (LNR). ICESIM, a steady state, one-dimensional river ice process model originally developed in 1973 by Acres International Limited (now Hatch), is selected for this study.ICESIM is originally developed in FORTRAN and is capable of predicting the progression and stabilization of river ice cover. Step 5: Improvement of Hatch-MH’s river ice simulation model: ICESIM model is converted to Matlab as the first step of the model improvements. A Graphical User Interface (GUI) is designed for the program which facilitates the assessment of model performance during the simulation leads to a more user-friendly model to operate. The new model, ICESIMAT is calibrated and evaluated based on the conducted field studies. Simulation capabilities of ICESIMAT are improved in the form of extended or additional subroutines to enhance its capabilities in the simulation of river ice processes and sediment transport. The current version of ICESIMAT is a steady state model, capable of simulating river ice , river hydrodynamic characteristics and sediment transport along the study reach. Though the model is restricted in the terms of the dimensions of the simulation (only one dimensional) its lower computational cost, permits a longer study reach to be simulated (in the scale of hundred kilometers instead of couple hundred meters in three dimensional simulation). ICESIM model is unable to simulate the break-up period which reduces the model capability in the simulation of the complete cycle of river ice. New subroutines are designed and added to extend the model capability to include simulation of ice processes during the ice cover break-up and finally to calculate the sediment transport under the ice cover. Step 6: As the final step, the new subroutines are adjusted and linked to the main improved code, providing a new framework for dynamic ice cover simulation, more prepared for further future improvements both in terms of conceptual and programming aspects of the river ice modeling . The new Matlab basis of the code facilitates upgrading the model to include more complicated processes like river ice jam simulations. As the general result of this thesis, we have a better understanding of hydraulics and sediment transport processes in ice covered rivers ( direct and indirect measurements of river hydraulics characteristics), improved formulas for these processes (including more involving parameters) and a better version of the river ice simulation model (capable of simulating the complete river ice processes) for the contributors to this study in the industry.

Os conceitos de competência do escoamento e de capacidade de transporte foram desenvolvidos para escoamentos fluviais e acabam sendo transferidos para a modelagem de correntes de turbidez. No entanto, não é claro até que ponto o estabelecimento de analogias entre o ambiente fluvial e as correntes de turbidez pode ser realizado. Nesse contexto, o principal objetivo deste trabalho foi avaliar os conceitos de competência e de capacidade nas correntes de turbidez. O estudo foi desenvolvido experimentalmente, em um canal de inclinação variável de 4 m de comprimento, 40 cm de altura e 12 cm de largura. Um leito móvel foi construído em um trecho intermediário do canal utilizando sedimento - carvão mineral (ρ = 1405 kgm³) - com tamanho areia média. No total, 30 simulações de correntes de turbidez contínuas foram realizadas, utilizando o mesmo sedimento, porém com tamanho areia muito fina. Foram simuladas três declividades diferentes (6%, 0,3% e 0,015%), vazões de injeção entre 15 e 25 lmin, concentrações iniciais variando entre 0,03% e 5,5% e a duração dos ensaios ficou compreendida entre 3 e 30 min. O levantamento do perfil longitudinal do leito móvel, antes e após o ensaio, foi realizado e todo o sedimento foi coletado. Três quantidades distintas de sedimento foram quantificadas: a quantidade depositada antes do leito móvel, a quantidade depositada no leito móvel e a quantidade depositada depois do leito móvel. O cálculo da eficiência de transporte das correntes de turbidez no trecho do leito móvel, bem como das suas descargas sólidas, foi realizado. Também foram realizadas análises granulométricas do sedimento depositado antes e após do leito móvel. Por fim, parâmetros do fluxo foram obtidos (altura e velocidade) através de análises visuais e números adimensionais clássicos do transporte sólido em canais abertos foram avaliados. A partir da interpretação dos dados, conclui-se que os conceitos de competência do escoamento e de capacidade de transporte não foram capazes de refletir o diâmetro característico do sedimento transportado pelas correntes simuladas e a capacidade máxima de transporte desses fluxos, como normalmente ocorre em escoamentos fluviais. A relação entre a eficiência de transporte desses fluxos e a descarga sólida dos mesmos indicou que a partir de uma região a eficiência tornou-se invariante. Essa relação possui curvas com tendência logarítmica e as suas assíntotas ocorreram em valores de eficiência da ordem de 50% (inclinação de 6%) e de 40% (inclinações de 0,3% e 0,015%). A relação entre o diâmetro característico do sedimento depositado depois do leito e antes do leito indicou uma diminuição de cerca de 18% no tamanho dos grãos depositados a jusante. / The modeling of turbidity currents