La thèse a pour objectif l'étude des plages sableuses enclavées, i.e. contraintes aux extrémités par des caps rocheux, soumises aux vagues et à un cycle de marée morte-eau vive-eau. Une double approche complémentaire liant études numériques et observations et mesures in situ sur un site-atelier a été retenue. Deux modèles déterministes basés sur les différents processus physiques (hydrodynamiques, hydro-sédimentaires et morphodynamiques) en jeu dans la zone littorale et moyennés sur la verticale (2DH) on été sélectionnés. Le code XBeach (Roelvink et al., 2009) utilise un maillage aux différences finies et intègre certaines spécificités tels qu'un modèle de propagation d'un rouleau d'écume ("roller"), un modèle de génération d'ondes longues et un modèle de développement d'un contre-courant de retour "undertow". Le code TELEMAC (Hervouet, 2007) utilise un maillage aux éléments finis. Pour le site-atelier, les plages enclavées adjacentes de La Palue et de Lostmarc'h en Presqu'île de Crozon (Finistère) ont été choisies pour leur bathymétrie complexe (présence de chenaux de baïne et d'un saillant) et leur géométrie complexe (caps rocheux de longueurs diverses, îlot). Une année d'observations et de mesures in situ (levés topographiques, pose de courantomètres ADV) a été entreprise entre le mois de septembre 2014 et le mois de septembre 2015, par diverses conditions météorologiques.Dans un premier temps, la dynamique globale des plages enclavées, représentées schématiquement par une échancrure rectangulaire, a été appréhendée. En particulier, la genèse d'une barre de déferlement en zone subtidale sous l'influence des dimensions de l'échancrure rocheuse, d'un cycle de marée morte-eau vive-eau et de houles énergétiques a été illustrée à travers plusieurs séries de simulations numériques à l'aide du code XBeach.Dans un deuxième temps, la modélisation hydrodynamique des courants sur les plages de La Palue et de Lostmarc'h a été effectuée, avec l'objectif d'étudier l'influence des caractéristiques des houles (hauteur, période et direction) et de celles de l'échancrure rocheuse (longueur des caps, espacement des caps, inclinaison des pointes) sur la prédiction des courants. Une ouverture sur la modélisation aux éléments finis (TELEMAC) en comparaison à celle aux différences finies (XBeach) a été présentée. Le code XBeach a été validé sur le plan hydrodynamique grâce aux mesures in situ d'une campagne de terrain réalisée en janvier 2015. Une étude numérique hydro-sédimentaire sur une lunaison complète (du 01 septembre 2015 au 29 septembre 2015) a alors été entreprise à l'aide du code XBeach, afin d'étudier l'intensité et la direction des flux sédimentaires instantanés et résiduels au gré des différents climats de houle et de marée, et selon les différents secteurs typologiques des plages de La Palue et de Lotsmarc'h. Cela a permis de mettre en évidence les particularités de la dynamique des plages enclavées par rapport aux plages ouvertes "quasi-infinies", tels que le phénomène de "contournement de cap", mais aussi les similitudes. Les résultats du code XBeach, pour différentes configurations du modèle, ont par ailleurs été comparés aux levés topographiques réalisés au cours du mois de septembre 2015.Dans un troisième temps, après une analyse statistique approfondie des climats de houle au large des plages de La Palue et de Lostmarc'h sur une période de 7 ans, l'impact morphodynamique de tempêtes extrêmes (houles décennale et cinquentennale) conjugué à celui de la hausse du niveau marin induit par le changement climatique, a été estimé sous la forme d'une étude numérique à l'aide du code XBeach sur un profil transversal au sud de la plage de La Palue / In this thesis, waves and neap-spring tide action on sandy embayed beaches constrained by rocky headlands is studied. A complementary approach combining numerical modeling and in situ measurements is used. Two deterministic model based on major physical processes (hydrodynamics, hydro-sedimentary and morphodynamics) present in the coastal zone and averaged over the water height (2DH) have been selected. XBeach (Roelvink et al., 2009) employs a finite difference scheme and incorporates some specific processes such as a roller, wave groups and the associated bound long wave and a undertow. TELEMAC (Hervouet, 2007) is a finite element scheme. Nearby sandy embayed beaches of La Palue and Lostmarc'h (Crozon Peninsula, Finistère) have been chosen for their complex bathymetry (rip channels, tombolo) and geometry (headlands, small island). One year of in situ measurements (topographical surveys, ADV velocimeters) between septembre 2014 and september 2015 has been undertaken, with different waves and tide climates.Firstly, the overall dynamics of embayed beaches built schematically in the form of a rectangular indentation has been caught. Especially, subtidal sandbar formation under the influence of waves, neap-spring tide and rocky headlands has been studied through several sets of numerical simulations.Secondly, hydrodynamic modelling on the embayed beaches of La Palue and Lostmarc'h has been made with the aim