Manel Grifoll

Breaching of a barrier under extreme events. The role of morphodynamic simulations

ABSTRACT Gracia, V., García, M., Grifoll, M. and Sánchez-Arcilla, A., 2013. Breaching of a barrier beach under extreme events. The role of morphodynamic simulations A study of three breaching episodes of the Trabucador barrier beach (Spanish Mediterranean coast) is presented. The analysis is done from a modeling perspective using XBEACH and SWAN. The morphodynamic model (XBEACH) has been validated with the available information. The so obtained Brier Skill Score index has been 0.44 which can be considered as acceptable. Results indicate that in all cases breaching was taking place after 8 hours of wave action (storm) or less. Besides, four main parameters control the final morphodynamic response of the barrier: the storm intensity, the water level and the initial emerged topography and the storm duration. Major breaching occurs when swell waves are in coincidence with high water levels whereas if they are uncoupled the barrier tends to be breached by a diversity of smaller channels.

Formation of fine sediment deposit from a flash-flood river in the Mediterranean Sea

We identify the mechanisms controlling fine deposits on the inner-shelf in front of the Besos 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.

Numerical simulation of water mixing and renewals in the Barcelona harbour area: the winter season

In the present paper, we use numerical simulation to investigate currents, mixing and water renewal in Barcelona harbour under typical conditions of wind forcing for the winter season. This site is of particular importance due to the interplay between touristic and commercial activities, requiring detailed and high-definition studies of water quality within the harbour. We use Large Eddy Simulation (LES) which directly resolves the anisotropic and energetic large scales of motion and parametrizes the small, dissipative, ones. Small-scale turbulence is modelled by the anisotropic Smagorinsky model (ASM) to be employed in presence of large cell anisotropy. The complexity of the harbour is modelled using a combination of curvilinear, structured, non-staggered grid and the immersed boundary method. Boundary conditions for wind and currents at the inlets of the port are obtained from in-situ measurements. Analysis of the numerical results is carried out based on both instantaneous and time-averaged velocity fields. First- and second-order statistics, such as turbulent kinetic energy and horizontal and vertical eddy viscosities, are calculated and their spatial distribution is discussed. The study shows the presence of intense current in the narrow and elongated part of the harbour together with sub-surface along-shore elongated rolling structures (with a time scale of a few hours), and they contribute to the vertical water mixing. Time-averaged velocity field reveals intense upwelling and downwelling zones along the walls of the harbour. The analysis of second-order statistics shows strong inhomogeneity of turbulent kinetic energy and horizontal and vertical eddy viscosities in the horizontal plane, with larger values in the regions characterized by stronger currents. The water renewal within the port is quantified for particular sub-domain regions, showing that the complexity of the harbour is such that certain in-harbour basins have a water renewal of over five days, including the yacht marina area. The LES solution compares favourably with available current-meter data. The LES solution is also compared with a RANS solution obtained in literature for the same site under the same forcing conditions, the comparison demonstrating a large sensitivity of properties to model resolution and frictional parametrization.

Suspended sediment observations in the Barcelona inner-shelf during storms

ABSTRACT Grifoll, M., Gracia, V., Fernandez, J. and Espino, M. 2013. Suspended sediment observations in the Barcelona inner-shelf during storms. This contribution describes in detail the Suspended Sediment Concentration (SSC) obtained from an observational set at 24 m depth in the Catalan inner-shelf (North-western Mediterranean Sea) analysing the dominant processes affecting sediment dynamics during storm events. A clear correlation between SSC, energetic waves, and along-shelf currents events has been found. Peaks in river discharge associated to rainy events are also correlated with SSC measurements. Measured conditions highlight the different sediment dynamic situations present during short-term events such as “wet” and “dry” storms with similar wave height. SSC observed during “dry” conditions are correlated with wave action. On the other hand, SSC peaks during “wet” conditions are primarily caused by wave action, along-shelf current, and river discharge. Observed sediment fluxes during storms are larger in spring than during fall. Although the observations present similar characteristics to other inner-shelf regions in the NW Mediterranean Sea, the results reveal a non-negligible influence of the along-shelf current in sediment dynamics. The results presented here contribute to advance in the understanding of sediment dynamics in wave-dominated regions such as the Catalan shelf.

Characterization of bottom sediment resuspension events observed ina micro-tidal bay

In this study we investigate the variability in nearbottom turbidity in Alfacs Bay (in the northwestern Mediterranean Sea). The bay is characterized by a micro-tidal environment and seiching activity that may lead to flow velocities of more than 50 cm s−1. A set of current meters and optical sensors were mounted near the sea bottom to acquire synchronous hydrodynamic and optical information from the water column. The time-series observations showed an evident relation between seiche activity and sediment resuspension events. The observations of turbidity peaks are consistent with the node–antinode location of the fundamental and first resonance periods of the bay. The implementation of a coupled wave–current numerical model shows strong spatial variability of the potential resuspension locations. Strong wind events are also a mechanism responsible for the resuspension of fine sediment within the bay. This is confirmed by suspended sediment concentration maps derived from Sentinel-2 satellite imagery. We suggest that the sequence of resuspension events plays an important role in the suspended sediment concentration, meaning that the occurrence of sediment resuspension events may increase the suspended sediment in subsequent events. The suspended sediment events likely affect the ecological status of the bay and the sedimentary process over a long-term period.

Shelf circulation induced by an orographic wind jet

The dynamical response to cross-shelf wind-jet episodes is investigated. The study area is located at the northern margin of the Ebro Shelf, in the Northwestern (NW) Mediterranean Sea, where episodes of strong northwesterly wind occur. In this case, the wind is channeled through the Ebro Valley and intensifies upon reaching the sea, resulting in a wind jet. The wind-jet response in terms of water circulation and vertical density structure is investigated using a numerical model. The numerical outputs agree with water current observations from a high-frequency radar. Additionally, temperature, sea level, and wind measurements are also used for the skill assessment of the model. For the wind-jet episodes, the numerical results show a well-defined two-layer circulation in the cross-shelf direction, with the surface currents in the direction of the wind. This pattern is consistent with sea level set-down due to the wind effect. The comparison of the vertical structure response for different episodes revealed that the increase of stratification leads to an onshore displacement of the transition from inner shelf to mid-shelf. In general, the cross-shelf momentum balance during a wind-jet episode exhibits a balance between the frictional terms and the pressure gradient in shallow waters, shifting to a balance between the Coriolis force and the wind stress terms in deeper waters.

Numerical simulations and observations for offshore wind farms in a NW Mediterranean shelf

ABSTRACT Sánchez-Arcilla, A., González-Marco, D., Sospedra, J., Palomares, A., Sierra, J.P., Schuon, F., Espino. M., Grifoll, M., Pallarés, E., Jiménez, P.A. and Navarro, J. 2013. Meteo-oceanographic simulations and observations to assess the potential of offshore wind farm in a NW Mediterranean shelf. Renewable marine energy is important in squeezed Mediterranean coastal zones. Wind turbines deployed over the narrow Catalan continental shelf require accurate wind/wave/current fields for a reliable design, operation and maintenance. This paper presents the large (comparative to other open sea areas) errors in meteo-oceanographic predictions for semi enclosed domains such as the coastal sea off the Ebro Delta coast. The emphasis is on the sequence of high resolution coupled and nested models and the role of in situ collocated measurements for calibration and validation.