uses the concepts of competence and flow capacity that have been developed based on river hydraulics. However, the analogies between the fluvial environment and turbidity currents are not very clear. The main purpose of this study was to evaluate the competence and flow capacity of turbidity currents. The study was performed using an experimental channel 4 m long, 40 cm high, and 12 cm wide with variable slope. A mobile bed of medium sand-size - mineral coal (ρ=1405 kgm³) - was constructed in an intermediate area of the channel. In total, 30 simulations of continuous turbidity currents were performed and the sediment used in the turbidity currents was very fine sand-size mineral coal. The simulations were accomplished considering three different channel slopes (6%, 0.3% e 0.015%); injection discharge varied between 15 and 25 l/min, and initial volumetric concentrations varied between 0.03% and 5.5%. The simulations lasted between 3 and 30 minutes. The longitudinal profile of the mobile bed was analyzed before and after each simulation, and all the sediment was collected at the end. Three different amounts of sediment were quantified: the amount of sediment deposited before, on, and after the mobile bed. Based on this data, both the transport efficiency and solid discharge of the turbidity current on the mobile bed were determined. Particle-size distribution of the deposited sediment before and after the mobile bed was also carried out. Finally, flow parameters were obtained (high and velocity) considering visual evaluation and the classical dimensional numbers in sediment transport in open channel flows were analyzed. The data interpretation revealed that the competence and flow capacity parameters were not able to reflect the grain diameter transported by the simulated currents and a maximum flow capacity of these flows, as usually is observed in fluvial flows. The relation between the transport efficiency and solid discharge of these flows demonstrated that after a certain point, the efficiency became constant. This relation resulted in log trend curves with asymptotes located at efficiency values of approximately 50% (6% slope) and 40% (0.3% and 0.015% slopes). The characteristic diameter of the sediment deposited after the mobile bed presented a reduction of 18% when compared to the sediment deposited before the mobile bed.

Os conceitos de competência do escoamento e de capacidade de transporte foram desenvolvidos para escoamentos fluviais e acabam sendo transferidos para a modelagem de correntes de turbidez. No entanto, não é claro até que ponto o estabelecimento de analogias entre o ambiente fluvial e as correntes de turbidez pode ser realizado. Nesse contexto, o principal objetivo deste trabalho foi avaliar os conceitos de competência e de capacidade nas correntes de turbidez. O estudo foi desenvolvido experimentalmente, em um canal de inclinação variável de 4 m de comprimento, 40 cm de altura e 12 cm de largura. Um leito móvel foi construído em um trecho intermediário do canal utilizando sedimento - carvão mineral (ρ = 1405 kgm³) - com tamanho areia média. No total, 30 simulações de correntes de turbidez contínuas foram realizadas, utilizando o mesmo sedimento, porém com tamanho areia muito fina. Foram simuladas três declividades diferentes (6%, 0,3% e 0,015%), vazões de injeção entre 15 e 25 lmin, concentrações iniciais variando entre 0,03% e 5,5% e a duração dos ensaios ficou compreendida entre 3 e 30 min. O levantamento do perfil longitudinal do leito móvel, antes e após o ensaio, foi realizado e todo o sedimento foi coletado. Três quantidades distintas de sedimento foram quantificadas: a quantidade depositada antes do leito móvel, a quantidade depositada no leito móvel e a quantidade depositada depois do leito móvel. O cálculo da eficiência de transporte das correntes de turbidez no trecho do leito móvel, bem como das suas descargas sólidas, foi realizado. Também foram realizadas análises granulométricas do sedimento depositado antes e após do leito móvel. Por fim, parâmetros do fluxo foram obtidos (altura e velocidade) através de análises visuais e números adimensionais clássicos do transporte sólido em canais abertos foram avaliados. A partir da interpretação dos dados, conclui-se que os conceitos de competência do escoamento e de capacidade de transporte não foram capazes de refletir o diâmetro característico do sedimento transportado pelas correntes simuladas e a capacidade máxima de transporte desses fluxos, como normalmente ocorre em escoamentos fluviais. A relação entre a eficiência de transporte desses fluxos e a descarga sólida dos mesmos indicou que a partir de uma região a eficiência tornou-se invariante. Essa relação possui curvas com tendência logarítmica e as suas assíntotas ocorreram em valores de eficiência da ordem de 50% (inclinação de 6%) e de 40% (inclinações de 0,3% e 0,015%). A relação entre o diâmetro característico do sedimento depositado depois do leito e antes do leito indicou uma diminuição de cerca de 18% no tamanho dos grãos depositados a jusante. / The modeling of turbidity currents uses the concepts of competence and flow capacity that have been developed based on river hydraulics. However, the analogies between the fluvial environment and turbidity currents are not very clear. The main