APPENDIX A

ROC Model Discussion

Genwest (2012) uses the ROC model to estimate the thickness of the heaviest concentrations of oil, which would be targeted and encountered by skimmers. ROC incorporates weathering (Galt, 2011) and spreading (Galt and Overstreet, 2011) of oil based on spill volume, oil properties, and environmental conditions. On the basis of the committee’s discussions with the report authors, weathering was included in the ROC model runs that were used to develop the nominal average oil thickness by day in the Genwest (2012) report. Weathering includes evaporation, emulsification (formation of mousse by entrainment of seawater into the oil), dissolution, and degradation (both biodegradation and photo-oxidation). The floating oil volume of light crude oils, which contain substantial fractions of volatiles, would decrease rapidly in the first hours to days after release, so the average oil thickness should also decrease rapidly. Evaporative losses even for medium and heavy crudes can also be substantial (up to 30% of the mass/volume) over the first 3 days after release. Dissolution of the slick and degradation would not change oil volume or thickness significantly over hours to a few days. On the other hand, emulsification would increase oil thickness. ROC appropriately includes these changes with weathering algorithms used in state-of-the-art oil spill models.

ROC considers the bounding area of a contiguous oil slick, which initially spreads radially by gravitational slumping controlled by viscous forces. To account for wind and wave motion and oil droplet formation and entrainment due to wave motion, the circular area of spread oil is extended in the downwind axis by 3% of wind speed (a common rule of thumb for oil drift rate, range 2-6%; ASCE Task Committee on Modeling Oil Spills, 1996) plus an additional 0.5% of the wind speed if the wind is greater than 6 m/s (to represent the additional downwind velocity expected in windrows associated with Langmuir cells). This approximates for the two-dimensional model in ROC the three-dimensional process (described based on field data by Elliott, 1986), where oil entrained into underlying water is left behind the leading edge of the floating oil because the oil floating on the water surface moves at roughly 3% of wind speed, whereas droplets in underlying water move slower. The smallest droplets remain under water the longest and undergo little wind transport, whereas progressively larger droplets resurface behind the leading edge sooner (due to higher buoyancy). This results in a comet-shaped slick with thick oil at the downwind end and sheen trailing behind. This was noted first by Elliott et al. (1986) and more recently through numerical simulations by the Boufadel group (Boufadel et al., 2006, 2007). These spreading processes are modeled explicitly in three-dimensional oil spill models such as SIMAP (French-McCay, 2004) and presumably in OSCAR (Reed et al., 1995; Aamo et al., 1997).

The correction of the contiguous oil slick thickness to account for the leading edge of thicker oil does appear to be included in the ROC model runs used to develop the nominal average oil thickness. In committee discussions with the Genwest report authors, they confirmed that this spreading correction was in fact included. Details of those calculations (i.e., what factor or factors were used) are not available.

Figures 16-18 in Galt and Overstreet (2011) present estimated oil thickness at Langmuir convergences relative to initial spill thickness for a range of wind conditions and oil densities. Their conclusion was “that under most conditions of wind- and windrow-spacing, spilled oil could collect in convergences comprising only 20%, or less, of the original spills’ area.” In ROC



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26 REVIEW OF THE EDRC REPORT APPENDIX A ROC Model Discussion Genwest (2012) uses the ROC model to estimate the thickness of the heaviest concentrations of oil, which would be targeted and encountered by skimmers. ROC incorporates weathering (Galt, 2011) and spreading (Galt and Overstreet, 2011) of oil based on spill volume, oil properties, and environmental conditions. On the basis of the committee’s discussions with the report authors, weathering was included in the ROC model runs that were used to develop the nominal average oil thickness by day in the Genwest (2012) report. Weathering includes evaporation, emulsification (formation of mousse by entrainment of seawater into the oil), dissolution, and degradation (both biodegradation and photo-oxidation). The floating oil volume of light crude oils, which contain substantial fractions of volatiles, would decrease rapidly in the first hours to days after release, so the average oil thickness should also decrease rapidly. Evaporative losses even for medium and heavy crudes can also be substantial (up to 30% of the mass/volume) over the first 3 days after release. Dissolution of the slick and degradation would not change oil volume or thickness significantly over hours to a few days. On the other hand, emulsification would increase oil thickness. ROC appropriately includes these changes with weathering algorithms used in state-of-the-art oil spill models. ROC considers the bounding area of a contiguous oil slick, which initially spreads radially by gravitational slumping controlled by viscous forces. To account for wind and wave motion and oil droplet formation and entrainment due to wave motion, the circular area of spread oil is extended in the downwind axis by 3% of wind speed (a common rule of thumb for oil drift rate, range 2-6%; ASCE Task Committee on Modeling Oil Spills, 1996) plus an additional 0.5% of the wind speed if the wind is greater than 6 m/s (to represent the additional downwind velocity expected in windrows associated with Langmuir cells). This approximates for the two- dimensional model in ROC the three-dimensional process (described based on field data by Elliott, 1986), where oil entrained into underlying water is left behind the leading edge of the floating oil because the oil floating on the water surface moves at roughly 3% of wind speed, whereas droplets in underlying water move slower. The smallest droplets remain under water the longest and undergo little wind transport, whereas progressively larger droplets resurface behind the leading edge sooner (due to higher buoyancy). This results in a comet-shaped slick with thick oil at the downwind end and sheen trailing behind. This was noted first by Elliott et al. (1986) and more recently through numerical simulations by the Boufadel group (Boufadel et al., 2006, 2007). These spreading processes are modeled explicitly in three-dimensional oil spill models such as SIMAP (French-McCay, 2004) and presumably in OSCAR (Reed et al., 1995; Aamo et al., 1997). The correction of the contiguous oil slick thickness to account for the leading edge of thicker oil does appear to be included in the ROC model runs used to develop the nominal average oil thickness. In committee discussions with the Genwest report authors, they confirmed that this spreading correction was in fact included. Details of those calculations (i.e., what factor or factors were used) are not available. Figures 16-18 in Galt and Overstreet (2011) present estimated oil thickness at Langmuir convergences relative to initial spill thickness for a range of wind conditions and oil densities. Their conclusion was “that under most conditions of wind- and windrow-spacing, spilled oil could collect in convergences comprising only 20%, or less, of the original spills’ area.” In ROC

