Search

International audience; Foredunes are efficient natural coastal defenses acting as protective barriers during storm events. They also have the capacity to be an ecosystem hosting significant biodiversity. The economic development and/or recreational use of the foredune commonly results in a modification of natural functioning and the concomitant mixing of natural and anthropogenic processes. While the impact of human interventions on the short term evolution of coastal dunes is reasonably well understood, relatively less is known on their imprint at a scale of several decades. The Truc Vert beach-dune system (SW France), which has been exposed to various dune management strategies for more than a century, provides a relevant site to explore the respective contributions of natural and anthropogenic processes on coastal foredune evolution and the current coastal dune landscape. For this purpose, the coastal dune system was investigated using several approaches that combine ground penetrating radar (GPR), topographic data, aerial photographs and historical maps.A 20-m thick GPR sequence provides a stratigraphic record from which we detail ~150-year period of coastal dune change, including the initiation of the foredune. Results show a mixture of radar facies typical of natural aeolian erosion or deposition and radar facies that are the signature of human actions. These anthropogenic works include a large fence emplaced in 1860 to build and fix the foredune, and intense mechanical reshaping of the dune profile by bulldozers in 1972 followed by an intensive planting of vegetation. These various management strategies had a profound influence on coastal dune changes and, in turn, on the current coastal dune landscape. Historic archives documenting coastal dune works were critical to discriminate some of the radar facies, which could be wrongfully interpreted as natural erosion or deposition facies. Therefore, these results demonstrate the importance of coupling GPR and historical documentation wherever possible to determine, in part or fully, the contributions of human interventions and actions in modern dune evolution and morphological development.

Robin, N.; Billy, J.; Castelle, B.; Hesp, P.; Laporte-Fauret, Q.; Nicolae-Lerma, A.; Marieu, V.; Rosebery, D.; Bujan, S.; Destribats, B., and Michalet., 2020. Beach-dune recovery from the extreme 2013-2014 storms erosion at Truc Vert Beach, Southwest France: New insights from ground-penetrating radar. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 588–592. Coconut Creek (Florida), ISSN 0749-0208.Foredunes are efficient natural coastal defenses acting as protective barriers during storm events. A striking example is the response of coastal dunes along the Aquitaine Coast (SW France) to the 2013-2014 winter, which was the most energetic winter since at least the mid-20th century in most of the NE Atlantic. The aim of this study is to analyze the (partial) foredune recovery from the 2013-2014 winter at Truc Vert beach using ground-penetrating radar (GPR) and through the coastal dune internal structure. Geophysical data was compared with semi-monthly DGPS topographic data of the beach, UAV-photogrammetry and Airborne LiDAR surveys of the coastal dune. Three main recovery phases were identified from the 2013-2014 winter: (i) aggradation of the upper-beach and dune front-crest during the post-storm summer; (ii) distinct phases of incipient dune-foot development with dune-crest stability during the following year, and (iii) since 2016, aggradation of both dune-foot and dune front-crest tending towards an equilibrium profile. This study shows that GPR surveys, combined with chronological topographic data, is an efficient tool to provide comprehensive understanding of the beach-dune system evolution at a decadal scale. This approach opens new perspectives to document foredune recovery timing and magnitude along sandy coasts.

International audience

International audience; The internal architecture of a beach-ridge system can provide clues into the processes involved in its formation. Detailed investigation of the mixed sand-and-gravel beach-ridge plain on the Miquelon-Langlade Barrier (northwest Atlantic Ocean, south of Newfoundland) provides a better understanding of such sedimentary systems in terms of internal architecture, depositional processes and driving mechanisms. Investigations of this regressive paraglacial barrier included the topographic (RTK-GPS) and ground-penetrating radar (100 to 500 MHz antennae) data collection, orthophotographs and optically stimulated luminescence (OSL) dating. The Miquelon-Langlade beach-ridge plain is composed of two opposing progradational systems, reflecting contrasting eastward and westward hydrodynamic exposures. Four ridge sets, each with concave planform shapes, define the 500–1400-m wide eastward prograding plain along the sheltered eastern coast. Two ridge-sets with linear and fan-shaped planforms define the 200–500-m wide south-westward prograding plain along the energetic western coast.Beach-ridge units overlay a basal sedimentary unit that disrupted incoming wave energy and altered the planform morphology of the prograding plain. The internal architecture pattern of individual beach ridges are characterized by sigmoidal configurations with seaward-dipping beds (2.3–4.7° true dip-angles). It is proposed that these wave-built facies are deposited as beach berms, likely by fair-weather waves. The elevations of these facies are determined by the sea level elevation and the swash height of constructive waves during their formation. Beach ridges are overlain by either peat resulting from the flooding of topographically low-lying areas, or by aeolian sand deposits of varying heights and morphologies (relict foredunes).Despite the differences in wave exposure along each coast, the sedimentary composition and internal architecture of the two progradational systems are broadly similar; bed dip angles of the individual beach ridges differ by only 1–2°, steeper along the exposed westward-facing coast. Overall, the Miquelon-Langlade beach-ridge plain highlights the primary influences of wave exposure, inherited topography, and proximal sediment sources (from erosion of moraines) on coastal progradational patterns.

International audience; Estuarine tidal bars emplaced in estuaries are complex sediment bodies composed of sand and mud provided by rivers. This study focuses on the Plassac Tidal Bar, which is the unique lobate-shape tidal bar of the Gironde Estuary, located at the very upstream extremity of the bay-head delta, because no detailed sedimentological studies have been made on such geomorphological category of tidal bar. Its stratigraphy was investigated using a dense grid of 65 very high resolution seismic profiles, ground-truthed by 6 to 9 meter-long cores and time-controlled by historical bathymetric surveys. The base of the tidal bar consists of a tidal erosional surface. This surface has eroded muddy tidalites, corresponding to the maximum flooding surface of the Gironde Estuary. The bar is composed of two vertical sequences of facies, composed of thickening-up cross stratified fine-to medium-grained sand beds at the top and decimeter-thick mud-layers at the base. Those mud layers constitute the master bedding of the bar and are dipping in a channelward direction, perpendicularly to the tidal flows and to the sediment transport directions. One phase of lateral accretion of the Plassac Tidal Bar is correlated with variations in fluvial discharge of the last decades and suggests a climate control on the internal architecture of this sandbar. Low water periods lead to the upstream migration of the turbidity maximum and to the deposition and consolidation of the muddy master bedding planes. During the subsequent flood periods, the muddy master bedding planes are partially eroded and then buried by the fluvially-sourced sandy mini-flood lobes that merge with the tidal bar. The internal architecture of the Plassac Tidal Bar is controlled by a lateral accretion process, its master bedding surfaces which record this process are dipping perpendicularly to the axis of the tidal flows. All the other tidal bars of the bay-head delta (around a dozen) exhibit an elongated morphology and prograde longitudinally by seaward frontal accretion occurring in the axis of the tidal flows. The master bedding surfaces which record this process are seaward dipping along the axis of the tidal flows. Those variations in internal accretion surfaces orientation of tidal bars are of considerable importance and must be taken into account for studies of ancient tidal deposits, paleoenvironment reconstructions and for heterogeneities characterization and quantification of reservoir properties in these types of sandstones.