purpose of this study was to evaluate the competence and flow capacity of turbidity currents. The study was performed using an experimental channel 4 m long, 40 cm high, and 12 cm wide with variable slope. A mobile bed of medium sand-size - mineral coal (ρ=1405 kgm³) - was constructed in an intermediate area of the channel. In total, 30 simulations of continuous turbidity currents were performed and the sediment used in the turbidity currents was very fine sand-size mineral coal. The simulations were accomplished considering three different channel slopes (6%, 0.3% e 0.015%); injection discharge varied between 15 and 25 l/min, and initial volumetric concentrations varied between 0.03% and 5.5%. The simulations lasted between 3 and 30 minutes. The longitudinal profile of the mobile bed was analyzed before and after each simulation, and all the sediment was collected at the end. Three different amounts of sediment were quantified: the amount of sediment deposited before, on, and after the mobile bed. Based on this data, both the transport efficiency and solid discharge of the turbidity current on the mobile bed were determined. Particle-size distribution of the deposited sediment before and after the mobile bed was also carried out. Finally, flow parameters were obtained (high and velocity) considering visual evaluation and the classical dimensional numbers in sediment transport in open channel flows were analyzed. The data interpretation revealed that the competence and flow capacity parameters were not able to reflect the grain diameter transported by the simulated currents and a maximum flow capacity of these flows, as usually is observed in fluvial flows. The relation between the transport efficiency and solid discharge of these flows demonstrated that after a certain point, the efficiency became constant. This relation resulted in log trend curves with asymptotes located at efficiency values of approximately 50% (6% slope) and 40% (0.3% and 0.015% slopes). The characteristic diameter of the sediment deposited after the mobile bed presented a reduction of 18% when compared to the sediment deposited before the mobile bed.

Os conceitos de competência do escoamento e de capacidade de transporte foram desenvolvidos para escoamentos fluviais e acabam sendo transferidos para a modelagem de correntes de turbidez. No entanto, não é claro até que ponto o estabelecimento de analogias entre o ambiente fluvial e as correntes de turbidez pode ser realizado. Nesse contexto, o principal objetivo deste trabalho foi avaliar os conceitos de competência e de capacidade nas correntes de turbidez. O estudo foi desenvolvido experimentalmente, em um canal de inclinação variável de 4 m de comprimento, 40 cm de altura e 12 cm de largura. Um leito móvel foi construído em um trecho intermediário do canal utilizando sedimento - carvão mineral (ρ = 1405 kgm³) - com tamanho areia média. No total, 30 simulações de correntes de turbidez contínuas foram realizadas, utilizando o mesmo sedimento, porém com tamanho areia muito fina. Foram simuladas três declividades diferentes (6%, 0,3% e 0,015%), vazões de injeção entre 15 e 25 lmin, concentrações iniciais variando entre 0,03% e 5,5% e a duração dos ensaios ficou compreendida entre 3 e 30 min. O levantamento do perfil longitudinal do leito móvel, antes e após o ensaio, foi realizado e todo o sedimento foi coletado. Três quantidades distintas de sedimento foram quantificadas: a quantidade depositada antes do leito móvel, a quantidade depositada no leito móvel e a quantidade depositada depois do leito móvel. O cálculo da eficiência de transporte das correntes de turbidez no trecho do leito móvel, bem como das suas descargas sólidas, foi realizado. Também foram realizadas análises granulométricas do sedimento depositado antes e após do leito móvel. Por fim, parâmetros do fluxo foram obtidos (altura e velocidade) através de análises visuais e números adimensionais clássicos do transporte sólido em canais abertos foram avaliados. A partir da interpretação dos dados, conclui-se que os conceitos de competência do escoamento e de capacidade de transporte não foram capazes de refletir o diâmetro característico do sedimento transportado pelas correntes simuladas e a capacidade máxima de transporte desses fluxos, como normalmente ocorre em escoamentos fluviais. A relação entre a eficiência de transporte desses fluxos e a descarga sólida dos mesmos indicou que a partir de uma região a eficiência tornou-se invariante. Essa relação possui curvas com tendência logarítmica e as suas assíntotas ocorreram em valores de eficiência da ordem de 50% (inclinação de 6%) e de 40% (inclinações de 0,3% e 0,015%). A relação entre o diâmetro característico do sedimento depositado depois do leito e antes do leito indicou uma diminuição de cerca de 18% no tamanho dos grãos depositados a jusante. / The modeling of turbidity currents uses the concepts of competence and flow capacity that have been developed based on river hydraulics. However, the analogies between the fluvial environment and turbidity currents are not very clear. The main purpose of this study was to evaluate the competence and flow capacity of turbidity currents. The study was performed using an experimental channel 4 m long, 40 cm high, and 12 cm wide with variable slope. A mobile