Nicolas Aleman

African humid periods triggered the reactivation of a large river system in Western Sahara.

The Sahara experienced several humid episodes during the late Quaternary, associated with the development of vast fluvial networks and enhanced freshwater delivery to the surrounding ocean margins. In particular, marine sediment records off Western Sahara indicate deposition of river-borne material at those times, implying sustained fluvial discharges along the West African margin. Today, however, no major river exists in this area; therefore, the origin of these sediments remains unclear. Here, using orbital radar satellite imagery, we present geomorphological data that reveal the existence of a large buried paleodrainage network on the Mauritanian coast. On the basis of evidence from the literature, we propose that reactivation of this major paleoriver during past humid periods contributed to the delivery of sediments to the Tropical Atlantic margin. This finding provides new insights for the interpretation of terrigenous sediment records off Western Africa, with important implications for our understanding of the paleohydrological history of the Sahara.

Net offshore bar migration variability at a regional scale: Inter-site comparison (Languedoc-Roussillon, France)

ABSTRACT Aleman, N., Robin, N., Certain, R., Barusseau, J.-P. and Gervais, M., 2013. Net offshore bar migration variability at a regional scale: Inter-site comparison (Languedoc-Roussillon, France). The Languedoc-Roussillon coastline is a large unit stretching out over 200 km of sandy low coast in a wave dominated environment. The nearshore is characterized by a quasi continuous system of double sandbar that displays a wide range of typology. The interannual sandbar dynamic was investigated using 2D bathymetric profiles and 3D LiDAR imagery. This study has allowed determining the sandbar systems affected by the Net Offshore Migration (NOM). At a regional scale, conditions necessary to NOM development depend on the sandbar morphologies (crescentic or straight), the wave energy and the associated coastal orientation (low or high energy, sheltered area), the coastal structures (harbour and coastal defences) and the nearshore sedimentary budget. The areas where the NOM occurs show cycle dynamic differences. This regional inter-site comparison highlights that nearshore morphology and bar parameters seem to influence the nearshore bar behaviour. Interaction between the nearshore slope, width of the bar zone and the migration rate control the NOM duration. The sandbar volume and the regional wave climate influence also the migration rate of the system. On the Languedoc-Roussillon coast, the sediment grain size does not appear to influence the seaward bar migration.

Wave-driven circulation over a double nearshore bar system during storm conditions

ABSTRACT Robin, N., Certain, R., Bouchette, F., Anthony, E.J., Meulé, S., and Aleman, N., 2014. Wave-driven circulation over a double nearshore bar system during storm conditions. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 084–089, ISSN 0749-0208. Current profiles and waves were recorded from a multi-instrumented transect over a double nearshore bar system in the Gulf of Lions, NW Mediterranean Sea (France) during storm conditions with shoreface significant wave heights of up to 3.2 m. The results constitute a preliminary analysis aimed at constraining the 3D nearshore circulation in a microtidal system. Significant time changes in the vertical distribution of nearshore velocities were observed, forced by the wind/wave conditions. Such vertical changes have been highlighted by theoretical velocity profiles in the literature, but our study demonstrates much larger variability than has hitherto been shown. Another result obtained was that the hydrodynamic pattern observed in the inner trough was distinct from that observed along the seaward flank of the inner bar. For a well-defined threshold in wave height, velocities in the trough increased abruptly and earlier, and remained strong over a longer time than those on the seaward flank. The trough thus behaves essentially as a drain for water piled against the shore. This behavior is altered by the width of the surf zone (and not only by the significant wave height), which modulates the mean current velocity. These results are a useful preliminary step in improving numerical modeling of the complex surf-zone circulation over bar-trough systems.

Apport de la géophysique terrestre (géo-radar, sismique) pour l’aide à la gestion du trait de côte

1. CEFREM-UMR 5110, Université de Perpignan Via Domitia, 52 Avenue Paul Alduy, 66000 Perpignan, France. nicolas.robin@univ-perp.fr 2. BRGM, Université de Montpellier, 1039 rue de Pinville, 34000 Montpellier, France. 3. Marseille Université, CNRS, IRD, Collège de France, CEREGE, Aix-en-P., France. 4. BRGM, 3 avenue Claude Guillemin, BP 36009, 45060 Orléans Cedex 2, France. 5. BRGM, 117 avenue de Luminy, BP 168, 13276 Marseille Cedex 9, France.