Manuel Espino

A new risk assessment method for water quality degradation in harbour domains, using hydrodynamic models

This contribution presents a method for assessing the risk of water degradation in harbour domains. The method describes a normalized Index of Risk, ranging from 0 to 1, which determines the risk of water degradation due to a pollution event. A branch-decision scheme of decision-making theories was implemented in order to obtain this index. This method evaluates the cost of each decision as a function of the vulnerability, proximity and toxicity of the potential contaminant. A novel feature of this method is that the risk is defined by considering the physical behaviour of the harbour (i.e. water circulation patterns). Regions where water residence time is high are considered more vulnerable to pollutant releases. This method could be implemented from an operational perspective, in which case an oceanographic operational system could be used to obtain current forecasts which in turn would be used to forecast risk maps.

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.

Feasibility and usefulness of steady-state calculations of the mean circulation in the vicinity of the Ebro mouth. Model tests against field data

Abstract A steady-state quasi-3D finite element model is forced with in situ measured wind conditions in order to obtain an estimate of the mean circulation off the Ebro delta and to assess its consequences upon water fluxes. The model is spectral in the vertical direction and is run with a reduced number of vertical degrees of freedom (modes). The boundary conditions prescribed on the ocean contours account for the existence of a slope jet. The hypothesis is that the direct wind forcing on the upper layer and the shelf mesoscale circulation are the main mechanisms driving the flow in the area and that it should be possible to reproduce the main current features even in the vicinity of the Ebro river mouth with this relatively simple model setup, as the usual river freshwater discharge rates are rather low. This is clearly different from the case of other region of freshwater influence systems, in which the river plume dynamics and the related density currents play relevant roles. The model results are compared with surface drifter trajectories obtained during field campaigns.

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.

Finite element modelling of the thermal outflow of three power plants in Huelva Estuary

A simple explicit, characteristic-based finite element method for the numerical simulation of the dispersion of the thermal outflow in Huelva estuary is considered in this paper. The derivation involves a local Taylor expansion of the convection-diffusion equation. The numerical model used was originally designed to simulate the dispersion of pollutants in the open sea and has now been reconfigured to simulate the dispersion of the thermal outflow of some power plants which may be constructed in Huelva estuary in the near future. Numerical results obtained are presented.

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.

Comparison between nested grids and unstructured grids for a high-resolution wave forecasting system in the western Mediterranean sea

ABSTRACT Traditionally wave modelling uses a downscaling process by means of successive nested grids to obtain high-resolution wave fields near the coast. This supposes an uncertain error due to internal boundary conditions and a long computational time. Unstructured grids avoid multiple meshes and thus the problem of internal boundary conditions. In the present study high-resolution wave simulations are analysed for a full year where high-resolution meteorological models were available in the Catalan coast. This coastal case presents sharp gradients in bathymetry and orography and therefore correspondingly sharp variations in the wind and wave fields. Simulations with SWAN v.4091A using a traditional nested sequence and a regional unstructured grid have been compared. Also a local unstructured grid nested in an operational forecast system is included in the analysis. The obtained simulations are compared to wave observations from buoys near the coast; almost no differences are found between the unstructured grids and the regular grids. Simultaneously, tests have been carried out in order to analyse the computational time required for each of the alternatives, showing a decrease to less than half the time when working with regional unstructured grids and maintaining the forecast accuracy and coastal resolution with respect to the downscaling system.

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.

Operational Forecasting as a Tool for Managing Pollutant Dispersion and Recovery

The fate and concentration of plastic pollutants in coastal seas depends on meteo-oceanographic factors and the multiple entry points in the domain. This will in turn control the implications for water quality, transmission of various biological diseases and the efficiency of recovery. In this paper we shall present a robust numerical model linked sequence that has been prepared jointly by Puertos del Estado and LIM-UPC. It is based on advanced, coupled meteo-oceanographic models that account for the main physical mechanisms responsible for circulation and dispersion. From here concentrations, times of renewal and trajectories can be derived with enough resolution so as to capture the important gradients in topography and bathymetry characteristic of coastal seas. The validation of these forecasts will require suitable data that address the various fields numerically predicted. This includes wind, atmospheric pressure, waves, currents and also the variations in mean water level. Such an observational effort needs also initial and boundary conditions that for dynamically narrow shelfs can shape the water, sediment and pollutant fluxes in the platform. In the paper we shall discuss the nested sequence that has been prepared and is now run pre-operationally for a set of Canary island harbours, where the Plocan data can provide valuable boundary information for model assessments. The simulated current dispersion patterns will be the basis to discuss impacts and recovery policy so as to maintain a water quality status in harbour areas and adjacent coastal tracts that comply with regulations and the requirements of coastal users.

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.