Forest harvesting is blamed for a perceived increase in the flashiness and turbidity of the Cascapedia River's water. This has raised questions over the source of the sediment (harvest parcels, roads, or stream banks) and its potential impact on the sedimentology of the river. The objectives of this research are twofold. The first is to ascertain if harvesting operations are associated to a widening of low-order tributaries, creating a source of sediment. The second is to determine if variations in the sedimentology along four segments of the Cascapedia can be associated to harvesting operation intensity. Firstly, analysis of stream width in low-order tributaries shows that, once the variations associated with basin area and D50 are removed and within the range of harvesting in our dataset, there appears to be a 25% increase in width associated with the harvesting activities of the last five years, as well as with road density, both in a 60 m stream buffer for a number of the sampled streams. Secondly, the models relating harvesting intensity and changes in sedimentology are sensitive to a few sites or contrary to theory. Future studies should determine the underlying hydrological processes responsible for stream enlargement and the process of sediment deposition.

Thesis (Ph. D.)--Centre Universitaire de Perpignan, 1977. / Errata sheet inserted, v. 1. Includes bibliographical references (Vol. 1, p. [i]-xxxix).

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Areas under intensive tobacco crop cultivation have been impacting the water balance and increasing soil erosion in Southern Brazil. Application of large amounts of mineral fertilizers, higher than the tobacco requirement, contributes to excessive phosphorus loads in soil and water bodies. The assessment of potential environmental impacts due to changes in land use and management practices is necessary to achieve the sustainable management of natural resources such as soil and water. In this study the hydrological processes, the sediment yield and the phosphorus transfer in the Arroio Lino watershed were evaluated by using the Soil and Water Assessment Tool (SWAT) model. The small watershed (4.18 km2 ) is located in Southern Brazil and its main land use is tobacco crop under conventional tillage. Measured streamflow and sediment yield at the watershed outlet were used for model streamflow sensitivity analysis, calibration and validation. Phosphorus loads at four sub-watersheds (A1, A2, B, C) and at the watershed outlet were used for model sensitivity analysis and calibration. A Latin Hypercube (LH) and One-factor- At-a-Time (OAT) sensitivity analysis was performed on input variables. Model calibration was performed with the Shuffled Complex Evolution Algorithm-Uncertainty Analysis (SCE-UA). Time series plots and standard statistical measures were used to verify model predictions. Adaptations of the model parameters for the reality of the region in which the watershed is located were made during the calibration and validation of the model. The predicted monthly streamflow matched the observed values, with a Nash Sutcliffe coefficient (NSE) of 0.87 for calibration and 0.76 for validation. In an attempt to account for sediment transport and deposition processes across the landscape, the SWAT model was modified to simulate landscape sediment transport capacity. The new deposition routine performed better during calibration (NSE of 0.70) than SWAT standard version (NSE of -0.14) in the studied watershed. The modified model provided reasonable simulations of sediment transport across the landscape positions. The results suggest that the integration of the sediment deposition routine in SWAT increases model predictions accuracy in steeper areas, while at the same time significantly improves the ability to predict spatial distribution of sediment deposition areas. The predicted P loads are in the order of magnitude of the measured ones, however, the model failed to predict the P loads in three sub-watersheds (A1, A2 and B). Although occupying only 29% of the total land cover, cropland is the primary source of nutrients in the watershed (80%). After calibration of streamflow, sediment yield and phosphorus loads, the simulation of distinct management scenarios was done for the Arroio Lino watershed. Three scenarios