Sediment Transport in the Gulf of Lion (NW Med.): Impact of Extreme Meteorological Events

In situ observations were combined with 3D modeling to gain understanding and quantify the shelf to slope transfer in the Gulf of Lion (NW Mediterranean Sea) within the framework of the Eurostrataform program. The outputs of the hydrodynamic sediment transport coupled model were compared to data (near-bottom current and suspended sediment concentration) collected at the head of seven submarine canyons and at a 27 m deep site on the shelf over a 6-month period (Nov. 03 May 04). The comparisons provided a reasonable validation of the model reproducing the recorded spatial and temporal variations and suggested that it can be used to identify the controlling processes and to estimate the transport. 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, one of the largest Mediterranean river, during an exceptional flood accompanying a severe marine storm in early December. Our study reveals that marine storms played a crucial role on the sediment dispersal on the shelf and on the off- shelf export. Storms occurring in early December and late February resuspended a very large amount of shelf sediments (> 5 M tons). Erosion was controlled by waves on the inner shelf and by energetic currents on the outer shelf. Model outputs indicate that the shelf cyclonic circulation induced by these onshore winds transported resuspended sediments and river-borne particles to the south-western end of the shelf. Suspended sediment was then essentially exported alongshore towards the Catalan shelf and into the Cap Creus Canyon that incises the slope close to the shore. Transfer within the canyon was associated to an important downwelling process. In autumn (early December), the downwelling was restricted to the upper slope due to buoyancy effects, and produced afterwards significant shelf sediment deposition in the canyon head. Conversely, during the winter storm (late February), the downwelling interacted with dense water cascading that enhanced the transfer along the bottom and swept resuspended sediments towards deeper areas of the canyon. Transfer taking place mostly during stormy events was estimated to reach about ~ 7 M tons during the study period. Most of the particulate matter delivered by the Rhone River (~ 5 M tons) was mostly entrapped on the eastern shelf, where the wave and current energy was weaker.