Ana J. Abascal

Application of HF radar currents to oil spill modelling

In this work, the benefits of high-frequency (HF) radar currents for oil spill modeling and trajectory analysis of floating objects are analyzed. The HF radar performance is evaluated by means of comparison between a drifter buoy trajectory and the one simulated using a Lagrangian trajectory model. A methodology to optimize the transport model performance and to calculate the search area of the predicted positions is proposed. This method is applied to data collected during the Galicia HF Radar Experience. This experiment was carried out to explore the capabilities of this technology for operational monitoring along the Spanish coast. Two long-range HF radar stations were installed and operated between November 2005 and February 2006 on the Galician coast. In addition, a drifter buoy was released inside the coverage area of the radar. The HF radar currents, as well as numerical wind data were used to simulate the buoy trajectory using the TESEO oil spill transport model. In order to evaluate the contribution of HF radar currents to trajectory analysis, two simulation alternatives were carried out. In the first one, wind data were used to simulate the motion of the buoy. In the second alternative, surface currents from the HF radar were also taken into account. For each alternative, the model was calibrated by means of the global optimization algorithm SCEM-UA (Shuffled Complex Evolution Metropolis) in order to obtain the probability density function of the model parameters. The buoy trajectory was computed for 24h intervals using a Monte Carlo approach based on the results provided in the calibration process. A bivariate kernel estimator was applied to determine the 95% confidence areas. The analysis performed showed that simulated trajectories integrating HF radar currents are more accurate than those obtained considering only wind numerical data. After a 24h period, the error in the final simulated position improves using HF radar currents. Averaging the information from all the simulated daily periods, the mean search and rescue area calculated using HF radar currents, is reduced by approximately a 62% in comparison with the search area calculated without these data. These results show the positive contribution of HF radar currents for trajectory analysis, and demonstrate that these data combined with atmospheric forecast models, are of value for trajectory analysis of oil spills or floating objects.

Calibration of a Lagrangian Transport Model Using Drifting Buoys Deployed during the Prestige Oil Spill

Abstract The experience acquired in the Prestige crisis management has demonstrated the importance of forecasting oil slick trajectories to plan an effective oil spill response. To have a reliable prediction system, we need to perform a detailed calibration and validation of the oil spill transport model. In this work, the Lagrangian transport model, PICHI, developed by the University of Cantabria during the Prestige accident, is calibrated by means of an automatic calibration methodology. The shuffled complex evolution method, developed by the University of Arizona (SCE-UA), is applied to estimate the optimal coefficients of the model. The calibration of the model has been carried out using 13 buoys deployed in the Bay of Biscay during the Prestige accident as well as coetaneous meteorological and oceanographic data. Moreover, reanalysis data collected in the Spanish ESEOO project framework has also been used. Results suggest that buoys outside the continental slope were mainly driven by wind, whereas ocean currents played an important role in the motion of the buoys located over the continental slope and shelf. According to these findings, the final calibration of the coefficients is performed considering different buoy data. The methodology applied to this broad buoy database, has allowed us to calibrate the model, taking into account the relative importance of the forcings in buoy movement as well as the dynamics associated with each area.

Analysis of the reliability of a statistical oil spill response model

A statistical oil spill response model is developed and validated by means of actual oil slick observations reported during the Prestige accident and trajectories of drifter buoys. The model is based on the analysis of a database of hypothetical oil spill scenarios simulated by means of a Lagrangian transport model. To carry out the simulations, a re-analysis database consisting of 44-year hindcast dataset of wind and waves and climatologic daily mean surface currents is used. The number of scenarios required to obtain statistically reliable results is investigated, finding that 200 scenarios provide an optimal balance between the accuracy of the results and the computational effort. The reliability of the model was analyzed by comparing the actual data with the numerical results. The agreement found between actual and numerical data shows that the developed statistical oil spill model is a valuable tool to support spill response planning.

Stochastic Lagrangian trajectory model for drifting objects in the ocean

The prediction of drifting object trajectories in the ocean is a complex problem plagued with uncertainties. This problem is usually solved simulating the possible trajectories based on wind and advective numerical and/or instrumental data in real time, which are incorporated into Lagrangian trajectory models. However, both data and Lagrangian models are approximations of reality and when comparing trajectory data collected from drifter exercises with respect to Lagrangian models results, they differ considerably. This paper introduces a stochastic Lagrangian trajectory model that allows quantifying the uncertainties related to: (i) the wind and currents numerical and/or instrumental data, and (ii) the Lagrangian trajectory model. These uncertainties are accounted for within the model through random model parameters. The quantification of these uncertainties consists in an estimation problem, where the parameters of the probability distribution functions of the random variables are estimated based on drifter exercise data. Particularly, it is assumed that estimated parameters maximize the likelihood of our model to reproduce the trajectories from the exercise. Once the probability distribution parameters are estimated, they can be used to simulate different trajectories, obtaining location probability density functions at different times. The advantage of this method is that it allows: (i) site specific calibration, and (ii) comparing uncertainties related to different wind and currents predictive tools. The proposed method is applied to data collected during the DRIFTER Project (eranet AMPERA, VI Programa Marco), showing very good predictive skills.