of management practices: conventional tillage (CT), minimum tillage (MT) and no-tillage cultivation (NT) with reduction of 50% of fertilizer rate application were tested over a period of 30 years. No-tillage practices did not significantly affect water yield, but greatly affected sediment yield due to reduction of soil erosion. The soluble phosphorus losses decreased mainly when the fertilizer doses decreased. The simulation results suggest that conventional tillage practices should be replaced by less intensive tillage practices in order to minimize environmental impacts caused by a single land use. / Áreas com culturas de tabaco cultivadas intensivamente, no sul do Brasil, vêm afetando a qualidade da água e aumentando a erosão do solo nesta região. A aplicação de grandes quantidades de fertilizantes minerais, maiores do que a exigência da cultura, contribui para a excessiva carga de fósforo nos solos e corpos d'água. A avaliação dos impactos potenciais das mudanças no uso e manejo do solo é fundamental para propiciar uma gestão sustentável dos recursos naturais como solo e água. Neste estudo foram avaliados os processos hidrológicos, a produção de sedimentos e a transferência de fósforo na bacia hidrográfica do Arroio Lino, por meio do modelo Soil and Water Assessment Tool (SWAT). A pequena bacia (4,18 km2 ) localiza-se no sul do Brasil e seu principal uso da terra é a cultura de fumo sob plantio convencional. As vazões e a produção de sedimentos medidos no exutório da bacia foram utilizadas para análise de sensibilidade, calibração e validação dos parâmetros do modelo. Cargas de fósforo em quatro sub-bacias (A1, A2, B, C) e no exutório foram utilizados para a análise de sensibilidade e calibração do modelo. A análise de sensibilidade foi feita com o uso de um algoritmo que combina as técnicas de Hipercubo Latino (LH) e One-factor-At-a-Time (OAT). A calibração foi realizada com o algoritmo Shuffled Complex Evolution (SCE-UA). Análises gráficas e medidas estatísticas foram utilizadas para verificar as previsões do modelo. Adaptações nos parâmetros do modelo foram feitas durante as etapas de calibração e validação, tendo em vista a realidade da região em que a bacia está localizada. Na simulação de vazões mensais obtiveram-se valores do índice de eficiência de Nash e Sutcliffe (NSE) de 0,87 na calibração e 0,76 na validação. Com o intuito de representar melhor os processos de deposição de sedimentos nas encostas, o modelo SWAT foi modificado com a incorporação de um termo de capacidade de transporte de sedimentos na paisagem. Com a nova rotina de sedimentos obteve-se um melhor desempenho durante a calibração (NSE de 0,70) em relação ao modelo SWAT original (NSE de -0,14). As simulações com o modelo modificado foram satisfatórias para o transporte de sedimentos em diferentes posições da paisagem. Os resultados sugerem que a integração da rotina de deposição de sedimentos no SWAT aumenta a precisão do modelo de previsões em áreas mais íngremes e, ao mesmo tempo, melhora significativamente a capacidade de prever a distribuição espacial das áreas de deposição de sedimentos. As previsões de cargas de fósforo foram na ordem de grandeza das cargas medidas; no entanto, o modelo não conseguiu prever satisfatoriamente as cargas de fósforo em três sub-bacias (A1, A2 e B). Apesar das lavouras ocuparem apenas 29% da área total da bacia, de acordo com a simulação elas são a principal fonte de nutrientes na bacia hidrográfica (80%). Após a calibração da vazão, da produção de sedimentos e de fósforo, foi realizada a simulação de cenários de manejo do solo na bacia hidrográfica do Arroio Lino. Três cenários de práticas de manejo: preparo convencional (PC), cultivo mínimo (CM) e plantio direto de cultivo (NT) com redução de 50% da taxa de aplicação de fertilizantes foram testados durante um período de 30 anos. A prática de plantio direto não afetou significativamente o escoamento, no entanto afetou fortemente a produção de sedimentos devido à redução da erosão do solo. Houve redução das perdas de fósforo principalmente devido à redução das doses de fertilizantes. Os resultados indicam que as práticas de preparo convencional do solo deveriam ser substituídas por práticas de cultivo mínimo ou direto, a fim de minimizar os impactos ambientais causados por um determinado uso do solo.

