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Oil in the Sea III: Inputs, Fates, and Effects
PHOTO 16 A spill of roughly 320,000 gallons of south Louisiana crude in May 1997, streaming across the broken marshes of Lake Barre. There is very little substrate exposed thus the oil is being pushed through the submerged vegetation by wind. (Photo courtesy of Jacqui Michel, Research Planning, Inc.)
is the same in both the vertical and the horizontal (order of tens of meters), the vertical component is more important because it is strong compared to other normal vertical mixing processes in the ocean. In contrast, LC-induced horizontal dispersion is weak compared to other horizontal mixing processes. Rye (2001) shows aircraft observations from numerous spills that indicate LC horizontal dispersion, but the effects are relatively small scale. It is interesting to note that Rye’s (2001) comparison focused solely on horizontal scales and not vertical, presumably because of the lack of good data in the vertical.
Lehr and Simecek-Beatty (2001) point out that LC may well be as important at enhancing vertical dispersion as wave breaking. Theory suggests that wave breaking will drive oil droplets roughly one wave height into the water column, whereas LC could drive smaller near-neutrally buoyant droplets tens of meters down, perhaps as far as the base of the mixed layer. Given this, it is an apparent paradox that state-of-the-art oil spill trajectory models include vertical dispersion due to wave breaking but not LC. The primary reason for this is that there is presently no relatively simple verified algorithm to include LC in a spill model. A realistic model would have to not only include a physical model of the Langmuir cell hydrodynamics but also to consider the buoyancy of the oil droplets and hence the droplet-sized distribution. None of these are well understood.
In summary, LC is a potentially important mechanism whose effects have been seen in real spills but are not presently well understood. Further measurements are clearly needed especially with regard to the efficiency of LC in enhancing vertical dispersion and subsequent hydrocarbon dissolution. If further research demonstrates the importance of LC compared to other processes, then a relatively simple LC algorithm should be developed and incorporated into oil spill trajectory models. LC effects on cleanup strategies are another potentially fruitful topic of research.
Dispersion is a mixing process caused by the turbulence field in the ocean. It is the process that would cause a liter of instantaneously released dyed water to expand over time and eventually dissipate in the ocean. Without dispersion, advection would move that liter downstream, but the volume of dyed water would not change over time. Dispersion occurs in both the horizontal and the vertical directions, but because the hydrodynamic processes in the vertical and horizontal are often quite different, a distinction is usually made.
In oil spill modeling, horizontal dispersion is often combined with “spreading,” but they are fundamentally different processes characterized by different length and time scales. A liter of oil dumped on a tabletop will spread but it will not