Development of a GIS-based oil spill risk assessment system

The Prestige crisis proved the importance of developing scientific- and application- oriented activities which allow us to improve the oil spill preparedness and response systems having efficient tools to minimize the spill impact in case of an emergency. In this work a methodology has been developed in which oil spill risk is calculated assuming its dependency on the hazard, H, and vulnerability, V, components. To estimate the probability of an oil spill reaching a specific target area, H, an approach based on numerically generated data has been used. Regarding the other risk component, the oil spill vulnerability V, a new approach is presented which focuses on the integration in one single index of physical, biological and socio-economic aspects of the coast. To illustrate the presented methodology it has been applied to the Cantabrian coast, Northern coast of Spain (Bay of Biscay) where a user-friendly application which incorporates a Geographic Information System has been built. This application integrates the two components of the oil spill risk, H and V, to support spill response planning along this coast.

A HIGH RESOLUTION OPERATIONAL OIL SPILL MODEL AT SANTANDER BAY (SPAIN): IMPLEMENTATION AND VALIDATION

ABSTRACT A High Resolution Operational Oceanography System that provides decision makers with short-term (within 48 hours) oil spill trajectory forecasting at local scale, has been developed in the...

Operational oil spill trajectory modelling using HF radar currents: A northwest European continental shelf case study

This paper presents a novel operational oil spill modelling system based on HF radar currents, implemented in a northwest European shelf sea. The system integrates Open Modal Analysis (OMA), Short Term Prediction algorithms (STPS) and an oil spill model to simulate oil spill trajectories. A set of 18 buoys was used to assess the accuracy of the system for trajectory forecast and to evaluate the benefits of HF radar data compared to the use of currents from a hydrodynamic model (HDM). The results showed that simulated trajectories using OMA currents were more accurate than those obtained using a HDM. After 48h the mean error was reduced by 40%. The forecast skill of the STPS method was valid up to 6h ahead. The analysis performed shows the benefits of HF radar data for operational oil spill modelling, which could be easily implemented in other regions with HF radar coverage.

AMBEMAR-DSS: A Decision Support System for the Environmental Impact Assessment of Marine Renewable Energies

The high energy demand and the threat of climate change have led to a remarkable development of renewable energies, initially through technologies applied to the terrestrial environment and, recently, through the awakening of marine renewable energies. However, the development of these types of projects is often hampered by failure to pass the corresponding environmental impact assessment process. The complexity of working in the marine environment and the uncertainties associated with assessing the impacts of such projects make it difficult to carry out objective and precise environmental impact assessments. AMBEMAR-DSS seeks to establish a basis for understanding and agreement between the different stakeholders (project developers, public administrations, environmental organizations and the public in general), in order to find solutions that allow the development of marine renewable energies, minimizing their environmental cost. For this purpose, a DSS is proposed which, based on cartographic information and using objective and quantifiable criteria, allows comparative assessments and analyses between different project alternatives. The analytical procedures used by the system include, among others, hydrodynamic modeling tools and visual impact simulators. In addition, impacts on marine species are assessed taking into account intrinsic ecological and biological aspects. The magnitude of the impacts is quantified by means of fuzzy logic operations and the integration of all the elements is carried out by an interactive multi-criteria analysis. The results are shown in tables, graphs and figures of easy interpretation and can be also visualized geographically by means of a cartographic viewer. The system identifies the main impacts generated in the different phases of the project and allows establishing adequate mitigation measures in search of optimized solutions. The establishment of the assessment criteria has been based on the abundant, but dispersed, scientific literature on the various elements of the system and having the opinion of experts in the various fields. Nevertheless, the DSS developed constitutes a preliminary basis on which to build and improve a system with the input of researchers, promoters and experts from different disciplines.

A high-resolution operational forecast system for oil spill response in Belfast Lough

This paper presents a high-resolution operational forecast system for providing support to oil spill response in Belfast Lough. The system comprises an operational oceanographic module coupled to an oil spill forecast module that is integrated in a user-friendly web application. The oceanographic module is based on Delft3D model which uses daily boundary conditions and meteorological forcing obtained from COPERNICUS and from the UK Meteorological Office. Downscaled currents and meteorological forecasts are used to provide short-term oil spill fate and trajectory predictions at local scales. Both components of the system are calibrated and validated with observational data, including ADCP data, sea level, temperature and salinity measurements and drifting buoys released in the study area. The transport model is calibrated using a novel methodology to obtain the model coefficients that optimize the numerical simulations. The results obtained show the good performance of the system and its capability for oil spill forecast.

SHORT-TERM AND MEDIUM-TERM FORECAST OF OIL SPILL TRAJECTORIES: APPLICATION TO LOCAL AND REGIONAL SCALES

ABSTRACT This paper presents two methodologies to provide short-term and medium-term forecast of oil spill trajectories at local and regional scales. For short-term predictions (within 48 hours), a...