of study influence of waves characteristics (height, period and direction) and indentation's ones (headlands length, spacing between headlands, headlands inclination) on the prediction of currents. A simulation using finite element scheme (TELEMAC) has been achieved and compared to those using finite difference scheme (XBeach). XBeach currents predictions at two locations on the beaches of La Palue and Lostmarc'h over the period from the 21st January 2015 to the 23rd January 2015 have been compared to in situ measurements to validate the model. Whereupon, a numerical hydro-sedimentary simulation (using XBeach) over a full lunation from the 1st September 2015 to the 29th September 2015 has been undertaken on these two beaches, with the aim of study the intensity and direction of sediment flux (instantaneous and residual) for various waves and tide climates and for different morphological areas. This work has helped to highlight the dynamics features of embayed beaches compared to open beaches ones, such as headland sand bypassing, but also similarities. Moreover, XBeach morphodynamics predictions have been compared to in situ measurements for different model setup.Thirdly, after an in-depth statistical analysis of wave climates over a 7 year period offshore of La Palue and Lostmarc'h beaches, the morphodynamic impact of extreme storm waves (ten-yearly and fifty-yearly) combined to a sea level rise induced by climatic change on a cross-shore bottom profile south of La Palue has been estimated through a numerical study (using XBeach)

Ce travail de recherche s’inscrit dans le contexte actuel d’accroissement de la vulnérabilité de l’Homme et des activités humaines face aux risques littoraux (aléas érosion et submersion marine), en lien avec les changements environnementaux globaux (élévation du niveau moyen relatif des océans et augmentation de la fréquence et de la virulence des tempêtes) et à la concentration de la population sur le littoral. Suite à la tempête Xynthia du mois de février 2010, l’Etat français s’est doté d’une Stratégie Nationale de Gestion Intégrée du Trait de Côte visant à mettre en place de nouvelles approches en matière de gestion du trait de côte, et notamment, favoriser le repli stratégique contre la défense côtière à tout prix. La mise en place de cette stratégie doit avant tout s’appuyer sur la mise en place d’un réseau d’observation et de suivi de l’évolution du trait de côte à l’échelle de la France. Ce travail s’inscrit dans cette problématique. Il a eu pour objectif d’étudier les dynamiques du trait de côte des plages sableuses et des cordons de galets (formes d’accumulation) de la Bretagne, à différentes échelles spatio-temporelles. Sur le long terme, l’analyse a consisté dans un premier temps à établir une typologie des côtes d’accumulation suivant différents critères morphosédimentaires ; la cinématique du trait de côte a ensuite été analysée à l’échelle des soixante dernières années (1950-2010) à partir d’une étude diachronique par photogrammétrie. Sur le moyen terme, l’analyse s’est faite dans le cadre des suivis topomorphologiques réalisés à l’échelle pluriannuelle à pluridécennale, de plusieurs sites ateliers de l’ODC (IUEM) et/ou du SNO Dynalitt. L’étude des changements morphosédimentaires s’est accompagnée d’une analyse de données de houle, de marée, de vent, et de pression atmosphérique, dans le but d’expliquer la part de ces forçages dans l’alternance de phases d’érosion et de périodes de régénération. Enfin sur le court terme, cette analyse s’est basée sur des levés topomorphologiques à haute fréquence réalisés au cours de l’hiver tempétueux 2013-2014. A cela, s’est ajoutée une étude expérimentale visant à quantifier le runup, dans le but d’analyser l’impact des niveaux d’eau extrêmes à la côte sur l’érosion du trait de côte. De manière globale, la quantification des variations du trait de côte sur le long terme a mis en évidence une tendance à l’érosion (27 % de linéaire étudié), une progradation du trait de côte non négligeable (20,5 % de linéaire) ainsi qu’une importante artificialisation du trait de côte (34,5 % de linéaire). La similitude des réponses morphosédimentaires des cordons littoraux étudiés aux conditions météomarines a permis d’identifier cinq phases distinctes. Trois phases morphogènes (de 1998 à 2003 ; de l’hiver 2006/07 au printemps 2008 ; de l’hiver 2012/13 à aujourd’hui), caractérisées par une érosion importante du trait de côte combinée à une haute fréquence des épisodes tempétueux ; et deux phases au cours desquelles les conditions météomarines ont été plus clémentes (de 2003 à l’hiver 2006/07 ; du printemps 2008 à l’hiver 2012/13), permettant la régénération des cordons littoraux. Au cours de l’hiver 2013-2014, trois tempêtes ont été particulièrement morphogènes en matière d’érosion du trait de côte car elles ont été combinées à de fortes marées de vive-eau : la tempête du 4 et 5 janvier, celle du 1 au 3 février, ainsi que celle du 2 au 3 mars. Durant cet hiver, le recul moyen pour tous les sites étudiés a atteint -6,3 m, avec un maximum d’environ -30,1 m et un minimum de -0,2 