Samuel Meulé

Sediment Dynamics in the Bay of Marseille, Gulf of Lions (France): Hydrodynamic Forcing vs. Bed Erodibility

Abstract The present contribution aims to provide an insight into the sediment dynamics of the Bay of Marseille (BoM), France, an area characterised by a mostly rocky, steep-sloped, and protected shoreline. In terms of sediment composition, the northern part of the study area is dominated by fines, whereas sands of mean grain size 0.2 < d50 < 1 mm occur at most other locations. Posidonia oceanica meadows occupy many nearshore areas. Critical bed-shear stress was determined through tests in a unidirectional flume using 15 natural cores. Measured values ranged from 0.04 N/m2 to 0.46 N/m2 and were expressed as a function of the logarithm of d50. Mud content had no observable effect on sediment cohesion, whereas a weak positive trend between critical shear stress and clay content was discerned. Autonomous benthic stations (Acoustic Doppler Current Profiler/Conductivity, Temperature, Depth/Turbidity/ALTUS Data Collection System) were deployed for periods of 2–3 months, and several suspension and erosion events were identified for significant wave heights exceeding 1.5 m. Grain-size trends showed that the protected BoM appeared to favour accumulation of sediments advected from the surrounding areas. The southern sector of the BoM was found to be the area with the highest sediment agitation and erosion rates but with weak transport trends. The northern sector, exposed to dominant SW waves, was shown to be the second most active area in terms of sediment dynamics, whereas the several Posidonia oceanica meadows and sheltered locations along the study area appeared to be possible sediment sinks. The latter included locations close to port installations and the western part of the Cortiou area, known to have water quality issues, which, according to the present findings, are linked to low sediment mobility and/or dispersion of pollutants.

Sedimentation and erosion patterns in a low shoot-density Zostera noltii meadow in the fetch-limited Berre lagoon, Mediterranean France

ABSTRACT A.-E. Paquier, S. Meulé, E.J. Anthony, G. Bernard, 2014. Sedimentation and erosion patterns in a low shoot-density Zostera noltii meadow in the fetch-limited Berre Lagoon, Mediterranean France. 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. 563–567, ISSN 0749-0208 Seagrass meadows fulfill several coastal ecosystem services that include coastal protection, provision of shelter for fishes and fish nesting sites, and water oxygenation. These aspects are hinged in part on the capacity of these meadows to attenuate waves and to slow down currents. Berre lagoon (area: 155 km²) is a fetch-limited, micro-tidal brackish water body on the French Mediterranean coast. At the turn of the 20th century, the lagoon was occupied by extensive meadows of Zostera marina and Zostera noltii. Urban and industrial pollution and freshwater diversion into the lagoon contributed to the complete disappearance of Zostera marina while Zostera noltii declined dramatically. Since the 1970s, these sources of perturbation have been drastically reduced but with no appreciable effect on Zostera noltii. A study was carried out on a receding Zostera noltii meadow occupying a small shallow bay (< 2 m deep) fronting a beach over a 14-month period in order to monitor both patterns of shoot density and erosion and accretion in the bay. Berre lagoon experiences short-fetch waves generated by northwesterly “Mistral” winds that exhibit a seasonal pattern. Shoot density shows a markedly seasonal trend that does not appear to be related to bed changes, which evened out over the study period. The data show that bed changes are less marked over the Zostera noltii meadow, and are more important in non-colonized areas, as well as along the beach, which exhibits sediment rotation. These patterns are probably reflecting the influence of the meadow on wave dissipation patterns.

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.

105. SEDIMENT DYNAMICS IN THE BAY OF MARSEILLE

The present contribution aims to give insight into the sediment dynamics of the Bay of Marseille, under the general context of assessing the origin and fate of pollutants. Towards the objectives of the study, bed cover information was obtained from sediment samples and erodibility tests in flumes; while in-situ measurements (ADCP/ /Turbidity/bed level) were combined with satellite images and hydrodynamic models. The results showed that a) the estimated critical bed shear stress was in general lower than the one obtained following a theoretical Shields approach; b) sediment suspension was wave dominated and for conditions beyond the 90% annual exceedence values, bottom sediments can be active along most of the study area. Sinks and sources are scarce and fine suspended sediments from the Rhone River plume can episodically contribute to the sediment budget of the Bay of

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.