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LETTER REPORT 27 (based on the data in Figures 16-18 of Galt and Overstreet, 2011), the inclusion of Langmuir cell convergences (windrows) amounts to a factor of 5 or more (up to 100 times) increase in thickness compared with a mean over the area encompassing one or more cells. The scale of Langmuir cells is taken as three times the mixed- layer depth (Galt and Overstreet, 2011), which is typically 10 to 30 m. Thus, Langmuir cell windrows would be about 30 to 90 m apart (about 100 to 300 ft). If a skimmer’s swath width is order 100-1,000 ft and its speed is 0.75 kt based on ERSP, the skimmer would sweep 0.2 to 2 square miles in a 12-hour operational period. Thus, skimmers would be crossing both windrows and open water, and large swath widths are much larger than the windrow spacing. In addition, Langmuir cells are not always present; they will appear after wind blows in one direction for a relatively long time (many hours or more). Thus, this analysis indicates that inclusion in ROC of a factor 5-100 increase in thickness due to Langmuir cells would overestimate the average oil thickness that would be encountered by a skimmer. However, based on discussions with the Genwest report authors, the ROC model runs used to develop the nominal average or representative oil thickness did not include consideration of thickening of oil in windrows due to Langmuir circulation. This is appropriate for the reasons described above. Unfortunately, the Genwest report is not clear on this point; it merely cites Galt and Overstreet (2011), which describes the basis and inclusion of such correction factors in ROC.

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28 REVIEW OF THE EDRC REPORT REFERENCES Aamo, O., M. Reed, and A. Lewis. 1997. Regional Contingency Planning Using the OSCAR Oil Spill Contingency and Response Model. Trondheim, Norway: SINTEF Applied Chemistry, Environmental Engineering. Available online at www.itk.ntnu.no/ansatte/ Aamo_Ole.Morten/pdf/region.pdf. Accessed August 28, 2013. ASCE Task Committee on Modeling Oil Spills. 1996. State-of-the-art review of modeling transport and fate of oil spills. Journal of Hydraulic Engineering 122:594-609. Boufadel, M.C., R. Bechtel, J. Weaver. 2006. The movement of oil under non-breaking waves. Marine Pollution Bulletin 52:1056-1065. Boufadel, M.C., K. Du, V.J. Kaku, and J. Weaver. 2007. Lagrangian simulation of oil droplets transport due to regular waves. Environmental Modelling & Software 22:978-986. Elliott, A.J. 1986. Shear diffusion and the spread of oil in the surface layers of the North Sea. Deutsche Hydrografische Zeitschrift 39:113-137. Elliott, A.J., N. Hurford, and C.J. Penn. 1986. Shear diffusion and the spreading of oil slicks. Marine Pollution Bulletin 17:308-313. French-McCay, D.P. 2004. Oil spill impact modeling: Development and validation. Environmental Toxicology and Chemistry 23:2441-2456. Galt, J.A. 2011. Oil Weathering Technical Documentation and Recommended Use Strategies. Edmonds, WA: Genwest Systems, Inc. Available online at http://www.genwest.com/DRAFT%20ROC%20OilWx%20TechDoc.pdf. Accessed August 26. 2013. Galt, J.A. and R. Overstreet. 2011. Development of Spreading Algorithms for the ROC. Edmonds, WA: Genwest Systems, Inc. Available online at http://www.genwest.com/ DRAFT%20ROC%20Spreading%20TechDoc.pdf. Accessed August 26, 2013. Genwest (Genwest Systems, Inc.). 2012. EDRC Project Final Report. Edmonds, WA: Genwest Systems, Inc. Available online at http://www.bsee.gov/uploadedFiles/BSEE/ Research_and_Training/Technology_Assessment_and_Research/tarprojects/600- 699/Genwest_EDRC-Project_Final_Report-1.pdf. Accessed August 26, 2013. Reed, M., O.M. Aamo, and P.S. Daling. 1995. Quantitative analysis of alternate oil spill response strategies using OSCAR. Spill Science & Technology 2:67-74.