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Sediment Transport in the Gulf of Lion (NW Mediterranean): Impact of Extreme Meteorological Events (storm, flood)

Ulses Caroline

In situ observations were combined with 3D modeling to gain understanding and quantify the sediment transport in the Gulf of Lion (NW Mediterranean Sea). The outputs of the hydrodynamic - sediment transport coupled model were compared to near-bottom current and suspended sediment concentration measurements collected at the head of seven submarine canyons and shallow shelf site, over a 6-month period (Nov. 03 - May 04). The comparisons provided a reasonable validation of the model that reproduces the observed spatial and temporal variations. Regarding sediment dynamics, the study period was marked by an unusual occurrence of marine storms and high river inputs. The major solid and liquid discharges were supplied by the Rhone, the largest Mediterranean River, during an exceptional flood accompanying a severe marine storm in early December. Our study reveals (i) that most of the particulate matter delivered by the Rhone was mostly entrapped on the prodelta, and (ii) that marine storms played a crucial role on the sediment dispersal on the shelf and the off-shelf export. Storms occurring in early December and late February resuspended a very large amount of shelf sediment (> 8 M tons). Erosion was controlled by waves on the inner shelf and by energetic currents on the outer shelf. Resuspended sediments and river-borne particles were transported to the south-western end of the shelf by a cyclonic circulation induced by these onshore winds and essentially exported alongshore towards the Catalan shelf and into the Cap Creus Canyon that incises the slope close to the shore. Exportation taking place mostly during stormy events was estimated to reach about 8.8 M tons during the study period.