Turbulent bedload transport represents the main contribution to the riverbed morphological evolution, and associates the non-trivial collective granular behavior with a turbulent fluid flow. Therefore, its description is both a scientific challenge and a societal issue. The present numerical approach focuses on the granular phase characterization, and considers idealized steady uniform bedload transport, with monodisperse spherical beads and a unidirectional fluid flow. This simplified configuration allows to study the underlying physical mechanisms.A minimal coupled numerical model is proposed, associating a three dimensional discrete element method with a one-dimensional volume-averaged fluid momentum balance resolution. The model is compared with classical experimental results of dimensionless sediment transport rate as a function of the Shields number. The comparison is extended to granular depth profiles of solid volume fraction, solid velocity and sediment transport rate density in quasi-2D bedload transport configurations. Parameter sensitivity analysis evidenced the importance of the fluid-particle phase coupling, and showed a robust agreement of the model with the experiments. The validated model is further used to analyze the granular depth structure in bedload transport. Varying the channel inclination angle and the specific density, it is shown that the classical Shields number and dimensionless sediment transport rate formulations do not take appropriately into account the effects of these two parameters. Analyzing the solid depth profiles and the continuous two-phase flow equations, the neglected fluid flow inside the granular bed is identified as the missing contribution. Its importance is enhanced near the transition to debris flow. A rescaling of the Shields number is proposed and is shown to make all the data collapse onto a master curve when considering the dimensionless sediment transport rate as a function of the modified Shields number. Lastly, the bedload transport granular rheology is characterized by computing locally the stress tensor as a function of the depth. The lowermost part is shown to follow a creeping regime and exhibits signature of non-local effects. The dense granular flow on the top of it, is well described by the mu(I) rheology and is observed to persist up to unexpectedly high inertial numbers. It is characterized by the co-existence of frictional and collisional contributions. The transition from dense to dilute granular flow is controlled by the Shields number, the slope and the specific density. Saltation is observed in the uppermost granular layer. These findings improve the understanding of bedload transport granular mechanisms and challenge the existing granular rheologies. / Turbulent bedload transport represents the main contribution to the riverbed morphological evolution, and associates the non-trivial collective granular behavior with a turbulent fluid flow. Therefore, its description is both a scientific challenge and a societal issue. The present numerical approach focuses on the granular phase characterization, and considers idealized steady uniform bedload transport, with monodisperse spherical beads and a unidirectional fluid flow. This simplified configuration allows to study the underlying physical mechanisms.A minimal coupled numerical model is proposed, associating a three dimensional discrete element method with a one-dimensional volume-averaged fluid momentum balance resolution. The model is compared with classical experimental results of dimensionless sediment transport rate as a function of the Shields number. The comparison is extended to granular depth profiles of solid volume fraction, solid velocity and sediment transport rate density in quasi-2D bedload transport configurations. Parameter sensitivity analysis evidenced the importance of the fluid-particle phase coupling, and showed a robust agreement of the model with the experiments. The validated model is further used to analyze the granular depth structure in bedload transport. Varying the channel inclination angle and the specific density, it is shown that the classical Shields number and dimensionless sediment transport rate formulations do not take appropriately into account the effects of these two parameters. Analyzing the solid depth profiles and the continuous two-phase flow equations, the neglected fluid flow inside the granular bed is identified as the missing contribution. Its importance is enhanced near the transition to debris flow. A rescaling of the Shields number is proposed and is shown to make all the data collapse onto a master curve when considering the dimensionless sediment transport rate as a function of the modified Shields number. Lastly, the bedload transport granular rheology is characterized by computing locally the stress tensor as a function of the depth. The lowermost part is shown to follow a creeping regime and exhibits signature of non-local effects. The dense granular flow on the top of it, is well described by the mu(I) rheology and is observed to persist up to unexpectedly high inertial numbers. It is characterized by the co-existence of frictional and collisional contributions. The transition from dense to dilute granular flow is controlled by the Shields number, the slope and the specific density. Saltation is observed in the uppermost granular layer. These findings improve the understanding of bedload transport granular mechanisms and challenge the existing granular rheologies.

