Sediment budget in the southern inlet of the Gironde Estuary (SW France)

The southern inlet of the Gironde estuary is a secondary sedimentary entity separated from the main channel by an emerging shoal. This sandy coastal system evolves under the influence of strong tidal currents and high energy atlantic swells. Severe erosion occurs along its coastline and important sediment displacements were observed on a long-term scale. Two main processes are involved in the sediment budget of this system: longshore drift and sediment loss to the open sea

Morphodynamique des plages sableuses

La morphodynamique des plages sableuses et l\u27évolution côtière à court et moyen termes sont un domaine de recherche primordial, notamment dans le contexte actuel du réchauffement planétaire et de l\u27érosion qu\u27il engendre. Si, en général, le phénomène de recul du trait de côte est bien documenté avec ses variabilités spatiales et temporelles, les processus physiques impliqués en sont encore très mal connus. Il est donc difficile à l\u27heure actuelle de mettre en place une océanographie côtière opérationnelle et prévisionnelle. Cet article présente succinctement les connaissances actuelles en les illustrant de quelques résultats obtenus sur la côte aquitaine dans le cadre du Programme national d\u27environnement côtier (PNEC) de l\u27INSU/CNRS, et propose quelques pistes de recherche

Morphodynamic behaviour of a tidal inlet system in a mixed-energy environment

This paper deals with the morphodynamic evolution of the Arcachon lagoon inlet (South West France) subjected to a mixed-energy regime. Patterns of erosion and deposition within this inlet are strongly influenced by ebb-tidal delta processes. Using an extensive data set, we propose a two stage conceptual model explaining the interactions between this transitional inlet and the adjacent shorelines. When only one channel is present, it is observed that: i) the longshore drift bypasses the inlet towards the south; ii)each year, 109 m3 of sediment is eroded from the ebb-tidal delta. At the same time, large migrating swash bars accrete along the downdrift beach causing a progradation of the shoreline. With the opening of a new channel to the north, most of the longshore transport will be trapped in the inlet and the downdrift beaches are submitted to strong erosion

Swash platform morphology in the ebb-tidal delta of the Barra Nova inlet, South Portugal

A fieldwork study of an artificially created inlet cut in the barrier island system of the Ria Formosa (South Portugal) was carried out from January to March 1999. This study focused on the updrift swash platform associated with the ebb-tidal delta. The area was monitored during a month with fair weather conditions studying morphodynamics, sedimentary dynamics and hydrodynamic measurements. The results were used to establish a sedimentary budget of the swash platform and to quantify the morphodynamics. A strong accretion was observed on the swash platform, which corresponded to the volume of sediment provided by the longshore drift on the adjacent updrift coast. A redistribution of sediment was observed on the platform, changing the orientation of the main swash platform axis. The swash bars forming this area are very dynamic (20 m migration in 8 days), and the entire swash platform migrated toward the inlet resulting in progressive closure of the system. Characteristics of the inlet and morphodynamics of the swash platform were used to validate a conceptual model of by-passing by ebb-tidal delta breaching for this system

Application of grain size trend analysis for the determination of sediment transport pathways in intertidal areas

In this paper, we apply the Gao and Collins procedure of grain size trend analysis for the first time in an intertidal sedimentary system. Our purpose is to evaluate a reliable method to determine residual transport directions in intertidal environments, where sediment movement is not well understood. Residual transport patterns can be determined by grain size trend analysis if the following criteria are applied: (1) a single sediment transport unit must be precisely defined and investigated; (2) the thickness of sampling has to be chosen in order to investigate the most recent sedimentary event; and (3) a regular mesh grid is sampled to avoid bias in the calculation of grain size trends.The Gao and Collins method was undertaken simultaneously with a topographic survey and a tracer experiment, during consecutive meteorological conditions of fair weather and a storm event. The topographic changes observed between these two situations and the directions of sediment movement deduced from tracer tracking are coherent with the residual transport pattern calculated from the grain size trend analysis. The method was tested on a ridge and runnel system which corresponds to the intertidal part of a macrotidal beach, called La Salie beach, located along the southwest French coast, in a high wave energy environment. This study permits us to recognize two different residual transport patterns for this intertidal ridge and runnel system; one for fair weather and a second for storm conditions. During fair weather, specific sites are subject to sand accretion, while during storm conditions these sand stocks are subsequently spread out over the whole area, and remain partly on the beach

Morphodynamics of ridge and runnel systems during summer

Morphological analysis of ridge and runnel systems is carried out for a 1.6 km long straight shoreline at the Truc Vert Beach (French Atlantic Coast). Foreshore has been investigated through high-resolution shoreline cartography and topographic surveys recorded during summers, from 1999 to 2001. Hydrodynamic data are from the VAG-ATLAwave model and a TRIAXYS wave buoy. 1999 shoreline maps show three rhythmical ridge and runnel systems with an average wavelength of 480 m. The SSW-NNE trend bars were about one meter high. Runnels were SW-NE oriented. The crosshore profiles of 1999 point out the shoreward sediment\u27s transfer of both berm and bar. During summer 2000, bars and channels are disrupted, whereas 2001 surveys show an irregular and double system of nearshore and foreshore bars. The shoreline map analysis underlines a conceptual model of ridge and runnel systems described by four phases: from the nearshore bar formation to the bar welding to the foreshore and system organization. These rythmical systems migrate longshore to the south

Sediment transport pattern at the Barra Nova inlet, south Portugal: a conceptual model

The Barra Nova inlet, in south Portugal, is known to migrate progressively southeastwards under wave action. The morphodynamics of this system during a representative year suggests that this long-term evolution is dependent on a seasonal behavior of the tidal inlet which can be described through a three-stage model of post-storm, transition and extended fair-weather conditions. Processes involved in this evolution indicate that the historical migration of the Barra Nova is not dependent on the longshore drift forcing constricting the channel on the updrift coast, but rather on the adjustment of the system to a major erosion of the downdrift coast during short storm events