bed of medium sand-size - mineral coal (ρ=1405 kgm³) - was constructed in an intermediate area of the channel. In total, 30 simulations of continuous turbidity currents were performed and the sediment used in the turbidity currents was very fine sand-size mineral coal. The simulations were accomplished considering three different channel slopes (6%, 0.3% e 0.015%); injection discharge varied between 15 and 25 l/min, and initial volumetric concentrations varied between 0.03% and 5.5%. The simulations lasted between 3 and 30 minutes. The longitudinal profile of the mobile bed was analyzed before and after each simulation, and all the sediment was collected at the end. Three different amounts of sediment were quantified: the amount of sediment deposited before, on, and after the mobile bed. Based on this data, both the transport efficiency and solid discharge of the turbidity current on the mobile bed were determined. Particle-size distribution of the deposited sediment before and after the mobile bed was also carried out. Finally, flow parameters were obtained (high and velocity) considering visual evaluation and the classical dimensional numbers in sediment transport in open channel flows were analyzed. The data interpretation revealed that the competence and flow capacity parameters were not able to reflect the grain diameter transported by the simulated currents and a maximum flow capacity of these flows, as usually is observed in fluvial flows. The relation between the transport efficiency and solid discharge of these flows demonstrated that after a certain point, the efficiency became constant. This relation resulted in log trend curves with asymptotes located at efficiency values of approximately 50% (6% slope) and 40% (0.3% and 0.015% slopes). The characteristic diameter of the sediment deposited after the mobile bed presented a reduction of 18% when compared to the sediment deposited before the mobile bed.

Le réseau de digues français s'étend sur 9000 km et protège moult enjeux humains et socio-économiques. Les surverses se produisent lorsque le niveau d'eau dépasse la crête de la digue et sont responsables de nombreuses ruptures d'ouvrages. La prévision des caractéristiques des brèches (vitesse de développement, taille finale et hydrogramme) reste délicate, d'autant que les effets de la dynamique de la rivière n'ont jamais été étudiés.Cette étude expérimentale est basée sur cinq essais réalisés en canal sur des modèles physiques de digue cohésifs. Les phases classiques du développement de brèches ainsi que le mécanisme de headcutting ont été observés. L'érosion s'est manifestée à trois échelles spatiales (grain, mm et cm) et la durée des essais a varié entre 1h30 et 10 jours selon les sols. Nous avons constaté que le développement d'une brèche et de la fosse d'érosion devient asymétrique en présence d'une vitesse longitudinale amont. Des essais géotechniques originaux menés à l'IRPHE ont permis de relever des différences importantes sur le mode et la cinétique d'érosion entre les sols testés, toujours en bon accord avec les observations menées sur les modèles de digues.Des essais en brèche de largeur fixe nous ont mis à jour l'émergence d'une hydraulique particulière dans la brèche lorsque le vitesse longitudinale incidente augmente (ressauts, recirculations) qui a un impact sur l'hydrogramme de brèche.Enfin, des simulations menées sous RUBAR20 ont permis de retrouver de nombreuses observations expérimentales tant sur le fonctionnement hydraulique des brèches que sur la sensibilité de la cinétique d'érosion à la contrainte critique d'érosion et à l'érodibilité. / France's embankment network stretches over 9000~km, protecting people and infrastructures. Overtopping occurs when water level rises above the embankment's crest, often leading to breach development. Reliable forecasting of breaches' characteristics (widening rate, final geometry and breach hydrograph) remains a challenging task. Furthermore, the effects of river dynamics have never been studied.This work is based on a campain of five overtopping tests performed on scaled homogeneous cohesive model embankments, carried out in an eight meters recirculated flume. The classical steps of dike breaching were observed, as wall as headcutting. Depending on soil type and water content, erosion occured at three distinct scales (sand grain, mm, cm) while the experiments' durations ranged from 1h30 to 250~h. Evidence was brought that river dynamics can result in a completely asymmetric development of both the breach and scour pit. Simple geotechnical tests showed notable differences on erosion modes and rates among tested soils, consistently with the flume experiments.The study of fixed-width breaches showed that distinguish hydraulic phenomena such as hydraulic jumps and recirculations arise when lateral flow speed increases on the river side, which comes with an important drop in breach flow.In addition, bidimensional numerical simulations offered a practical and satisfying way of modelling breach hydraulics. In particular, the strong surface deformation and important velocity gradients in the vicinity of the breach were well-rendered. Elements relative to breach formation sensitivity to erosion parameters (threshold stress and erodibility coefficient) are also provided.