m. / This research is part of current context of increasing of human beings and human activities vulnerability face to coastal risks (erosion hazards and marine submersion), linked to global environmental changes (global relative sea level rise and growth of storms’ frequency and intensity) and the current concentration of the population on the coastline. Following the storm Xynthia (February 2010), France has adopted a national strategy dealing with coastal erosion and shoreline retreat, in order to define a new integrated national strategy of coastline management, promoting the relocation of stakes and properties. This strategy focuses on the monitoring of shoreline changes and the identification of coastal risks due to erosion.This work fall within this issue. The main objective was to study sedimentary coasts dynamics (sand dunes and gravel barriers) of Brittany, following different spatial and temporal scales. In the long term, the analysis consisted, first of all, in establishing a typology of sedimentary coasts according to several morpho-sedimentary criteria; then, coastline kinematic has been analyzed over the last sixty years (1950-2010) through a diachronic study using photogrammetry. In the medium term, analysis was carried out as part of topographic surveys performed at multi-annual to multi-decadal scale, on several Observatoire du Domaine Côtier (IUEM) and/or Service National d’Observation - Dynalitt survey sites. The morpho-sedimentary study was combined with a wave, tide, wind and atmospheric pressure analysis, in order to explain the role of these forcing in the erosion and recovery successive phases. Finally, in the short term, analysis was based on high frequency of topomorphological surveys carried out during the 2013-2014 stormy winter. Additionally, an experimental study to quantify runup process, in order to analyze extreme water levels impact on shoreline erosion.Overall, the quantification of coastline variations over the long term revealed an erosion tendency on 27% of the studied linear, a non-negligible progression of the shoreline on 20.5% of the studied linear, and a significant shoreline anthropogenic impact (34.5% of the studied linear). The similarity of the morpho-sedimentary responses of the littoral strands studied to the meteorological conditions allowed to identify five distinct phases. Three morphogenetic phases (1998 to 2003; winter 2006/07 to spring 2008; winter 2012/13 to the present), characterized by severe shoreline erosion combined with a high frequency of storm episodes, and two phases (2003 to winter 2006/07 ; spring 2008 to winter 2012/13) characterized by milder weather conditions, allowing shoreline recovery. During 2013-2014 winter, three storms were particularly morphogenetic for sedimentary coasts, because they were combined with high spring tides: January 4-5, February 1-3, and March 2-3. During this winter, the average retreat for all studied sites reached -6,3 m, with a maximum of -30,1 m and a minimum of -0,2 m.

Braided channels arise due to high sediment availability in conjunction with regular competent flows and readily erodible banks. Together, these boundary conditions lead to the deposition and reworking of a network of transient bars that characterize the braided planform. However, quantifying the geomorphic response of braided systems to alterations in these boundary conditions is not straightforward, as channels adjust over a wide range of timescales, rendering traditional field-based observation intractable. As such, the development of simple yet robust relationships between channel morphology and sediment transport has the potential to allow predictions of channel response to altered hydrologic or sediment regimes. In this research, I first use laboratory flume experiments to relate particle travel distance during floods (termed particle path length) and the spacing of channel bars in braided rivers (Chapter 2), finding that deposition sites for sediment in transport can be readily predicted by the characteristic confluence-diffluence spacing in a reach. I then use the relationship between path length and channel morphology to build a simple, open-source morphodynamic model for braided rivers that computes sediment transport using path-length distributions derived from bar spacing (Chapter 3). I explore the validity of this model, specifically noting that its modular framework allows exploration of process representations in morphodynamic modeling in ways existing models do not. Finally, I employ the model to determine the role of sediment supply in braided channel bar morphodynamics (Chapter 4). Specifically, I address the relative roles of sediment sourced from upstream versus sediment sourced from within a braided reach in terms of channel morphodynamics at decadal timescales. This research demonstrates that simple scaling relationships, while necessarily imperfect, nevertheless provide insight into morphodynamic processes in braided rivers, while also allowing predictions of channel response to sediment or hydrologic forcing at the timescales of channel adjustment.