L'évolution des caractéristiques des particules en suspension au cours de leur transfert au sein des bassins versants est encore mal connue. Ceci limite actuellement notre aptitude à prédire correctement l'érosion ou les flux de matières en suspension (MES) et rend difficile la proposition de pratiques de gestion adaptées aux réglementations en vigueur. A partir d'expériences en milieu contrôlé, nous avons montré que les particules de sols ont tendance à se désagréger en milieu turbulent. Il semble de plus qu'une augmentation de la concentration en suspension accélère cette désagrégation et conduise à des particules plus fines. Même si l'évolution des particules à l'échelle horaire semble rester de second ordre derrière les caractéristiques des sols sources, il est indispensable de pouvoir vérifier si les particules en suspension se comportent de la même manière en conditions naturelles où des interactions complexes entre processus peuvent avoir lieu. Cependant l'absence de méthode de mesure permettant le suivi des propriétés de transport des sédiments en écoulements très concentrés (de 1 plusieurs centaines de grammes par litre) dans les bassins élémentaires limite notre capacité à hiérarchiser les processus à considérer pour modéliser le transfert sédimentaire ou améliorer la gestion opérationnelle des sédiments. Afin de répondre à ce besoin instrumental, nous avons développé un Système de Caractérisation des Agrégats et des Flocs (SCAF). Cet instrument est conçu pour être incorporé dans les stations de suivi hydro-sédimentaire. La mesure est réalisée immédiatement après prélèvement d'un échantillon de la suspension à caractériser par une série de capteurs optiques qui suivent l'évolution de l'absorbance optique durant la sédimentation de l'échantillon. Nous proposons une méthode de traitement des données optiques donnant accès à la distribution des vitesses de chute de la suspension ainsi qu'à un indice de floculation qui renseigne sur la capacité des MES à floculer durant leur sédimentation. Les distributions de vitesses de chute mesurées sont validées sur une large gamme de matériaux et de régimes de sédimentation, afin de couvrir la variabilité des types de matériaux et des concentrations observées en milieu naturel. Pour des sédiments non cohésifs ou floculant peu durant leur sédimentation les mesures du SCAF s'ajustent sur celles issues des autres méthodes. Pour les suspensions qui floculent durant leur sédimentation, la plupart des méthodes de mesure classiques conduisent à des vitesses de chute non représentatives de la suspension. Nous avons montré que les variations des propriétés optiques des matières en suspension lors de leur floculation impactent nos mesures. La méthode proposée permet cependant de quantifier l'augmentation des vitesses de chute avec la floculation, et d'encadrer l'incertitude des mesures. Pour les mesures à forte concentration (>10 g/l), un front d'entravement peut se former durant la sédimentation, le SCAF mesurant alors précisément les vitesses de chute du front. La mesure des distributions des vitesses de chute et de la cohésion des particules en suspension peut permettre d'identifier différentes populations de particules formant une suspension (grains de sables, flocs, matières fines...). Le suivi de telles informations au sein de bassins versants ouvre de nouvelles perspectives pour aborder la connectivité sédimentaire et s'orienter vers une gestion optimale des flux de MES. / Little is known about the processes that govern the evolution of suspended particle characteristics during their transport through a river basin. This is a main limitation for modelling erosion severity or suspended solids (SS) fluxes. It also leads to difficulties to propose management policies adapted to environmental legislation. Based on experiments in controlled environments, we have shown that soil particles tend to disaggregate in turbulent flows. The increase in SS concentration was associated to an increase of the disaggregation of SS particles, leading to smaller final particle sizes. Laboratory experiments also showed that the variability of the particle sizes due to their evolution over one hour was smaller than the variability due to the soil type from which the particles originated. However it is important to ensure that the suspended particles behave in the same way in natural conditions, where complex interactions between hydraulic, chemical and biological processes can influence their evolution. Up to now no measurement method allows measuring continuously the suspended sediment properties in highly concentrated fluids (from one to hundreds grams per liter), such as those observed in headwater catchments during runoff events. This severely limits the possibility to identify the processes that are important to consider in numerical models. The Aggregate and Floc Characterization System (SCAF) has been developed in order to measure SS properties for a wide range of SS concentrations. It was designed to be easily incorporated into sequential samplers. Immediately after the collection of a sample from the river, the sedimentation of the suspension is recorded by continuous measurements of the absorbance by a series of optical sensors. A method was proposed to processes the raw optical data in order to obtain the settling velocity distribution of the suspension. It also provides a flocculation index representing the tendency of the particles to flocculate during their sedimentation. The calculated settling velocity distributions were validated on a large range of materials and settling regimes in order to cover the natural variability of suspended sediments. For sediments that hardly flocculate during their sedimentation or are non-cohesive, the measurements of the SCAF were similar to those from other methods. In the case of suspensions that strongly flocculate during sedimentation, most of the classical methods give non-representative falling velocities. In this case, the optical property of the particles may vary during settling, affecting the optical measurement. The proposed method allowed quantifying the increase of settling velocity induced by flocculation, and provided confidence intervals for the settling velocities. For high SS concentrations ( > 10 g/l), a settling front can be formed during the sedimentation, which is well characterized by the SCAF. The measurement of the settling velocity distributions and of the flocculation index can be used to identify different particle populations (sand grains, flocs, individual particles) forming a suspension. Monitoring these properties in watersheds offers new insights to explore sediment connectivity within river basins and to optimize water management strategies.

