Grifoll Manel

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  • Article
    Formation of fine sediment deposit from a flash flood river in the Mediterranean Sea
    (John Wiley & Sons, 2014-09-09) Grifoll, Manel ; Gracia, Vicenc ; Aretxabaleta, Alfredo L. ; Guillen, Jorge ; Espino, Manuel ; Warner, John C.
    We identify the mechanisms controlling fine deposits on the inner-shelf in front of the Besòs River, in the northwestern Mediterranean Sea. This river is characterized by a flash flood regime discharging large amounts of water (more than 20 times the mean water discharge) and sediment in very short periods lasting from hours to few days. Numerical model output was compared with bottom sediment observations and used to characterize the multiple spatial and temporal scales involved in offshore sediment deposit formation. A high-resolution (50 m grid size) coupled hydrodynamic-wave-sediment transport model was applied to the initial stages of the sediment dispersal after a storm-related flood event. After the flood, sediment accumulation was predominantly confined to an area near the coastline as a result of preferential deposition during the final stage of the storm. Subsequent reworking occurred due to wave-induced bottom shear stress that resuspended fine materials, with seaward flow exporting them toward the midshelf. Wave characteristics, sediment availability, and shelf circulation determined the transport after the reworking and the final sediment deposition location. One year simulations of the regional area revealed a prevalent southwestward average flow with increased intensity downstream. The circulation pattern was consistent with the observed fine deposit depocenter being shifted southward from the river mouth. At the southern edge, bathymetry controlled the fine deposition by inducing near-bottom flow convergence enhancing bottom shear stress. According to the short-term and long-term analyses, a seasonal pattern in the fine deposit formation is expected.
  • Article
    Along-shelf current variability on the Catalan inner-shelf (NW Mediterranean)
    (American Geophysical Union, 2012-09-22) Grifoll, Manel ; Aretxabaleta, Alfredo L. ; Espino, Manuel ; Warner, John C.
    We examine the circulation over the inner-shelf of the Catalan Sea using observations of currents obtained from three Acoustic Doppler Current Profilers (two at 24 m and one at 50 m) during March–April 2011. The along-shelf current fluctuations during that period are mainly controlled by local wind stress on short time scales and by remote pressure gradients on synoptic time scales. Different forcing mechanisms are involved in the along-shelf momentum balance. During storm conditions, wind stress, sea level gradients and the nonlinear terms dominate the balance. During weak wind conditions, the momentum balance is controlled by the pressure gradient, while during periods of moderate wind in the presence of considerable stratification, the balance is established between the Coriolis and wind stress terms. Vertical variations of velocity are affected by the strong observed density gradient. The increased vertical shear is accompanied by the development of stratified conditions due to local heating when the wind is not able to counteract (and break) stratification. The occasional influence of the Besòs River plume is observed in time scales of hours to days in a limited area in near the city of Barcelona. The area affected by the plume depends on the vertical extent of the fresher layer, the fast river discharge peak, and the relaxation of cross-shore velocities after northeast storm events. This contribution provides a first interpretation of the inner-shelf dynamics in the Catalan Sea.
  • Article
    Seasonal circulation over the Catalan inner-shelf (northwest Mediterranean Sea)
    (John Wiley & Sons, 2013-10-25) Grifoll, Manel ; Aretxabaleta, Alfredo L. ; Pelegri, Josep L. ; Espino, Manuel ; Warner, John C. ; Sanchez-Arcilla, Agustin
    This study characterizes the seasonal cycle of the Catalan inner-shelf circulation using observations and complementary numerical results. The relation between seasonal circulation and forcing mechanisms is explored through the depth-averaged momentum balance, for the period between May 2010 and April 2011, when velocity observations were partially available. The monthly-mean along-shelf flow is mainly controlled by the along-shelf pressure gradient and by surface and bottom stresses. During summer, fall, and winter, the along-shelf momentum balance is dominated by the barotropic pressure gradient and local winds. During spring, both wind stress and pressure gradient act in the same direction and are compensated by bottom stress. In the cross-shelf direction the dominant forces are in geostrophic balance, consistent with dynamic altimetry data.
  • Article
    Temporal evolution of the momentum balance terms and frictional adjustment observed over the inner shelf during a storm
    (Copernicus Publications on behalf of the European Geosciences Union, 2016-01-18) Grifoll, Manel ; Aretxabaleta, Alfredo L. ; Pelegri, Josep L. ; Espino, Manuel
    We investigate the rapidly changing equilibrium between the momentum sources and sinks during the passage of a single two-peak storm over the Catalan inner shelf (NW Mediterranean Sea). Velocity measurements at 24 m water depth are taken as representative of the inner shelf, and the cross-shelf variability is explored with measurements at 50 m water depth. During both wind pulses, the flow accelerated at 24 m until shortly after the wind maxima, when the bottom stress was able to compensate for the wind stress. Concurrently, the sea level also responded, with the pressure-gradient force opposing the wind stress. Before, during and after the second wind pulse, there were velocity fluctuations with both super- and sub-inertial periods likely associated with transient coastal waves. Throughout the storm, the Coriolis force and wave radiation stresses were relatively unimportant in the along-shelf momentum balance. The frictional adjustment timescale was around 10 h, consistent with the e-folding time obtained from bottom drag parameterizations. The momentum evolution at 50 m showed a larger influence of the Coriolis force at the expense of a decreased frictional relevance, typical in the transition from the inner to the mid-shelf.