[pt] Atrelado à densa ocupação dessas áreas, identifica-se que parte de suas feições naturalmente protetoras da linha de costa são recorrentemente suprimidas da paisagem, dando lugar a obras de engenharia rígidas que passam a ocupar o perfil dinâmico das praias. Com isso, em casos de eventos de tempestades, as praias tornam-se susceptíveis a ação das ondas, bem como a população que habita a beira mar. Nesse sentido, o presente trabalho tem como objetivo caracterizar sazonalmente o comportamento morfodinâmico e sedimentar do arco praial localizado entre o Leblon e Arpoador e sua suspectibilidade aos efeitos das ressacas, fornecendo subsídios para a gestão do litoral. Para a obtenção dos dados foram aquisitados 32 perfis topográficos de praia entre 2018-2021, com o método das balizas de Emery; análises nas areias das praias (granulometria e morfoscopia); análises morfoscópicas; cálculos do volume emerso dos sedimentos e levantamento de alertas de ressacas emitidos pela Marinha do Brasil e demais veículos midiáticos. As extremidades do arco praial (pontos LB01 e AR01) apresentaram maior variabilidade morfológica e na largura da praia variação da largura da praia.; o ponto LB02 (próximo ao canal de Jardim de Alah) apresenta-se como o mais susceptível às ondas de tempestades; e o ponto IP01 (localizado no meio do arco), é considerado o mais estável devido à presença de ilhas em sua frente. Os sedimentos emersos do arco praial exibiram uma sazonalidade marcada por acúmulo no verão e perda subsequente dos mesmos no inverno. O papel das ondas é essencial na dinâmica do arco praial, com incidência predomiante de Sudoeste (tempestade) e Sudeste (tempo bom), o que possibilita a formação de correntes de deriva litorânea importantes na distribuição Leste-Oeste rotacional de sedimentos no arco praial. Conclui-se que o arco praial apresenta certo equilibrio dinâmico, uma vez que os perfis topográficos mostram grande variabilidade de largura e tal dinâmica pode ser corroborada pela relação de ganha e perda do volume de sedimentos do arco. A areia média é predominante porém percebe-se uma distinta distribuição dos grãos no arco praial: observou areias médias a grossas na porção oeste (Leblon) e areias médias a finas na porção leste (Arpoador). A distribuição sedimentar se mostra diferente também em termos de grau de arredondamento (no pós-praia os grãos são subarredondados e na face de praia são subangulares), seleção do grão e maturidade. / [en] Linked to the dense occupation of these areas, it is identified that part of its features that naturally protect the coastline are recurrently suppressed from the landscape, giving way to rigid engineering works that begin to occupy the dynamic profile of the beaches. With that, in cases of storm events, the beaches become susceptible to wave action, as well as the population that inhabits the seaside. In this sense, the present work aims to seasonally characterize the morphodynamic and sedimentary behavior of the coast located between Leblon and Arpoador and its susceptibility to the effects of storm surges, providing subsidies for the management of the coast. To obtain the data, 32 topographic beach profiles were acquired between 2018- 2021, using the Emery method; beach sand analyzes (granulometry and morphoscopy); morphoscopic analysis; calculations of the emerged volume of sediments and survey of storm surge alerts issued by the Brazilian Navy and other media outlets. The ends of the beach (points LB01 and AR01) showed greater morphological variability and variation in beach width in beach width.; point LB02 (close to the Jardim de Alah channel) is the most susceptible to storm waves; and point IP01 (located in the middle of the arc), is considered the most stable due to the presence of islands in front of it. Emerged sediments from the coast exhibited a seasonality marked by accumulation in summer and subsequent loss in winter. The role of waves is essential in the dynamics of the beach arc, with predominant incidence from the Southwest (storm) and Southeast (clear weather), which enables the formation of longshore drift currents that are important in the rotational East-West distribution of sediments in the beach arc. It is concluded that the Leblon-Arpoador coast presents a certain dynamic balance, since the topographic profiles show great variability in width and such dynamics can be corroborated by the relationship between gain and loss in the volume of sediments in the arch. Medium sand is predominant, but a distinct distribution of grains can be seen in the coast: medium to coarse sand was observed in the western portion (Leblon) and medium to fine sand in the eastern portion (Arpoador). The sedimentary distribution is also different in terms of degree of roundness (in the backshore the grains are subrounded and in the beach face they are subangular), grain selection and maturity.