Barrages de corrections torrentielles et plages de dépôts jouent un rôle clés dans la protection contre les crues des torrents. Leurs gestionnaires ont pour mission de réduire les risques d'inondations, mais doivent désormais aussi minimiser les impacts environnementaux liés aux ouvrages de protection. Ceci nécessite une meilleure compréhension des effets des barrages de corrections torrentielles et des plages de dépôts sur le transport sédimentaire des torrents. Cette thèse s'inscrit dans cet objectif et se décompose en deux parties. Sa section sur l'état de l'art présente: i) les différents effets des barrages de correction torrentielle sur la production et le transfert sédimentaire; ii) des descriptions des processus hydrauliques et de sédimentation ayant lieu dans les plages de dépôts; et iii) les processus liés à la production et au transfert de bois d'embâcle. Une nouvelle méthode de quantification de la production sédimentaire des torrents complète cet état de l’art.La seconde partie de cette thèse présente le travail réalisé en banc d’essai expérimental. Une première série d’expérience a permis de mettre en évidence un transport par charriage plus régulier lorsque des barrages de correction torrentielle sont ajoutés à un bief alluvial. Une seconde série d’essais a été réalisée sur un modèle générique de plage de dépôt dans l’objectif d’en caractériser les écoulements. Pour cela, une nouvelle procédure de mesure et de reconstruction par approche inverse a été développée. Il en résulte une description des caractéristiques d’un écoulement proche du régime critique, ainsi que des mécanismes de rétrocontrôle entre morphologie et hydraulique pendant la phase de dépôt. / Check dams and open check dams are key structures in torrent hazard protection. Their managers must mitigate flood hazards, but now must also minimize the environmental impacts of these protection structures. This requires to improve the knowledge on the effects of check dams and open check dams on the sediment transport, and this thesis forms a contribution towards this end.The section on the current state of research reviews i) the diverse effects of check dams on sediment production and transfer; ii) descriptions of the hydraulics and sedimentation processes occurring in open check dams; and iii) woody debris production and trapping processes. This state of the art is completed with proposition of new bedload transport estimation methods, specifically developed for paved streams experiencing external supply or armour breaking.Experimental results are then provided. Firstly, flume experiments highlight the emergence of a more regular bedload transport when check dams are built in alluvial reaches. In a second stage, experiments were performed on a generic Froude scale model of an open check dam basin in order to capture the features of laterally-unconstrained, highly mobile flows. A new flow measurement and inverse-reconstruction procedure has been developed. A preliminary analysis of the results describes flows that tend toward a critical regime and the occurrence of feedback mechanisms between geomorphology and hydraulics during massive bedload deposition.

Une approche modélisante du potentiel TSL le long des plages du golfe du Lion est proposée sur la base du modèle d’ingénierie côtière Unibest-LT (Deltares). Sont exploités en entrée de modèle des données de houle de 1979 à 2010 (base de données ANEMOC-2 produite par le Cerema à partir d’une approche modélisante également) conjointement à des données topo-bathymétriques haute résolution LiDAR (base de données Litto3D produite conjointement par le SHOM et l’IGN). Le potentiel de TSL annuel est calculé pour 157 profils topo-bathymétriques couvrant l’ensemble des ∿250 km de littoral étudié dans ce travail de thèse.En premier lieu, le potentiel de TSL annuel résultant est d’abord estimé sur la base d’une année de houle type, établie à partir de 31 ans de houle horaire entre 1979 et 2010, puis pour chaque année entre 1979 et 2010. La différence observée avec le potentiel de TSL annuel résultat « type » permet de discrétiser les années en trois situations : anomalie positive, anomalie négative ou proche de la normale. Par ailleurs, un lien est clairement établi entre la variabilité interannuelle identifiée dans les simulations et les tempêtes. En outre, cette analyse interannuelle menée à l’échelle des plages du golfe du Lion amène à affiner les limites des cellules de dérive littorale potentielle, pour lesquelles trois scénarios d’organisation sont établis : habituel, occasionnel, exceptionnel.Enfin, le lien entre potentiel de TSL annuel résultant et variations du rivage est testé, conformément au large corpus bibliographique qui existe à ce sujet en ingénierie côtière. Cependant, les résultats s’avèrent décevants : une remise en cause de la théorie sous-jacente est proposée. / Wave data from 1979 to 2010 derived from a large-scale modelling database (ANEMOC-2) were used together with high resolution topo-bathymetric LiDAR data as inputs within the coastal engineering model Unibest-LT (Uniform Beach Sediment - Longshore Transport). The spatial and temporal coverage of these data offers a unique opportunity to carry out a comprehensive study of potential longshore transport intensity and direction.Over the 250 km-long stretch of coast covering the study area, the longshore drift directions computed with the 30-year mean wave climate closely match those of previous findings based on experimental geomorphological methods.Potential LST rates are then computed for each year individually over the 1979-2010 period. Deviations from the 30-year mean LST rates are used to identify 3 cases: positive anomalies, negative anomalies and close to normal. Storm contributions to longshore transport are brought into light: inter-annual variability in LST rates is strongly correlated to >4 m waves occurences.The relationship between LST rates gradients alongshore and shoreline changes is also investigated : despite the significant amout of papers on that subject, results showned in this thesis appears to be disappointing.In the light of these findings, coastal drift cell boundaries are defined taking into account the 30-year mean potential LST rates and the inter-annual variability. This thesis thus highlights the good potential of longshore transport modelling in yielding coherent results that are essential from a shoreline management perspective for future coastal sustainability.

