Abstract:
Oil spill research has significantly increased mainly as
a result of the severe consequences experienced from
industry accidents. Oil spill models are currently able
to simulate the processes that determine the fate of oil
slicks, playing an important role in disaster prevention,
control and mitigation, generating valuable information
for decision makers and the population in general. On
the other hand, satellite Synthetic Aperture Radar (SAR)
imagery has demonstrated significant potential in accidental
oil spill detection, when they are accurately differentiated
from look-alikes. The combination of both
tools can lead to breakthroughs, particularly in the development
of Early Warning Systems (EWS). This paper
presents a hindcast simulation of the oil slick resulting
from the Motor Tanker (MT) Cercal oil spill, listed
by the Portuguese Navy as one of the major oil spills in
the Portuguese Atlantic Coast. The accident took place
nearby Leix˜oes Harbour, North of the Douro River, Porto
(Portugal) on the 2nd of October 1994. The oil slick
was segmented from available European Remote Sensing
(ERS) satellite SAR images, using an algorithm based on
a simplified version of the K-means clustering formulation.
The image-acquired information, added to the initial
conditions and forcings, provided the necessary inputs
for the oil spill model. Simulations were made considering
the tri-dimensional hydrodynamics in a crossscale
domain, from the interior of the Douro River Estuary
to the open-ocean on the Iberian Atlantic shelf. Atmospheric
forcings (from ECMWF - the European Centre
for Medium-Range Weather Forecasts and NOAA -
the National Oceanic and Atmospheric Administration),
river forcings (from SNIRH - the Portuguese National
Information System of the Hydric Resources) and tidal
forcings (from LNEC - the National Laboratory for Civil
Engineering), including baroclinic gradients (NOAA),
were considered. The lack of data for validation purposes
only allowed the use of the two-dimensional surface
plume transport model VOILS [1] with the oil spreading
formulation enabled. The remaining oil weathering processes
(evaporation, emulsification, dispersion and dissolution
in the water column) and shoreline retention were
disregarded. The computational structure of the model
is based on Eulerian-Lagrangian formulations, horizontal
unstructured mesh discretization and it is soft-coupled
with the tri-dimensional hydrodynamic model SELFE -
Semi-Implicit Eulerian Lagrangian Finite Element [15]
that uses hybrid sigma-Z coordinates in the vertical. The
preliminary results of this hindcast simulation for the
Cercal oil spill are presented and compared with available
satellite SAR images. The forcings used play an important
role in the final results. During the late stage spreading
phases of the oil, about one month after the spill,
the Douro River outflow is best seen in the SAR images.
The morphology of the river outflow is discussed according
to traditional coastal dynamics, and compared with
model results. In addition to several interesting physical
features that were identified, we report on the generation
of Internal Solitary Waves (ISW) in the vicinity
of the Douro River Plume (DRP). It is well known that
trains of short-period internal waves can be generated by
river plumes (such as the Columbia River). The internal
structure of the observed internal waves (elevation
waves or mode-2 versus mode-1 internal waves) is discussed
based on the SAR signatures and available stratification.
The present work has been conducted under an
FCT - Fundac¸ ˜ao para a Ciˆencia e a Tecnologia / MCTES
- Minist´erio da Ciˆencia, Tecnologia e Ensino Superior
(PIDDAC - Programa de Investimentos e Despesas de
Desenvolvimento da Administrac¸ ˜ao Central) Portuguese
funded project entitled PAC:MAN Pollution accidents in
coastal areas: a Risk management system (PTDC/AACAMB/
113469/2008).