aground. This effort was supported by ERDA contract DY-76-S-02-4047 . The 

 model was written to allow for either a Monte Carlo prediction of the likely 

 spill location as a function of time, sampling randomly from the 10-year 

 average monthly wind rose for that area, or a deterministic hindcast of the 

 spill location using actual wind measurement for the period of interest. 

 Monte Carlo predictions of the location of the leading edge of the spill 5, 

 10, 15, 20, 25 and 30 days following the breakup of the Avgo Merohant were 

 generated for URI planning purposes shortly after the ship broke up. These 

 predictions, as well as a 30-day deterministic hindcast, are presented here. 

 In all figures, the forecast limits of oil by the USCG Oceanographic Unit 

 (Section 2.3.1) are included for comparison. 



Model Runs 



Two cases of the deterministic model were run. The first (Figure VII- 

 22, Appendix VII) included wind-driven currents only. It was assumed that 

 the wind induced a surface drift in the direction of the wind at 3.5% of the 

 wind velocity. This drift is accounted for by about 1.5% for the wind- 

 induced water motion and about 2.0% for the relative oil-to-water motion 

 (Smith, 1974). No Coriolis forces were included. The second case (Figure 

 VII-23) included wind-driven currents and tidal currents which were added 

 vector ially to yield the spill's overall movement. In both cases, starting 

 date was December 18, and 30-day trajectories were calculated. 



The model was run using 3-hour time steps, i.e., the spill was moved at 

 the determined rate and in the determined direction for a period of time 

 corresponding to 3 hours before the wind was changed. The necessary wind 

 data were obtained from the Great Point Coast Guard Station on Nantucket 

 Island. The tidal currents were taken from the National Oceanic and Atmos- 

 pheric Administration navigation chart 13006 (4/76). 



As in the deterministic case, the Monte Carlo runs were made both for 

 wind-driven currents and for wind-driven currents added vectorially to the 

 local tidal currents. The wind speed and direction for the Monte Carlo runs 

 were randomly sampled so that the probability of obtaining a given wind 

 magnitude and direction equals the probability that such conditions are 

 observed in the appropriate month during the past 10 years. These 10-year 

 averaged data (U.S. Naval Weather Service Command, 1970) list the probability 

 by month of obtaining the wind velocity in each of six ranges for each of 

 eight wind directions (N, NE, E, etc.). Once a direction and magnitude had 

 been chosen for the wind, the wind-induced drift of the oil was calculated 

 and added to the tidal current. The slick was then moved at this drift rate 

 for 2 hours, at which time the procedure was repeated. Figures VII-24 and 

 VII-25 depict five representative spill trajectories for the wind-only and 

 the wind-plus-tldal-current case, respectively. Figures VII-26 to VII-28 

 and Figure 2-14 represent the 5-, 10-, 20-, and 30-day Monte Carlo predic- 

 tions corresponding to 200 trajectories in which the tidal current was in- 

 cluded. Figure VII-29 represents, for comparison, the 30-day Monte Carlo 

 prediction for which the tidal current was not included. 



In all the plots shown in the above figures and figure 2-13, the pro- 

 jected path represents that of the leading edge of the oil slick. Thus, 



59 



