Fine-grained sediment dynamics during a strong storm event in the inner-shelf of the Gulf of Lion (NW Mediterranean)

Special issue Impact of Natural and Trawling Events on Resuspension, dispersion and fate of POLlutants (INTERPOL).-- 18 pages, 10 figures, 4 tablesA 1-month survey, comprising moored instruments and high-frequency sampling, was carried out in a shallow Mediterranean embayment during the autumn of 1999, to assess the effect of strong and unpredictable meteorological events on the near-bed, fine-grained sediment dynamics. A 1DV Reynolds Averaged Navier-Stokes (RANS) model for the wave-current boundary layer [Guizien et al., 2003. 1DV bottom boundary layer modeling under combined wave and current: turbulent separation and phase lag effects. Journal of Geophysical Research 108(C1), 3016] is tested, against the recorded suspended sediment concentration data (SSC); it is used then to determine the sediment resuspension and flux in the embayment. A strong southeasterly storm occurred on November 12, 1999, which generated a large swell (Hs=7m, Tm=10s), a sea surface rise of about 0.5 m and near-bottom currents of up to 35 cm s-1. During the storm, the SSC increased throughout the whole of the water column and reached 70 mg l -1, at 25.8 m (0.5 m above the bottom). Numerical computations of SSC profiles, based upon local sandy sediment grain size distribution, are in good agreement with the measured SSC profiles during the onset of the storm over the first 2 h. These observations confirm that the measured SSC profiles, during the storm, resulted from the resuspension of the fine-grained fraction (<60 μm); that is consistent with the grain size of material collected in sediment traps. Following the first 2 h, numerical simulations suggest that bed armouring occurred, after the surficial fine-grained fraction was winnowed. Computations of mud fraction SSC, along a cross-shore transect, which displays a seaward-fining texture of the sediment, indicate that strong resuspension during this severe storm event only affected water depths shallower than 35 m. This water depth coincides approximately with the transition from sand to mud, on the Gulf of Lion shelf, which is located around 30 m. Computations of the horizontal flux of suspended sediment flux, due to the current alone, agree with the observations; these indicate that the flux integrated, over the bottom 4.1 m above the seabed, during the storm (14 300 kg m-2) is associated mainly with the fine-grained sediment fraction. The fine-grained fraction flux is at least 2.5 times larger than the coarser fractions flux. Likewise, whilst most of the coarse grained flux (99%) is confined within few tens of centimetres above the bottom, more than half of the fine-grained flux occurs above the bottom boundary layer. © 2005 Elsevier Ltd. All rights reservedThe authors acknowledge the support from the “Action de Recherche Thématique” no. 1 of the French “Programme National Environnement Côtier” and the European Commission under contracts MAS3–CTP96–0049 and EVK3–CT–2000–00023.Peer Reviewe