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Abstract: In situ observations were combined with 3D modeling to gain understanding of and to quantify the suspended sediment transport in the Gulf of Lions (NW Mediterranean Sea). The outputs of a 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 at a shallow shelf site, over a 6-month period (November 2003–May 2004). The comparisons provide a reasonable validation of the model that reproduces the observed spatial and time variations. The study period was marked by an unusual occurrence of marine storms and high river inputs. The major water and sediment discharges were supplied by the Rhone, the largest Mediterranean river, during an exceptional flood accompanying a severe marine storm in early December 2003. A second major storm, with moderate flooding, occurred in February 2004. The estimate of river input during the studied period was 5.9Mt. Our study reveals (i) that most of the particulate matter delivered by the Rhone was 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. The marine storms occurring in early December 2003 and late February 2004 resuspended a very large amount of shelf sediment (>8Mt). Erosion was controlled by waves on the inner shelf and by energetic currents on the outer shelf. Sediment deposition took place in the middle part of the shelf, between 50 and 100m depth. Resuspended sediments and river-borne particles were transported to the southwestern end of the shelf by a cyclonic circulation induced by these onshore winds and exported towards the Catalan shelf and into the Cap de Creus Canyon which incises the slope close to the shore. Export taking place mostly during marine storms was estimated to reach 9.1Mt during the study period. [Copyright &y& Elsevier]

Abstract: A 1-year survey of sediment dynamics on the Têt inner-shelf in the south-western part of the Gulf of Lions was conducted as part of the EUROSTRATAFORM program (European Margin Strata Formation) from October 2004 to November 2005. Several bottom instruments (ADCP, wave gauge and altimeters) were deployed at 28m water depth on the Têt prodelta to measure forcing responsible for sediment erosion and transport on the inner-shelf. The survey period was characterized by low inputs from the coastal rivers and the absence of severe E–SE marine storms, but intense cold dense shelf water formation along the inner-shelf due to strong and persistent N–NW continental winds. The strong (>20cm/s) and prolonged (4 months) southwards alongshore currents during the preconditioning and formation phases of dense shelf water generated continuous sediment erosion of several centimetres on the Têt prodelta and significant suspended-sediment transport. Hydrodynamical modelling shows that the core of the flow predominantly affects the inner-shelf, but occasionally spreads over the mid-shelf mud belt and the outer-shelf, due to the variable intensity of the NW wind (Tramontane). The impact of this N–NW wind-driven process in terms of resuspension rates and suspended-sediment transport clearly differs from that of extreme E–SE storms observed during the previous winter at the same site [Guillén, J., Bourrin, F., Palanques, A., Durrieu de Madron, X., Puig, P., Buscail, R., 2006. Sediment dynamics during wet and dry storm events on the Têt inner shelf (SW Gulf of Lions). Marine Geology 234 (1–4), 129–142.]. Bottom shear stresses and resuspension rates due to strong dense shelf water flow peaked at 0.7–1Pa and 0.3–0.4g/m2/s, respectively, while they reached values of 6–9Pa and 1.6–1.8g/m2/s, respectively, during the strong E–SE storms events. However, due to differing durations, both led to significant (few centimetres) erosion of the inner-shelf sediment. [Copyright &y& Elsevier]

Abstract: Shelf-to-canyon suspended sediment transport during major storms was studied at the southwestern end of the Gulf of Lions. Waves, near-bottom currents, temperature and water turbidity were measured on the inner shelf at 28-m water depth and in the Cap de Creus submarine canyon head at 300m depth from November 2003 to March 2004. Two major storm events producing waves H s>6m coming from the E–SE sector took place, the first on 3–4 December 2003 (max H s: 8.4m) and the second on 20–22 February 2004 (max H s: 7m). During these events, shelf water flowed downcanyon producing strong near-bottom currents on the canyon head due to storm-induced downwelling, which was enhanced by dense shelf water cascading in February 2004. These processes generated different pulses of downcanyon suspended sediment transport. During the peak of both storms, the highest waves and the increasing near-bottom currents resuspended sediment on the canyon head and the adjacent outer shelf causing the first downcanyon sediment transport pulses. The December event ended just after these first pulses, when the induced downwelling finished suddenly due to restoration of shelf water stratification. This event was too short to allow the sediment resuspended on the shallow shelf to reach the canyon head. In contrast, the February event, reinforced by dense shelf water cascading, was long enough to transfer resuspended sediment from shallow shelf areas to the canyon head in two different pulses at the end of the event. The downcanyon transport during these last two pulses was one order of magnitude higher than those during the December event and during the first pulses of the February event and accounted for more than half of the total downcanyon sediment transport during the fall 2003 and winter 2004 period. Major storm events, especially during winter vertical mixing periods, produce major episodes of shelf-to-canyon sediment transport at the southwestern end of the Gulf of Lions. Hydrographic structure and storm duration are important factors controlling off-shelf sediment transport during these events. [Copyright &y& Elsevier]