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J. Murawski and J. Woge Nielsen
nutrients or tracers, give complete concentration maps as a direct result. The trajectory method, in which the computational effort is limited to the actually affected
region, requires a post-processing step to transfer the spill state vector (particle positions and properties) into the information about the concentration of the pollutant.
The study presented below is mostly limited to the trajectory analysis.
11.2.3 Oil Fate
For passive tracers, the combination of advection and spreading fully describes the
time evolution of a spill. The resulting information is, however, not sufficient for
an adequate reproduction of the time history of an oil spill. Oil is a chemically and
physically active substance. It interacts not only with sea water but also with air
and (suspended) sediment. On top of that, it reacts on sunlight, impacts the biological environment, and, on a longer time scale, is impacted through biodegradation.
These interactions result in major transformations of the oil, leading to changes in
its composition and thereby in its chemical and physical properties (Fingas 2011).
To replicate the described processes to a certain extent, a superstructure is applied to the routines for trajectory calculations to simulate short-to-medium time
range chemical processes, weathering, horizontal spreading as a surface film, and
buoyant motion affecting the oil particles along their trajectories. These processes
exert a certain feedback on the advection, mainly through changes in the density
of oil particles, equivalently, their buoyancy (which is generally non-neutral anyway) relative to sea water and their tendency to form a surface film. A part of an
oil spill is often ‘mixed down’ by the action of waves in rough seas (see below) and
is located at a certain depth in the water column. This also happens when the spill
occurs at a certain depth. The water column in the seas and oceans often possess
vertically sheared systems of currents (see, e.g., the description of the Ekman spiral
in Chap. 2, Sect. 2.3.5). In these cases subsurface oil moves at a different speed and
direction than oil at the surface. The submerged part of the spill typically lags behind
the surface part. This mechanism, which in turn depends on oil properties (density,
viscosity), ambient water properties and on certain metocean conditions, elongates
the spill in the flow direction and may even split it up into isolated patches.
The spreading of oil over the sea surface is an anisotropic process, with different
mechanisms responsible for horizontal and vertical spreading. It is particularly important for the early stages of an oil spill. This anisotropy makes it appropriate to
assign to an oil particle a disc-like shape, characterized by its coordinates, volume
and thickness. Such overlapping discs may be added up to determine the thickness
of the surface film.
Horizontal spreading is an advective–diffusive process that undergoes three
phases until it ultimately results in the terminal thickness—the formation of a
micron-thin surface film. The initial spill radius r first increases by buoyancy-inertia
forces. As the slick grows thinner, the spreading moves into a buoyancy–viscous
regime. The final stage is governed by viscosity and surface tension (Fay 1971).
J. Murawski and J. Woge Nielsen
nutrients or tracers, give complete concentration maps as a direct result. The trajectory method, in which the computational effort is limited to the actually affected
region, requires a post-processing step to transfer the spill state vector (particle positions and properties) into the information about the concentration of the pollutant.
The study presented below is mostly limited to the trajectory analysis.
11.2.3 Oil Fate
For passive tracers, the combination of advection and spreading fully describes the
time evolution of a spill. The resulting information is, however, not sufficient for
an adequate reproduction of the time history of an oil spill. Oil is a chemically and
physically active substance. It interacts not only with sea water but also with air
and (suspended) sediment. On top of that, it reacts on sunlight, impacts the biological environment, and, on a longer time scale, is impacted through biodegradation.
These interactions result in major transformations of the oil, leading to changes in
its composition and thereby in its chemical and physical properties (Fingas 2011).
To replicate the described processes to a certain extent, a superstructure is applied to the routines for trajectory calculations to simulate short-to-medium time
range chemical processes, weathering, horizontal spreading as a surface film, and
buoyant motion affecting the oil particles along their trajectories. These processes
exert a certain feedback on the advection, mainly through changes in the density
of oil particles, equivalently, their buoyancy (which is generally non-neutral anyway) relative to sea water and their tendency to form a surface film. A part of an
oil spill is often ‘mixed down’ by the action of waves in rough seas (see below) and
is located at a certain depth in the water column. This also happens when the spill
occurs at a certain depth. The water column in the seas and oceans often possess
vertically sheared systems of currents (see, e.g., the description of the Ekman spiral
in Chap. 2, Sect. 2.3.5). In these cases subsurface oil moves at a different speed and
direction than oil at the surface. The submerged part of the spill typically lags behind
the surface part. This mechanism, which in turn depends on oil properties (density,
viscosity), ambient water properties and on certain metocean conditions, elongates
the spill in the flow direction and may even split it up into isolated patches.
The spreading of oil over the sea surface is an anisotropic process, with different
mechanisms responsible for horizontal and vertical spreading. It is particularly important for the early stages of an oil spill. This anisotropy makes it appropriate to
assign to an oil particle a disc-like shape, characterized by its coordinates, volume
and thickness. Such overlapping discs may be added up to determine the thickness
of the surface film.
Horizontal spreading is an advective–diffusive process that undergoes three
phases until it ultimately results in the terminal thickness—the formation of a
micron-thin surface film. The initial spill radius r first increases by buoyancy-inertia
forces. As the slick grows thinner, the spreading moves into a buoyancy–viscous
regime. The final stage is governed by viscosity and surface tension (Fay 1971).