VERTIMAR-2005
Symposium on Marine Accidental Oil Spills
Calibration of a transport model using drifting buoys deployed during the Prestige accident
S.CASTANEDO, A.J.ABASCAL, R. MEDINA andI.J.LOSADA
Ocean and Coastal Research Group, Universidadde Cantabria, Dpto. de Ciencias y Técnicasdel Agua y delMedioAmbiente, Av. de los Castros s/n 39005 Santander, Spain. Fax:34-942-201860.
E-mail:
- INTRODUCTION
On November 19, 2002 the single-hulled oil tanker Prestige broke in two at about 130 nautical miles off the Spanish coast, west south-west of Cape Finisterre (42º15’N, 12º08’W) and sank at a depth of about 3,500 meters. Along the Spanish coast several emergency spill response system were built during the crisis (Montero et al., 2003; Castanedo et al.,2004). In these response systems one important task was to establish operational forecasting systems for developing proper response strategies. Generally, the structure of these predictions systems was composed by collection of observations including oil slicks, numerical modelling to provide forecasts of wind, waves, currents and oil trajectories and finally data management and dissemination.
Regarding observations, overflight information provided by national and regional administrations was crucial to fight the Prestige oil spill. Moreover, among the decisions made during the management of the catastrophe, it was proposed to release Lagrangian floats to both track the biggest oil slicks position and trajectory and to provide some feedback and/or validation for the numerical models of currents and oil dispersion forecast. The deployment of drifting floats was organised by the National Spanish Research Council (CSIC) and AZTI Foundation using available ARGOS buoys used for oceanographic studies (García-Ladona et al., 2005).
The information provided by the drifting buoys was very useful to verify the oil spill models performance during the crisis. This kind of analysis was carried out periodically in order to update the model parameters by means of a trial-and-error procedure. Taking advantage of this valuable data base, a more exhaustive analysis using the drifters’ trajectories is being carried out. The aim of this study is to calibrate a Lagrangian particle-tracking trajectory algorithm and, at the same time, investigate about the relative importance that the different forcing (wind, wave, currents) have on the oil spill fate. The purpose of this work is to present this analysis.
- METHODOLOGY
In order to carry out the aforementioned work both a two dimensional Lagrangian oil spill model and a global optimisation routine are used. Besides, meteorological data from re-analysis corresponding to the period November 2002-November 2003 was provided by the INM (Spanish Meteorological Institute). Regarding oceanic currents, two sets of data are used: (1) the output of the NRLPOM Cantabric model, operated by the Naval Research Laboratory (USA) and sent daily from the NRL to State Ports of Spain (PE) during the crisis and (2) re-analysis currents from the MERCATOR model ( provided by PE in the framework of the ESEOO project.
The numerical model used solve the following vector equation: where xi is the particle coordinate, and ua and ud are the advective velocity and the random velocity fluctuations respectively at the particle coordinate. As is usual in this kind of models, advection is taken into account by means of the following vector sum: ua= ucurrents +CD* uwind+Cw*uwave-induced, where CD is the wind drag coefficient and Cw is the wave coefficient.
As was mentioned before, one of the objectives of this study is to calibrate the numerical model. In order to do this, the adequate coefficients CD and Cw have to be found to achieve the best agreement between the buoys trajectories and the predicted ones. Owing to the great amount of variables that the problem has and with the aim of reducing the complexity of the analysis, the first stage of the work consisted of applying a global optimisation routine (Duan et al, 1994). This algorithm is very efficient in the resolution of highly non-linear problems and it is widely used in the automatic calibration of watershed models. In figure 1, an example of the results obtained with this method is presented. In the oral presentation and in the final paper, a more detailed explanation of the procedure and information about the results will be shown.
Figure 1. Automatic calibration results. Case1 (January 2003). a) Wind drag coefficient against wind velocity; b) Wind drag coefficient variation; c) correlation coefficient for the velocity in the x (W-E) direction; d) correlation coefficient for the velocity in the y (S-N) direction
REFERENCES
Castanedo, S., Medina, R., Losada, I.J., Vidal, C., Méndez, F.J., Osorio, A., Juanes, J.A., Puente, A.The Prestige oil spill in Cantabria (Bay of Biscay). Part I: Operational forecasting system for quick response, risk assessment and protection of natural resources. Journal of Coastal Research. En revisión.
Duan, Q., Sorooshian, S., Gupta, V. (1994). Optimal use of the SCE-UA global optimization method for calibrating watershed models.Journal of Hydrology, 158, pp 265-284.
Garcia-Ladona, E., Font, J., Rio, E., Julia, A., Salat, J., Chic, O., Orfila, A., Alvarez, A., Basterretxea, G., Vizoso, G., Piro, O., Tintore, J., Castanedo, S., Coto, M.G., Herrera, J.L. (2005). The use of surface drifting floats in the monitoring of oil spills. The Prestige Case. 19th Biennial International Oil Spill Conference.
Montero, P., Blanco, J., Cabanas, J. M., Maneiro, J., Pazos, Y., Moroño, A., Balseiro, C. F., Carracedo, P., Gomez, B., Penabad, E., Pérez-Muñuzuri, V., Braunschweig, F., Fernandes, R., Leitao, P. C., Neves, R. (2003) Oil spill monitoring and forecasting on the Prestige- Nassau accident. Environment Canada’s 26th Artic and Marine Oil spill (AMOP) Technical Seminar. Otawa. Canada. Proc. Pp. 1013-1029.
Acknowledgements
This work is partially funded by the Ministerio de Educación y Ciencia (ESEOO project, VEM2003-C14-03).