2, INVESTIGATIONS OF PHYSICAL PROCESSES 



In order to determine the factors that cause oil spill transport and the 

 rates at which the spill changes physically with time, a series of studies 

 were conducted that combined both modeling and observation of transport pro- 

 cesses. The objective of this research is to upgrade models capable of 

 forecasting oil spill trajectories and/or probabilities of shoreline impact 

 under various environmental conditions. 



The motion of oil floating on water is determined by three factors: 

 currents, waves, and winds. While the oil is bodily carried by water cur- 

 rents, it is capable of sliding relative to the water in response to the 

 forces of waves and winds. It has been long known that oil placed on the 

 water surface will absorb the smaller and shorter waves by viscous decay and 

 act to calm the seas. Additionally, the irrotational nature of wave motion 

 generates a drift at the water surface (stokes drift), but this irrotational 

 motion does not satisfy boundary conditions at the free surface, and as a 

 result vorticity is generated. In the presence of an oil film, the vorticity 

 generated at the surface quickly propagates vertically through the oil be- 

 cause of its relatively high viscosity and increases the surface speed (Mil- 

 gram 1977) . This increase in speed is considerably larger than the contri- 

 bution caused by wave absorption. The mechanism by which oil gains velocity 

 is not fully understood, but it is a function of wave length and height, the 

 thickness of the oil, the physical properties of the oil, such as surface 

 tension and visocity, and the physical properties of the oil-water interface. 

 Wind, in addition to being the prime generator of waves, can also act di- 

 rectly on the surface oil, causing a transfer of energy from the wind to the 

 oil. 



Although many oil spills have occurred in cold-water environments, in- 

 struments have not been adequate to measure these spills and to characterize 

 their behavior and the surrounding environment in sufficient detail to pro- 

 vide all the data required to compare actual spill behavior with trajectory 

 model forecasts. Such characterization requires measurements of environ- 

 mental conditions, both within and outside the spill, as well as a fixed 

 reference point to which one can relate spill movements. 



Throughout the course of the effort connected with the Argo Merchant, 

 NOAA and USCG researchers continually compared model-generated forecasts of 

 oil spill trajectories with the observed behavior of oil in the marine envi- 

 ronment. These comparisons were required to evaluate the accuracy of the oil 

 spill forecasts at various phases in the program, and to identify those ele- 

 ments of the modeling effort that needed refining and of the observational 

 program that needed strengthening. 



2.1 Techniques of Field Efforts 



Numerous agencies and institutions participated in studying the physical 

 processes that the oil from the Argo Merchant underwent. These included the 

 NOAA-USCG SOR Team, USCG, NASA, EPA, NOAA's Flight Operation Group and Data 

 Buoy Office, and Aero-Marine Surveys, Inc. (a BLM subcontractor), USGS , and 



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