Environmental Engineering Reference
In-Depth Information
meteorological data from the ocean is rare, it can be derived from spatial blending of
high-resolution satellite data (for example, seawinds instrument on the QuikSCAT
satellite- QSCAT) or numerical atmosphere models (for example, Fifth-Generation
Pennsylvania State University/National Center for Atmospheric Research mesoscale
model (known as MM5)). It is suggested that, in light winds, 3.5% of the wind speed
in the direction of the wind gives a good simulation of oil slick drift in offshore areas.
The deflection angles vary between 0 and 25
ⓦ
to the right/left of the wind direction
(northern/southern hemisphere) with a mean value of about 15
ⓦ
[
1
].
It is encouraging that a whole host of modern ocean circulation codes have enjoyed
considerable success in simulating 3D ocean circulation, such as POM [
3
]; ROMS
[
14
]; FVCOM [
6
]; ELCIRC [
17
] and SELFE [
18
]. Though their different numeri-
cal algorithms, the primitive equation are solved for describing the ocean motions
induced by winds, tides and baroclinic effects. One main challenge to ocean circu-
lation model is to resolve the complex geometry and bathymetry commonly found
in coastal and estuary area in an accurate, efficient and robust way. An unstructured
grid method used to discretize arbitrary geometries is a right key for this problem.
It becomes particularly important for oil spill model for the grid can be densified
in interested areas, which are near spill source points or highly sensitive to oil spill
pollution.
Although surface wave makes water particles perform periodic back-and-forth
excursions resulting in feeble net mass transport, they can modify ocean currents in
many ways, wave radiation stresses, Stokes drift, forcing in form of dissipation of
wave energy, and the surface/bottom drag coefficient. The progress in ocean wave
forecasting during the last twenty years is outstanding. Nowadays we are able to
estimate and forecast wave conditions, often with great accuracy, even with a detailed
description of the event. Compared with phase-resolved type ocean wave model (e.g.
Boussinesq equations and the mild slope equation), the phase-averaged model can
provide the great advantage in terms of computational efficiency for the grid size
is large enough to cover many wavelengths. The high-speed makes energy spectral
model is widely adopted in a large area for predicting oil spill trajectory. At present,
the third generation wave models (e.g. WAM, WAVEWATCH, SWAN) fully resolve
the physical processes such as wind input, propagation and dissipation [
4
]. Through
coupled with ocean circulation model, the spectral model is capable of resolving the
flow change within wavelength and wave period in the computation [
12
].
6.2.2 Lagrangian Particle Method
To accurately describe the oil slick after breaking up into small fragments due to
surface wave action and shear current, Lagrangian particle modelling of the transport
process of clusters of leaking oil has been a common technique in oil spill numerical
models for close to 20years.
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