Ce travail de thèse s'intéresse à l'effet d'une variation longitudinale de l'occupation du sol de la plaine d'inondation sur l'écoulement d'une rivière en débordement. Nous traitons le cas d'une transition entre une zone de prairie et une zone de forêt, et vice versa. Cette variation d'occupation du sol est associée à une transition de rugosité hydraulique entre une rugosité de fond (prairie fortement immergée) et des macro-rugosités émergées (arbres), modélisées respectivement par une moquette plastifiée et par un champ de cylindres. Ces écoulements sont étudiés en laboratoire dans un canal de dimension 18 m x 3 m. Dans un premier temps, nous considérons l'écoulement à travers un champ de cylindres émergents en lit simple, en étudiant l'effet du fond sur le sillage des cylindres et le phénomène de seiche (fortes oscillations de la surface libre). Dans un deuxième temps, nous nous penchons sur le développement vers l'uniformité d'un écoulement en lit composé de rugosité uniforme. La croissance asymétrique de la couche de mélange du lit composé, la propriété d'autosimilarité ainsi que l'organisation tridimensionnelle des structures turbulentes cohérentes associées à la couche de mélange sont analysées. Le troisième temps fait l'objet de la transition longitudinale de rugosité en lit composé, dont l'effet sur la couche de mélange et sur les structures cohérentes est discuté. Nous évaluons également les différentes contributions au transfert latéral de quantité de mouvement entre lit mineur et plaine d'inondation par diffusion turbulente, par échange de masse et par les courants secondaires / This PhD thesis investigates the effect of a longitudinal change in floodplain land use on an overflooding river flow. We consider a transition between a meadow and a woodland and vice versa. This change in land use is associated with a change in hydraulic roughness, between a bed roughness (highly submerged meadow) and emergent macro-roughnesses (trees), respectively modelled by a plastic artificial grass and an array of emergent cylinders. The flows are experimentally investigated in an 18 m x 3 m laboratory flume. In a first step, we investigate the flow through a cylinder array in a single channel, focusing on the effect of bed roughness on the cylinder wakes and on the seiche phenomenon (strong free surface oscillations). In a second step, we study the development towards flow uniformity of compound channel flows with a uniform hydraulic roughness on the floodplains. The asymmetrical growth of the compound channel mixing layer, the self-similarity property and the three-dimensional organisation of the turbulent coherent structures associated with the mixing layer are analysed. In a third step, we investigate the longitudinal change in roughness in compound channel configuration, which effects on mixing layer and on coherent structures are discussed. We also assess the contributions to lateral transfers of momentum between main channel and floodplain by turbulent diffusion, by mass exchange and by secondary currents

The subject of this diploma thesis is a comparison between two river hydraulics modeling approaches, the first one being combined 1D and 2D modeling, and the second one being solely 2D modeling. Both models are located at Bečva river, between its 17,282 and 28,419 river kilometer. The outputs of the simulations are water flow velocity maps, water depth maps, maps that show differences between flood areas calculated in both models, tables comparing channel water surface elevations and schematic long sections. In conclusion of the thesis, the outputs of the simulations are compared.