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ner slick will disperse more efficiently, and the mass balance increasingly shifts to
the water column. Visible on the water surface is an oil slick that contains only a
fraction of the oil, with the thickest part being central in the slick. This slick movement relative to the wind is less than expected, as most of the mass is underwater
and not affected by wind.
In less than ideal dispersion conditions, characterized by low wind speed and
high oil viscosity, the initial slick remains largely intact with a long comet-like tail
forming upwind. The tail continues to increase in length, while the thickness of the
downwind “main” slick gradually decreases. While thickness decreases, dispersion
efficiency slowly increases as relatively more small droplets are formed, and the
mass balance shifts slowly toward the water column. If the thickness decreases sufficiently, the oil slick behavior begins to transition toward that described above with
most of the oil mass in the water column and not affected by the wind.
These findings are consistent with in situ observations, where long slicks with a
thick portion downwind are found in conditions with low wind speeds and/or viscous oils and more favorable conditions create smaller slicks with the thickest portion in the center (Reed et al. 1994).
10.3 Influence of Individual Key Parameters on Dispersion
and Oil Slick Elongation
With model application, we can investigate the theoretical influence of different
parameters on the dispersion process and the development of the oil slick over time
with its eventual disappearance from the water surface.
The volume fraction of oil that is broken up into droplets that are sufficiently
small to be stably suspended can be calculated from oil properties, wind speed, and
initial oil layer thickness (Zeinstra-Helfrich et al. 2017). This variable, named dispersibility factor, represents the combined result of the different dispersion processes and provides a good indicator of dispersion success. In this chapter, the term
dispersibility refers to the favorability of conditions for creating small stable droplets without considering oil slick behavior over time.
When discussing oil slick behavior, the most relevant aspects are thickness profile, slick length, and the lifetime of the oil slick. This surface expression of a spill
can be summarized with the “time-integrated length of oil slick exceeding the
effects threshold of 25 μm” (Zeinstra-Helfrich et al. 2017). With equal oil slick
dimensions, this parameter correlates well with dispersibility factor: A highly dispersible slick will disappear quickly and have a low surface expression value. In
case of low dispersibility, a slick is present on the surface for a long period of time
and increases in size before disappearing, resulting in a high value for surface
expression.
In the following paragraphs, the influence of the different key parameters on the
oil dispersibility as well as oil slick behavior is discussed in more detail.
M. Zeinstra-Helfrich and A. J. Murk
ner slick will disperse more efficiently, and the mass balance increasingly shifts to
the water column. Visible on the water surface is an oil slick that contains only a
fraction of the oil, with the thickest part being central in the slick. This slick movement relative to the wind is less than expected, as most of the mass is underwater
and not affected by wind.
In less than ideal dispersion conditions, characterized by low wind speed and
high oil viscosity, the initial slick remains largely intact with a long comet-like tail
forming upwind. The tail continues to increase in length, while the thickness of the
downwind “main” slick gradually decreases. While thickness decreases, dispersion
efficiency slowly increases as relatively more small droplets are formed, and the
mass balance shifts slowly toward the water column. If the thickness decreases sufficiently, the oil slick behavior begins to transition toward that described above with
most of the oil mass in the water column and not affected by the wind.
These findings are consistent with in situ observations, where long slicks with a
thick portion downwind are found in conditions with low wind speeds and/or viscous oils and more favorable conditions create smaller slicks with the thickest portion in the center (Reed et al. 1994).
10.3 Influence of Individual Key Parameters on Dispersion
and Oil Slick Elongation
With model application, we can investigate the theoretical influence of different
parameters on the dispersion process and the development of the oil slick over time
with its eventual disappearance from the water surface.
The volume fraction of oil that is broken up into droplets that are sufficiently
small to be stably suspended can be calculated from oil properties, wind speed, and
initial oil layer thickness (Zeinstra-Helfrich et al. 2017). This variable, named dispersibility factor, represents the combined result of the different dispersion processes and provides a good indicator of dispersion success. In this chapter, the term
dispersibility refers to the favorability of conditions for creating small stable droplets without considering oil slick behavior over time.
When discussing oil slick behavior, the most relevant aspects are thickness profile, slick length, and the lifetime of the oil slick. This surface expression of a spill
can be summarized with the “time-integrated length of oil slick exceeding the
effects threshold of 25 μm” (Zeinstra-Helfrich et al. 2017). With equal oil slick
dimensions, this parameter correlates well with dispersibility factor: A highly dispersible slick will disappear quickly and have a low surface expression value. In
case of low dispersibility, a slick is present on the surface for a long period of time
and increases in size before disappearing, resulting in a high value for surface
expression.
In the following paragraphs, the influence of the different key parameters on the
oil dispersibility as well as oil slick behavior is discussed in more detail.
M. Zeinstra-Helfrich and A. J. Murk