Braided rivers represent one of the most complex forms of natural streams. Characterized by intense bed-load transport and highly dynamic channels, they carry significant naturalistic value and support a multiplicity of ecosystem services. Anthropogenic stressors and environmental changes put under stress hydro-morphological dynamics, biological processes, and ecosystem functioning and services of these fragile environments, necessitating integrated management and conservation strategies to preserve their biodiversity and ecological integrity. From a regulatory perspective, the two European Directives 2007/60/EC (the Floods Directive) and 2000/60/EC (the Water Framework Directive) identify and promote win--win measures that both reduce hydraulic risk and enhance the quality of water bodies. Some examples of win--win measures are river naturalization projects that not only restore river ecosystems to their natural state, enhancing biodiversity and ecosystem services but also provide flood protection, improve water quality, and offer recreational opportunities for local communities. This thesis contributes to the development of scientific knowledge in the previously mentioned areas, facilitating the know-how transfer of expertise from academia to the public institution. Building on these premises, this thesis aims to provide additional insights into the morphodynamics of braided rivers, offering new perspectives on the evolution of morphological indices during flood events and contributing valuable knowledge on how these complex systems respond to external stressors. The PhD thesis has been structured along three parts. The primary goal was to develop an innovative unsupervised algorithm for extracting the spatial and temporal evolution of braided river morphology. This computational framework is tailored for Sentinel--1 Synthetic Aperture Radar (SAR) data, overcoming the limitations imposed by weather conditions and day--night cicles. Moreover, it can be effortlessly adapted to additional SAR imagery databases. In cases where the water class covers only a minimal area of the entire scene, the histogram primarily represents the dry soil class. The framework faces this challenge employing a Self-Adaptive Thresholding Approach (SATA) to achieve a distinct bimodal distribution, enabling the accurate computation of threshold values for the 'dry soil' and 'water' classes. The tool, developed within the Python--API of Google Earth Engine (GEE), allowed us to assess the intra--event inundation dynamics, the estimation of the relationship between hydrometric level and wet area extension, and the assessment of bank erosion phenomena. The second chapter focuses on analyzing how morphological indices, such as the Total Braiding Intensity (TBI) index defined as the number of active channels, the Maximum Channel distance (MCD) defined as the distance between the most external channels, and the Cross-Sectional Cumulative Wetted Area (WA) defined as the sum of the wet area of all chanels in a cross section, correlate with discharge variations during flood events. To achieve this objective, the framework designed for Sentinel--1 images was adapted for use with high--definition imagery from the Italian COSMO--SkyMed satellite constellation. Leveraging the superior ground resolution of 3x3 meters provided by the Italian COSMO--SkyMed satellite constellation, we successfully segmented narrow secondary branches that remained undetected with Sentinel--1's 5x20 meter resolution. Thus obtained, the temporal evolution of the braiding system, enables us to evaluate the temporal evolution and the relationship between the TBI, MCD, and WA indices with increasing discharge values. The last part of the PhD thesis, deals with the assessment of the river bed grain size. The initial concept behind this PhD work was to analyze the potential of Synthetic Aperture Radar (SAR) data in assessing not only river morphology but also the pattern of patches with different grain size. While the initial two parts of the work addressed this, the final section's analysis of SAR data, unfortunately, did not provide significant results. Nevertheless, the subjects of surface roughness and the creation of spatially distributed grain size maps continue to hold significant scientific value in the fields of hydraulic and eco--hydraulic modeling and a key information for river management and renaturation projects. The principal role of this factor led us to slightly shift the research focus towards a detailed investigation of these elements, utilizing orthophotos, digital imagery, and corresponding analytical methods to model patterns of river roughness and grain size. A map illustrating the spatial pattern of grain size at the river reach scale was produced through regression analysis. This analysis correlated the texture properties derived from orthophoto tiles with the d50, d84, d90, and d95 grain size characteristics obtained from digital images, thereby providing considerable support for the implementation of detailed hydraulic models.

