­^¤åºK­n A time-independent finite-difference numerical scheme is developed to study the dynamic response of a submerged and a floating breakwater under the wave loading of a fully numerical force. The coupled surge, heave and pitch motion of a floating breakwater and the wave-structure interaction are included in the model. The numerical results are validated uses several bench mark studies and results available elsemlse. The wave reducing effect of a submerged and a floating breakwaters were analysis and discusse.

During the onslaught of a storm on the coasts of Taiwan in summer and autumn, large waves and storm surge have often caused beach erosion. In order to mitigate coastal disasters and erosion, soft-options have been promoted in recent years, to fulfill the purpose of shore protection, as well as to meet the new requirements of landscape, ecology and recreation. Consequently, semi-natural approaches have been adopted in stead of the conventional hard-structures. This thesis aims to report a study on the effects of detached submerged breakwater layouts and storm wave types on beach profile changes and berm retreat. In order to establish a practical procedure to assist the assessment of beach profile changes with submerged breakwaters during storm, we first collect and analyze the beach profile change data performed in large wave tanks (CE from the US and PI from Japan), and apply the well known SBEACH model to derive regression relationship between the two key parameters ( and ) in this model against the non-dimensional fall velocity ( ). The suggestion of Larson and Kraus (2000) to include hard bottom option in SBEACH with a set of modified and values, which may be different from that originally developed for a sandy beach environment, is then carried out using the beach profile changes results conducted experimentally with submerged breakwaters (Risio and Lisi, 2010). Consequently, we have conducted the numerical experiments systematically to study the beach profile changes using submerged breakwaters under various environmental combinations (with 10 different offshore distances, 10 breakwater heights, 10 crown widths and 4 types of storm wave conditions derived from storm return periods), from which a new set of and values are derived and used in SBEACH for the investigation of installing submerged breakwaters to mitigate potential beach erosion. Our numerical investigations using SBEACH for a beach with submerged breakwater reveal that: (1) Beach profile changes in erosion (0 m line and berm) and accretion (0 m line) due to regular waves are more significant than that of irregular waves. (2) Storm waves with a long return period cause more erosion to the shoreline (0 m line) and berm, while that with short return period may produce accretion to the shoreline. (3) The further a submerged breakwater away offshore, the more sediment transportation offshore and severe beach and berm erosion. (4).An increase of submerged breakwater height would result in accretion near the 0 line; except during a violent storm event, when an increase of berm height could reduce berm erosion. (5) An increase to the crown width of a submerged breakwater could only become effective to reduce wave energy and berm erosion, if an appropriate breakwater height is used; otherwise, a mere increase in width with insufficient height would increase wave height and berm erosion.

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Ces dernières années, de nouvelles stratégies ciblant un accompagnement de la mobilité du trait de côte plutôt que sa fixation ont vu le jour. Parmi celles-ci, les ouvrages atténuateur de houle visent à protéger la côte en dissipant l’énergie des vagues par déferlement bathymétrique, tout en restant invisibles depuis la plage. Leur utilisation a toutefois eu des effets contrastés et les processus hydro-sédimentaires induits par ces structures restent mal connus. L’objectif général de ce travail est de mieux comprendre les effets de ces atténuateurs de houle sur la morphodynamique littorale, notamment sur des sites où l’évolution de l’avant-côte est complexe et dynamique. L’observation des effets induits par un atténuateur de houle installé au lido de Sète (Golfe du Lion), sur la dynamique littorale est réalisée via un dispositif vidéo qui permet de caractériser l’évolution morphologique du système. En s’appuyant sur une méthode automatique de correction des images développée dans le cadre de cette thèse, l’estimation de la bathymétrie par inversion de la célérité des vagues et ses erreurs associées sont évaluées pour la première fois en Méditerranée. Les observations montrent que l’atténuateur de houle impacte de manière importante la morphologie et la dynamique des barres sableuses pré-littorales et révèlent que l’élargissement de la plage résulte principalement de son couplage avec la nouvelle forme de barre plus linéaire. Le modèle morphodynamique 2DBeach est ensuite implémenté sur Sète et sur un second site atelier en Australie où un récif artificiel de taille et de forme différente a été mis en place. Les simulations réalisées permettent de déterminer les circulations induites par ces ouvrages ainsi que les évolutions sédimentaires associées. Enfin, ce travail met en lumière les différents processus physiques contrôlant l’influence d’un atténuateur de houle sur les évolutions morphologiques des barres sableuses pré-littorales et du trait de côte, et renforce des connaissances essentielles à la gestion durable des plages sableuses. / In recent years, traditional coastal defense strategy has become increasingly unpopular as it is costly and lastingly scars the landscape with sometimes limited effectiveness or even adverse impact. Mimicking natural reefs, submerged breakwaters aims to protect the coast, decreasing wave energy through wave breaking offshore with the advantage of remaining invisible from the beach. The general objective of this work is to better understand the different morphodynamic processes that interact in the presence of these structures, especially for complex beach morphology with highly dynamic sandbars. The observation of the effects induced by a submerged breakwater deployed at the Lido of Sète (Gulf of Lions) on the morphological response is performed using a video monitoring system. Based on an automatic method for image correction developed in this thesis, a video-derived depth inversion algorithm was tested to infer nearshore bathymetry from remotely-sensed wave parameters. Our observations show that the submerged breakwater had a profound impact on the shoreline-sandbar system and suggest that, on barred beaches, the role of the sandbar is critical to shoreline response to the implementation of such a structure. The expected salient formation was not observed and, instead, shoreline coupled to the modified sandbar geometry, which resulted in a slight seaward migration of the shoreline in the lee of the structure. In order to characterize the nearshore circulation induced by these structures and to better assess sediment transport, the morphodynamic model 2DBeach was then implemented on Sète and at another beach in Australia where an artificial reef of different size and shape has been deployed. This work allows a better understanding of the influence of a submerged breakwater on the morphological evolution of sandbars and shoreline on time scales from storm to years, and provides new insight into nearshore system response to better design sustainable management of sandy beaches.