Orientador: Geraldo de Freitas Maciel / Resumo: No contexto de Desastres "Naturais", as corridas de lama têm sido objeto de estudo devido ao seu poder erosivo e, muitas vezes, destrutivo, acarretando perdas materiais vultosas e ceifando vidas. Nestes escoamentos, quando em condições favoráveis de vazão, inclinação e reologia do fluido, podem surgir instabilidades que se propagam em forma de trem de ondas na superfície, denominadas roll waves. A literatura acerca do assunto trata, geralmente, dos critérios de geração, estabilidade, e determinação das características principais do fenômeno, como amplitude, comprimento e celeridade de onda. Nesta dissertação buscou-se estudar a tensão de cisalhamento no fundo na presença de roll waves, em duas vertentes: a primeira apresenta as roll waves em água limpa, baseando-se no trabalho clássico de Dressler (1949); a segunda, dando continuidade aos trabalhos do Grupo de Pesquisa de Reologia de Materiais Viscosos e Viscoplástcos (Grupo RMVP), focou no estudo de roll waves desenvolvendo-se em fluidos do tipo Herschel-Bulkley, em duas situações - canal de fundo impermeável e fundo com condição de permeabilidade, grande contribuição desta dissertação. Os modelos matemáticos foram desenvolvidos com base nas equações de águas rasas, cuja implementação numérica permitiu confrontar resultados experimentais e numéricos, que apresentaram boa aderência. Para o modelo com condição de permeabilidade no fundo, verificou-se a influência do fator de porosidade nas características das roll waves (ampli... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Mudflows have been the focus of studies because of their erosive ability and often destructive power, causing material losses and taking away lives. Under favorable conditions of discharge, slope, disturbance, and rheology, these flows can develop a specific type of instability that is propagated downstream as shock waves, called roll waves. Most of the roll waves literature provides information on generation criteria, stability, and information of amplitudes, wavelengths, and celerity. This works brings a study of bottom shear stress in pulsating flows (roll waves) in 2 parts. In the first part, we present roll waves in clean water, based on Dressler’s work. In the second part, we continue the works of research team about roll waves developing in Herschel-Bulkley fluid under 2 conditions: impermeable bottom and porous bed. The mathematical models developed were based on shallow water equations. The results of these models were compared to Fluent and experimental results, showing a good agreement. For the mathematical model with porous bed condition, we evaluated the effect of porosity factor in properties of roll waves, and we observed that the amplitude of the roll wave usually increases, whereas the length and the celerity decrease with the presence of this factor. Finally, considering an impermeable bottom, the bottom shear stress presented an increase of 12% to 27%, depending on nature (laminar-turbulent) and dynamics (Froude number) of flow. Taking in account a porous b... (Complete abstract click electronic access below) / Mestre

Wave energy has a great potential in many coastal areas thanks to a number of advantages: the abundant resource, the highest energy density of all renewables, the greater availability factors than e.g. wind or solar energy; and the low environmental and particularly visual impact. In addition, a novel advantage will be investigated in this work: the possibility of a synergetic use for carbon-free energy production and coastal protection. In this context, wave energy can contribute not only to decarbonising the energy supply and reducing greenhouse emissions, but also to mitigating coastal erosion. In effect, wave farms will be deployed nearshore to generate electricity from wave energy, and therefore the leeward coast will be exposed to a milder wave climate, which can potentially mitigate coastal erosion. This thesis aims to determine the effectiveness of wave farms for combating coastal erosion by means of a suite of state-of-the-art process-based numerical models that are applied in several case studies (Perranporth Beach,UK; and Xago Beach, Spain) and at different time scales (from the short-term to the long-term). A wave propagation model, SWAN, is used to establish the effects of the wave farm on the wave conditions. The outcomes of SWAN will be coupled to XBeach, a costal processes model that is applied to analyse the effects of the milder wave conditions on the coast. In addition to these models, empirical classifications and analytical solutions are used as well to characterise the