El presente trabajo destaca la simulación numérica como herramienta capaz de reproducir y predecir los principales procesos que producen y mantienen los ecosistemas de llanuras de inundación. Para ello, la dinámica del flujo, la actividad geomorfológica, la sedimentación y la captura de nutrientes de la llanura de inundación, son evaluadas experimentalmente. Seguidamente, los datos experimentales son incluidos en un modelo numérico para desarrollar una herramienta completa de simulación capaz de predecir la dinámica del flujo, la actividad geomorfológica, la sedimentación, el aporte de nutrientes del río a la llanura de inundación así como la captación de los mismos. El estudio se desarrolla en un segmento del río Ebro (NE España), representativo de su tramo medio meandriforme. El estudio emplea un modelo numérico bidimensional (2D), basado en las ecuaciones 2D de las aguas poco profundas y calculado a partir del método de los volúmenes finitos. La calibración del modelo se basa en medidas experimentales realizadas en la llanura de inundación bajo dos caudales estacionarios. La validación del modelo se llevó a cabo comparando los resultados numéricos con las medidas experimentales de nivel de agua y extensión del área inundada realizadas durante los cinco eventos de riada ocurridos en el año 2007 (79+13 % área; 0.27+0.05m nivel de agua). Las simulaciones fueron utilizadas para analizar la dinámica actual de inundación de la llanura así como su actividad geomorfológica. Como resultado, se obtuvo que el presente régimen de caudales y sobretodo la estructura morfológica de la llanura, no resulta suficiente para generar una actividad geomorfológica tal que mantenga el mosaico cambiante de hábitats característico de los ecosistemas de llanura de inundación. La nueva formulación que describe la retención de nutrientes se ha desarrollado mediante la utilización de los datos experimentales, en condiciones de flujo controladas, semi-controladas y en el propio río Ebro. / González Sanchis, MDC. (2012). Flooding dynamics and nutrient retention in the middle Ebro floodplain: experimental assessment and numerical modeling [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/15242

Coastal foredunes are natural aeolian bedforms located landward of the backshore and which interact continuously with the beach. Traditionally, coastal dunes have been associated with onshore winds, however they can be found under more complex wind regimes where offshore winds are common such as the UK East coast, Northern Ireland and New Zealand. This research investigates the ways in which foredune-beach interactions occur under a complex wind regime at a range of overlapping temporal and spatial scales and is innovative in that it explicitly links small-scale processes and morphodynamic behaviour to large scale and long-term dynamics. The study area is the north Lincolnshire coast, East England. Detailed observations of airflow at three locations under varying wind regimes revealed considerable spatial variations in wind velocity and direction, however it was possible to determine a general model of how foredune topography deflected and modified airflow and the resultant geomorphological implications (i.e. erosion and deposition). During direct offshore and onshore winds, airflow remained attached and undeflected; and distinct zones of flow deceleration and acceleration could be identified. During oblique winds airflow was deflected to become more parallel to the dune crest. The field sites used are characterized by a seasonal erosion/accretion cycle and a series of increasingly complex models was developed and tested to determine whether it was possible to predict sand volume changes in the foredune-beach system based on a limited number of variables. The model predictions were tested against detailed digital terrain models at a seasonal timescale. The model prediction that best matched the observed (surveyed) sand volume changes included wind speed, direction, grain size, fetch effect controlled by beach inundation and angle of wind approach was accurate to within ±10% for 18 out of 48 tests at the seasonal scale and 6 out of 12 tests over periods of >5 years. A key variable influencing foredune-beach sand volume is the magnitude and frequency of storm surge events and this was not factored in to the model, but may explain the model-observation mismatch over the medium-term on two occasions. Over the past 120 years historical maps and aerial photographs indicate long-term foredune accretion of approximately 2 m year-1 at the three study sites (1891-2010). At this timescale, rates of coastal foredune accretion reflect the low occurrence of severe storm surges and suggest rapid post-storm recovery. The morphological response of the foredune-beach morphology is considered to be a combination of controlling and forcing factors. Process-responses within the system, associated with nearshore interactions and sediment transfer from the littoral drift, are compiled into a multi-scale morphodynamic model. Important to match appropriate dataset to scale of research question or management plan being explored. In the case of management, long-term records of past activity are necessary to predict the future but also to understand natural responses of system to short-term impact such as storm surge.

