This paper examines the regional longshore sediment transport pattern of the seaward coast of the United States and Gulf of Mexico from northern Maine to Tampa Bay, Florida. From previous studies it is known that along the coast there are variations in direction of sediment transport known as nodal zones as well as variations in sediment transport rate. Wave Information Study (WIS) hindcast data for the interval 1976 through 1995 (United States Army Corps of Engineers, 2003) provide a spatially continuous model of the regional longshore current directions in the study area. In chapter one, all available published field studies of longshore current direction and sediment transport directions and rates are compiled to create a description of the direction and, whenever possible, magnitude of longshore transport. A detailed compilation of regional and local published studies are provided in tables. An interpretation of sediment transport rates and directions is provided in eight regional maps of the study area. In chapter two the results of the literature compilations are compared with gross and net potential sediment transport directions and rates modeled using WIS hindcast data. The WIS deep-water wave characteristics are used to predict the directions and rate of longshore sediment transport at local outer coast positions using the method of Ashton et al. (2003a). The WIS-derived transport directions, including nodal zones, generally agree with the published field studies, although there are a few local inconsistencies particularly near inlets, shoals and irregular bathymetry. Trends in longshore transport rates, such as increases and decreases in gross transport rates are well represented by the WIS-derived potential transport rates. The discrepencies between the published field studies and WIS results are apparently primarily due to assumptions in the WIS model, such as assuming shore-parallel bathymetric contours.

Les barres sableuses pré-littorales ont un rôle fondamental en morphodynamique des plages soumises à l’action des vagues. Le déséquilibre permanent entre les flux sédimentaires induits vers laplage par les non linéarités des vagues et ceux induits vers le large par le courant de retour gouverne lamigration transversale des barres. Dans cette thèse, un nouveau modèle morphodynamique de profilde plage intégrant l’état de l’art des processus hydro-sédimentaires a été développé. Le faible coûten temps de calcul de ce modèle permet de réaliser des simulations à long terme, O(mois/années),de la morphologie de plages réelles ayant des caractéristiques variées (pente, type de déferlement,granularité). La simulation sur plusieurs jeux de données, de plages réelles et expérimentales, a permisd’identifier la contribution respective des principaux processus hydro-sedimentaires dans la dynamiquede la plage suivant les conditions de houle (e.g. Tempête, temps calme). Ces avancées scientifiques ontété intégrées à un modèle 2DH, ce qui a notamment permis de simuler pour la première fois sur des casacadémiques la formation d’une barre sableuse rectiligne à partir d’une plage parfaitement plane, suiviedu développement de corps sableux tridimensionnels. Ces résultats ouvrent la voie vers l’applicationde ce type de modèle aux plages naturelles soumises à une large variabilité de régimes de houle. / Sandbars are ubiquitous patterns along wave-dominated sandy coastlines and are key elementsin the global evolution of beaches. Cross-shore sandbar migrations are the result of the permanentimbalance between sediment flux driven by wave non-linearity and mean return current. In this thesis,we developed a new process-based beach profile model integrating the recent scientific advancesin term of hydrodynamics and sediment transport developed for beach morphodynamics. The lowcomputing time allows for long-term morphodynamic simulations (O months/years) of natural beachprofiles of diverse characteristics (beach slope, sediment grain size or type of wave breaking). Modelvalidations on several data sets, encompassing natural and experimental beach profile evolutions,highlight the respective contribution of the main hydrodynamic and sediment transport processesinvolved in specific cross-shore sandbar evolution relative to various wave conditions. Finally, all thecross-shore physical processes were integrated in a 2DH morphodynamic model, resulting for the firsttime in the simulation of a quasi-complete down state sequence showing alongshore bar generationwith subsequent spontaneous formation of transverse bar and rip morphology. These very encouragingresults pave the way for using this model to simulate 3-Dimensional evolutions of natural beachesforced by irregular wave conditions

A seasonal and long-term analysis of the vertical structure of currents in the nearshore is conducted to determine the role of the wind in driving currents and consequently affecting littoral transport processes. Approximately ten years (January, 2002 – October, 2011) of nearshore current profiles are examined using the data collected with an Acoustic Doppler Current Profiler (ADCP) installed off of Spessard Holland North Beach Park located in Melbourne Beach, Florida. Additionally, wind data collected with a directional anemometer from September, 2002, until October, 2008, are used to further characterize the long-term hydrodynamic forcing. With the shoreline oriented nominally 17o west of magnetic north, both the current profiles and the wind vectors have been rendered into longshore and cross-shore components. The water level record from a NOAA tide station located at the Trident Pier at nearby Port Canaveral is utilized in establishing the water depth and conditioning the data for statistical analysis. Monthly mean vertical profiles reveal that during the winter months the surface currents are usually toward the south, and toward the north in the summer. In spring and fall, they are mixed, demonstrating a clear seasonality in both direction and intensity of the longshore current. Subjecting the longshore and cross-shore current data to Empirical Orthogonal Function Analysis reveals that the first spatial Eigenfunction accounts for more than 98% of the variability in the vertical profile of the longshore current, and more than 86% of the variability in the profile of the cross-shore current. However, there is a rotation of the current to the right (clockwise) with the rotation angle increasing and the variance decreasing with depth below the surface. The spiral structure of the water column follows a surface Ekman veering, but for very shallow water. The upper layer of the current is almost aligned with the direction of the wind. Monthly correlations between 2-hour average time series of longshore current and 2-hour average time series of wind speed reveal the seasonal patterns of the wind and longshore current in which the upper layer of the water column is highly correlated with the longshore component of the wind speed for most of the year and slightly less correlated for the lower layer of the water column. Most importantly, on average, wave height (Hmo) is larger when the longshore current is heading to the south (Hmo=0.95 m) than when the current is going to the north (Hmo=0.73 m). Additionally, there is a stronger correlation between southerly directed currents and incident wave energy flux than northerly directed currents and wave energy flux. These results indicate that the net long-term north-to-south sediment transport known to characterize the region is heavily influenced by wind-driven currents.