alteration of the beach morphology due to the presence of a wave farm. The analysis of the wave farm impacts on the wave conditions and the beach morphology will be carried out through a set of ad hoc impact indicators. Parameters such as the reduction in the significant wave height, the performance of the wave farm, the effects on the seabed level and the erosion in the beach face area are defined to characterise these impacts. Moreover, the role played by the key design parameters of wave farms, e.g. farm-to-coast distance or layout, is also examined. The results from this analysis demonstrate that wave farms, in addition to their main purpose of generating carbon-free energy, are capable of reducing erosion at the coast. Storm-induced erosion is significantly reduced due to the presence of wave farms in the areas most at risk from this phenomenon. However, the effects of wave farms on the coast do not lend themselves to general statements, for they will depend on the wave farm design (WEC type, layout and farm-to-coast distance) and the characteristics of the area in question, as shown in this document for Perranporth and Xago. In summary, this synergy will improve the economic viability of wave farm projects through savings in conventional coastal defence measures, thereby fostering the development of this nascent renewable, reducing greenhouse gas emission and converging towards a more sustainable energy model. Thus, wave energy contributes to mitigating climate change by two means, one acting on the cause, the other on the effect: (i) by bringing down carbon emissions (cause) through its production of renewable energy, and (ii) by reducing coastal erosion (effect).

Ce travail de thèse concerne l'étude numérique du transport par charriage d'un lit de grain dans un écoulement de Couette laminaire. La méthode numérique employée est un modèle de type Euler-Lagrange, où la taille de la cellule de calcul eulérienne est supérieure mais du même ordre de grandeur que le diamètre moyen des grains. Elle repose pour la phase fluide Eulérienne sur les équations de Navier-Stokes moyennées par un terme de porosité et pour la phase solide Lagrangienne sur les équations de Newton résolues pour chaque particule via une méthode aux éléments discrets. Un terme d'interaction fluide-particule permet de coupler les deux phases. Dans un premier temps, des simulations en petit domaine sont effectuées afin d'étudier l'influence du nombre de Shields , du nombre de Reynolds particulaire Rep et du rapport de densité p/f sur le transport granulaire. Nous montrons, en particulier, que ce modèle numérique permet de retrouver le seuil de mise en mouvement du lit granulaire et les lois de débit granulaire fonction de comme obtenus dans la littérature. De plus, il est mis en évidence que ce débit granulaire q ne dépend que plus faiblement de Rep et p/f. Dans un second temps, des simulations sur de longs domaines permettent d'observer l'influence de ces mêmes paramètres sur le développement d'instabilités. Plus particulièrement, le développement de l'instabilité de lit sur les temps courts et sur les temps longs sont discutés. / This work deals with local numerical simulations of laminar shear flow eroding a bed of spherical particles. The numerical method used is an Euler-Lagrange model, based on the resolution of the Navier-Stokes equations, averaged over cells containing several particles, and Newton's equations for the solid phase using the discrete element method (DEM). The averaging procedure brings out a solid volume fraction term for the fluid phase, which mimics the porosity of the effective medium. A fluid-particle interaction term enables a two-way coupling. Firstly simulations are performed on a relatively small domain, allowing to reach a steady state without any instability development. The influence of the Shields number , the particle Reynolds number Rep and the density ratio p/f is observed independently from each other. Simulations agrees well with experiments and lower scale simulations, giving a granular transport largely governed by the Shields number compared to the other dimensionless numbers and a pretty good estimation of the threshold Shields number c, which delineates static and moving bed. Secondly, simulations on a larger domain are performed in order to capture the formation and the development of ripples. The influence of the three dimensionless numbers on the ripple characteristics, such as wavelength or amplitude, is investigated. These preliminary results seems to be in a reasonable agreement with experimental and simulation data. In particular, the relative influence of and Rep is discussed.