Ce travail porte sur l’étude de l’évolution morphologique dans la zone de swash à haute résolution spatiale et temporelle afin de pouvoir analyser l’influence de la houle. Cette étude s’appuie sur deux campagnes de mesures menées dans le grand canal à houle CIEM (Barcelone), l’une avec application d’un forçage aléatoire érosif de type JONSWAP et l’autre avec application d’une série de forçages bichromatiques. L’originalité de l’étude proposée ici consiste en l’analyse, par une méthode de stéréoscopie optique, de l’évolution du fond avec une excellente précision tout en quantifiant précisément l’évolution des fronts d’eau sur une surface continue, la majorité des techniques de mesure ne donnant en général accès qu’à des relevés ponctuels dans cette zone. La position du lit de sable est estimée avec une précision de quelques centaines de μm. Sur des temps longs, le forçage JONSWAP érosif engendre un recul global et quasi-homogène dans la zone de swash. Cependant, cette évolution ne semble pas influencer la réponse hydrodynamique qui reste similaire dans la zone de swash lorsque l’on répète la série. Ceci a été confirmé dans le cas des forçages bichromatiques, répétés sur des fonds différents. Au sein d’une série d’un forçage aléatoire, différentes échelles de temps sont observées : l’échelle gravitaire (période caractéristique du signal) et les ondes infragravitaires (large gamme d’événements plus longs). Cette coexistence de plusieurs échelles de temps a pu être associée à la forte variabilité du profil de plage, notamment dans la direction cross-shore. Le cas bichromatique montre quant à lui une convergence du profil de plage vers un état qui ne dépendrait que du forçage en entrée. L’interaction d’une seule onde infragravitaire avec l’onde gravitaire ne peut donc pas être responsable de la variabilité du fond observée avec le forçage aléatoire. En revanche, cette étude semble confirmer la présence d’événements particuliers à l’origine d’une forte modification du fond sableux. Le lien entre ces événements et l’interaction entre des événements hydrodynamiques spécifiques et la forme du fond n’est pas encore complètement établi. Enfin, pour le forçage aléatoire la variabilité transverse induite par les ondes courtes est forte alors qu’elle semble plus faible à l’échelle de l’onde longue. Pour le forçage bichromatique, cette variabilité transverse a également pu être observée mais son lien avec le forçage reste encore à élucider. / This study investigates the morphological evolution in the swash zone at high spatial and temporal resolution to be able to analyze the swell influence. The study is here based on two measurement campaigns in the large CIEM wave flume (Barcelona), using both a random erosive JONSWAP and bichromatics forcing. The originality of the proposed study lies on analysis, by an optical stereoscopic method, of bottom evolution with a very good precision while getting an accurate quantification of water fronts evolution on a continuous area, most of the measurement techniques generally allowing only to obtain punctual measurements in this area. The sand bed position is estimated with an accuracy of a few hundred μm. Over long periods, erosive JONSWAP forcing generates an overall and almost homogeneous backward movement in the swash area. However, this trend does not appear to influence the hydrodynamics response that remains similar in the swash area when the series is repeated. This was confirmed in the bichromatic forcings case, repeated on different bottoms. During a random forcing series, different time scales are observed : the gravity scale (characteristic period of the signal) and infragravity waves (wide range of longer events). This coexistence of several time scales could be associated with the observation of a strong variability of the beach profile, especially in the cross-shore direction. The bichromatic case shows meanwhile convergence of beach profile to a state that would depend only on forcing conditions. The interaction of a single infragravity wave with the gravitational wave can therefore not be responsible for the observed bottom variability with the random forcing. In contrast, the present study seems to confirm the presence of specific events responsible of a strong change in the sandy bottom. The link between these events and the hydrodynamic interaction between specific events and the bottom shape is not yet completely established. Finally, for the random forcing the transverse variability induced by short wave is strong as it seems weak at long wave time scale. For bichromatic forcing, the transverse variability has also been observed but its link with the forcing still remains to be elucidated.

Ce travail de thèse est une étude multi-échelles de temps de la morphodynamique d'une île barrière soumise à des conditions de forçages paroxysmaux de mousson, de tempêtes tropicales et de typhons. Cette étude se concentre dans un premier temps, sur un large travail de terrain effectué dans le cadre du projet KUN-SHEN. Sept mois de mesures (de novembre 2011 à janvier 2012 et de mai à septembre 2012) ont permis l'acquisition d'un jeu de données inédit dans la littérature, exhaustif et de haute résolution temporelle et spatiale. Des mesures hydrodynamiques du large jusqu’à la zone de jet de rive ont été acquises en parallèle d'un suivi topographique de la plage émergée. Les mesures de vagues ont été acquises à partir d'une bouée au large, de deux houlographes-courantomètres en avant-côte (mesures en continu à 2 Hz) ainsi que de pressiomètres enfouis dans la plage sub-aérienne (mesures en continu à 5 Hz). Les levés topo-bathymétriques ont été réalisés au D-GPS (résolution centimétrique) une fois par semaine pendant la saison de mousson et juste après et avant chaque événements de tempêtes extrêmes pendant la saison des typhons. Le travail d'analyse cible ensuite la morphodynamique de la zone de jet de rive et de la plage émergée sur un panel d’échelles spatio-temporelles allant de l'instantanée à annuelle. A l’échelle instantanée, ce sont les variations d’élévation de la surface libre de l'eau et du lit sableux (quelques secondes) qui sont observées au cours de chacune des phases (montant, apex, tombant) de la tempête tropicale Talim (juin 2012, Hs = 10.34 m et Ts = 14.6 s). Pour le même événement de tempête, la réponse morphologique de toute la plage émergée est ensuite décrite et quantifiée dans le détail à l’échelle événementielle (quelques jours). Les bilans sédimentaires de chacune des saisons sont ensuite quantifiés dans le but de caractériser la dynamique saisonnière (quelques mois) à annuelle d'une barrière sableuse soumise