Morphodynamical modelling of field-scale swash events

In the present work, data for three single swash events are selected from those available for an accretive tide that occurred at Le Truc Vert beach (France) during a measurement field campaign at that location. These events are primarily chosen because of the different final bed change they produced, namely variable accretion, seaward erosion/landward deposition and variable erosion along the swash zone. These data are compared to results obtained from a ‘state-of-the-art’ numerical fully-coupled 1D morphodynamical shallow water solver, driven by measurements made of those events in the lower swash/inner surf zone. It is found that the hydrodynamics is reasonably well represented, although the computed results exhibit reduced maximum inundations in comparison with the observed ones. The model reproduces the correct order of magnitude of the morphodynamic change after each event, and sometimes the pattern of erosion and deposition, but this change is generally underestimated. Sensitivity analyses are conducted with respect to more uncertain physical parameters and assumed initial conditions. They suggest that initial spatial distributions for velocity and pre-suspended sediment concentration play a key role in the quantitative and qualitative prediction of the bed change

Understanding coastal morphodynamic patterns from depth-averaged sediment concentration

This review highlights the important role of the depth-averaged sediment concentration (DASC) to understand the formation of a number of coastal morphodynamic features that have an alongshore rhythmic pattern: beach cusps, surf zone transverse and crescentic bars, and shoreface-connected sand ridges. We present a formulation and methodology, based on the knowledge of the DASC (which equals the sediment load divided by the water depth), that has been successfully used to understand the characteristics of these features. These sand bodies, relevant for coastal engineering and other disciplines, are located in different parts of the coastal zone and are characterized by different spatial and temporal scales, but the same technique can be used to understand them. Since the sand bodies occur in the presence of depth-averaged currents, the sediment transport approximately equals a sediment load times the current. Moreover, it is assumed that waves essentially mobilize the sediment, and the current increases this mobilization and advects the sediment. In such conditions, knowing the spatial distribution of the DASC and the depth-averaged currents induced by the forcing (waves, wind, and pressure gradients) over the patterns allows inferring the convergence/divergence of sediment transport. Deposition (erosion) occurs where the current flows from areas of high to low (low to high) values of DASC. The formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features, but the physical mechanisms for their migration, their finite-amplitude behavior and their decay can also be explored

Morphodynamique des plages à barres en domaine méso à macrotidal (exemple de la plage du Truc Vert, Gironde, France)

Pendant les 3 ans d'étude (1998-2001), la plage du Truc Vert a présenté plusieurs types morphologiques qui ont été assemblés en 3 groupes (I, II, III). Les types Ia et Ib (groupe I) sont formés par 3 à 4 systèmes barre-baïne ayant une longueur d'onde moyenne de 474m. Le groupe II est composé des systèmes irréguliers des types IIa, IIb, IIc et IId.Ces derniers montrent des barres intertidales coupées par des chenaux des baïnes. La position des barres qui peuvent être submergées, et la longueur de ces sytèmes sont très variables. Finalement, le groupe III correspond à une morphologie de plage plane caractérisée par un profil court et concave vers le haut avec de rares barres presque submergées. L'analyse des données hydrodynamiques a montré que l'évolution temporelle des différents types morphologiques dépend de l'énergie des vages (hauteur et période), de leur direction et de leur durée, ainsi que du contexte morphologique. A partir de ceci, il a été possible de déterminer 4 modèles morphodynamiques conceptuels concernat la mise en place, l'évolution longitudinale et la déstructuration des systèmes barre-baïne, ainsi que la formation d'une plage plane. Ces modèles composent un cycle évolutif qui débute par l'action de fortes vages d'O-NO et NO (de plus de 4,5 m de hauteur signifiactive et de plus de 8 s de période signifactive) et la mise en place d'une plage plane, caractérisant un transfert de sédiments vers la plage sous-marine. 7 à 10 jours plus tard, des barres sous-marines internes en croissant apparaissent (types IIb ou IIc). Ensuite, ces barres tendent à s'accrocher à la côte et, dans un délai de 13 à 18 jours, les systèmes barre-baïne apparaissent (types Ia ou Ib). Après leur mise en place, en association avec des vagues d'O-NO et NO (de moins de 2 m de hauteur et 8 s de période), ces systèmes migrent longitudinalement vers le sud (0,8 et 3,8 m/jour) au cours d'un été. Une migration vers le nord, associée à des vagues d'O-SO, a également été observée. Ce cycle évolutif peut être interrompu par des vagues de plus de 3 m de hauteur et 8 s de période provenant de l'O-NO et du NO, mais également par des vagues moins dynamiques mais plus fréquentes (entre 2 et 3 m de hauteur et 8 s de période mais d'une durée supérieure à 4 jours). Nous avons observé que le développement complet d'un cycle évolutif sur la côte girondine pouvait prendre de 13 jours à plus de 4 mois. Il apparaît donc que ces processus peuvent se développer au cours de courtes périodes, à la suite de fortes tempêtes, ou au cours de longes périodes, en conséquence de variations saisonnières.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF

Morphodynamics of nearshore rhythmic sandbars in a mixed-energy environment (SW France): 2. Physical forcing analysis

International audienceThe morphology and migration of rhythmic intertidal ridge and runnel systems, and subtidal crescentic bars that border the southwest coast of France were characterized using in situ surveys and maps obtained by remote-sensing methods. The period from 1986 to 2000 was investigated. A total of 35 km of coast was mapped. This data set shows several specificities, the origin of which are examined in the present report using hydrodynamic data. A complete analysis of the influence of wave climate on both the shape and the movements of these rhythmic sedimentary patterns was performed. In addition, SWAN and MORPHODYN-coupled numerical models were used to provide quantification of both wave breaking and longshore currents for wave parameters that were representative of the mean values and of the energetic conditions. This study demonstrated the short time response of intertidal systems to the wave forcing. When the offshore significant wave height ( Hs) was lower than 2.5 m, regular coastal ridge and runnel systems developed in the intertidal zone and migrated in the longshore-drift direction at a rate of 1.7-3.1 m day -1. By contrast, the ridge and runnel system morphology abruptly changed when the Hs exceeded 2.5 m, and after the storm, the typical ridge and runnel rhythmic topography was recovered within 5-9 days. The crescentic bars, which had a convex seaward shape, were affected by waves with Hs values greater than 3 m (slightly less for short waves). Depending on the wave orientation, the crescentic bars moved in the longshore-drift direction at a rate that reached 1 m day -1. The data suggested a slight negative correlation between the mean alongshore length of the crescentic bar and the mean Hs. Finally, it seemed that increasing the wave obliquity with respect to the coast resulted in the flattening of the crescentic bars. Thus, coupling Spot and in situ mapping to hydrodynamic records allow the characterization of coastal morphology and dynamics, with time and space samplings that are particularly well adapted to the little studied alongshore morphodynamics. This approach should improve the difficult parameterization of morphodynamic models in high-energy environments

The ECORS-Truc Vert’08 nearshore field experiment: Presentation of a three-dimensional morphologic system in a macro-tidal environment during consecutive extreme storm conditions

A large multi-institutional nearshore field experiment was conducted at Truc Vert, on the Atlantic coast of France in early 2008. Truc Vert’08 was designed to measure beach change on a long, sandy stretch of coast without engineering works with emphasis on large winter waves (offshore significant wave height up to 8 m), a three-dimensional morphology, and macro-tidal conditions. Nearshore wave transformation, circulation and bathymetric changes involve coupled processes at many spatial and temporal scales thus implying the need to improve our knowledge for the full spectrum of scales to achieve a comprehensive view of the natural system. This experiment is unique when compared with existing experiments because of the simultaneous investigation of processes at different scales, both spatially (from ripples to sand banks) and temporally (from single swash events to several spring-neap tidal cycles, including a major storm event). The purpose of this paper is to provide background information on the experiment by providing detailed presentation of the instrument layout and snapshots of preliminary results.Hydraulic EngineeringCivil Engineering and Geoscience