à deux types de forçages extrêmes différents (mousson/typhons). L'impact sur le front de plage, des groupes de tempêtes faibles à modérées pendant l'hiver est ainsi comparé à l'impact des tempêtes extrêmes pendant l'été. On souligne finalement 1) l'importance d'un jeu de mesures in-situ de bonne qualité dans un travail d'analyse morphodynamique, 2) l'essentialité de l’emboîtement des échelles spatio-temporelle ainsi que 3) le rôle du profil morphologique héritée dans la réponse morphologique d'une plage émergée lorsqu'elle est soumise à des conditions de forçages de vent et de vague extrêmes. / This work focuses on a multi-scale analysis of a sand barrier morphodynamics exposed to paroxysmal forcings of monsoon, tropical storms, and typhoons. The study provided a large dataset of in-situ measures acquired in the framework of the KUNSHEN project. Seven months of field work (from November 2011 to January 2012 and from May to September 2012) provided a new, exhaustive and hight resolution measures. Measures of hydrodynamic conditions from offshore to the swash zone are acquired as well as high frequency topographic surveys of the emerged beach. Waves monitoring are from an offshore buoy, two current profilers in the shoreface (continuous measures at 2 Hz) and pressure sensors buried in the sub-aerial beach (continuous measures at 5 Hz). The topo-bathymetric surveys were acquired using D-GPS (centimetric resolution) one time a week during winter season of monsoon and just before and after each extreme storm during summer season of typhoons.Then, analysis focus in the morphodynamics of the swash zone and the emerged beach on a range of scales from instantaneous to one year. At the instantaneous scale, we analyze variations of the water surface fluctuation and sand bed elevation (seconds) during each phases (rising, apex and falling) of the Talim tropical storm (June 2012, Hs = 10.34 m and Ts = 14.6 s). Concerning the same storm, we characterize the morphologic changes of the whole emerged beach at the evenemential scale (days). Sand balances of each seasons are quantified in order to characterize the dynamics from seasonal scale (months) to annual scale of a sand barrier driven by two types of extreme forcings (monsoon/typhoons). On the beach-front, impact of groups of winter weak storms is compared to the impact of extremes tropical storms and typhoons. To conclude, we highlight 1) importance of in-situ measures in morphodynamic studies, 2) the major role of scales stacking and 3) the role of herited beach profile in the morphologic responses of the emerged beach driven by extremes forcings of wind and wave.

Cette thèse porte sur une étude de la morphologie et des processus morphosédimentaires qui régissent la formation, la dynamique et l'évolution des flèches littorales sableuses caractéristiques de la côte sous dominance de la houle du Sénégal. Elle insiste sur la singularité de la côte sénégalaise à fort développement de flèches par rapport aux littoraux d'Afrique de l'Ouest à cordons massifs. Ces flèches sont construites par la dérive littorale mais l'on sait très peu de leur dynamique, notamment les apports de sables, leur redistribution par la dérive littorale, les relations entre l’allongement de ces flèches et les écoulements fluviaux et tidaux sur des embouchures fluviaux ou lagunaires déviées par ces formes, et les conditions favorisant leur rupture, aspect très important en aménagement littoral. La thèse, à travers une approche combinant la télédétection, l'expérimentation in situ impliquant des mesures de topométrie à haute résolution et d'hydrodynamique, et la modélisation conceptuelle, distingue des flèches à dérive libre et à dérive entravée et les mécanismes d'allongement continu ou restreint et de courbure de ces flèches. Ces aspects morphodynamiques intègrent aussi la relation entre ces flèches et les embouchures des cours d'eau qu'elles barrent, notamment celle du delta du fleuve Sénégal. Les flèches de la côte sénégalaise jouent un rôle important en matière de protection du littoral et d'aménagement des embouchures, et sont un maillon essentiel du transit de sables le long de cette côte. Les schémas d'aménagement de ces flèches et les embouchures associées doivent mieux intégrer la façon dont ces formes fonctionnent et leurs conditions de rupture. / This thesis is a study of the morphology and morphosedimentary processes that determine the formation, dynamics and evolution of the sand spits characterizing the wave-dominated coast of Senegal. The thesis insists on the singularity of the coast of Senegal, the numerous spits of which provide a contrast with the sandy coasts of West Africa that are characterized by massive beach-ridge complexes. These spits are constructed by longshore drift but little is known of their dynamics, notably sand supply patterns, the way sand is redistributed alongshore, the relationship between these spits and river and tidal flux at the river mouths or lagoon inlets they divert, and the conditions that favour spit breaching, a mechanism that has important implications for coastal management. Using an approach that combines remote sensing, field experiments aimed at acquiring high-resolution topometric and hydrodynamic data, and conceptual models, spits characterized by free drift are distinguished from those with limited elongation, and the mechanisms pertinent to each type and to spit lengthening and recurves are analyzed. These morphodynamic aspects also include a scrutiny of the relationship between these spits and the river mouths they divert, especially that of the Senegal River delta. The spits of the coast of Senegal play an important role in coastal protection and in the management of the river mouths. They are also an essential element in sand drift along this coast. Coastal zone management plans regarding these spits and their associated river mouths need to consider more closely how these spits work and the conditions that can lead to their breaching.