typhoon, approximating a 6-knot (3 m/sec) maximum drift (Ref. 54). Prior to the passage of 

 the typhoon, the current drift was relatively straight to the northwest at 2 to 3 knots ( 1 to 1 .5 

 m/sec). This demonstrates the dramatic change in both speed (2-3 to 6 knots) and direction 

 (northwest to northeast) that can occur for a significant period of time after a hurricane. 



Besides limited ship drift data and the single current measurement in the Gulf of 

 Mexico reported above, no other data giving current measurements in severe storms could be 

 located. Theoretical predictions of the effect of high winds on currents do exist, however; 

 some of the available hterature are given in Refs. 55 to 59. A recent paper by Forristall 

 gives a detailed discussion of the three-dimensional structure of storm-generated currents 

 (Ref. 60), where a theoretical model for three-dimensional wind-driven currents is also pro- 

 vided. The driving force comes from the boundary condition of wind shear at the surface. The 

 model solution comes from two major steps: first, a two-dimensional finite difference scheme 

 is used to solve for current transports and storm surge; second, the current profiles at selected 

 points are calculated from convolution integrals, which may be evaluated given a wind stress 

 and sea surface slope at those points. 



Some of the important conclusions follow. Wind-driven waves generally produce the 

 dominant forces on offshore structures; however, since the hydrodynamic force is propor- 

 tional to the square of the water particle velocity, a current of a few feet per second can 

 increase the total force by 50 percent over that caused by waves alone (Refs. 50 and 60). 



Solutions for a hurricane in the Gulf of Mexico were derived. An example solution was 

 conducted using given data from Hurricane Camile (August 1 969). Surface currents as higli as 

 15.5 knots (8 m/sec) were predicted; current speeds at other depths were as follows: 8.75 

 knots (4.5 m/sec) at 10 meters, 4.5 knots (2.3 m/sec) at 20 meters, 2.1 knots (1.1 m/sec) at 

 30 meters, and 1 knot (0.5 m/sec) at 50 meters. A current speed versus depth curve from 

 these data are given in Fig. E.2. These velocities were predicted for a station a few miles 

 east of the hurricane in 100 meters of water, based on a maximum wind velocity of approxi- 

 mately 1 22 knots (63 m/sec) and a maximum wave height of 22 meters (Ref. 6 1 ). The ef- 

 fect of sloping bathymetry was strong, producing primarily longshore currents, which for 

 deep water OFEF's will not be a consideration; thus, open ocean surface currents may be 

 somewhat less. The model demonstrates that a tight circular wind can spin the water col- 

 umn to velocities much higher than those generated by a linear wind, and that the outward 

 flow at the surface is balanced by an inward flow near the bottom. P. Black (personnel 

 communication) has estimated that there is an approximate 10:1 wind velocity to surface 

 current speed ratio in severe cyclonic storms. The predicted current in Forristall's model 

 above (Ref. 60) approximates this estimated ratio (122-knot winds to 15.5-knot surface 

 current). 



Because of the lack of measured data, models such as the one described above wiU 

 have to be used to estimate the current field in the event of a storm in the vicinity of an 

 OFEF. A program to obtain storm-generated current data has been started by the Shell 

 Development Company, Ref. 50, and as data are reported they should be incorporated into 

 OFEF engineering design. Table E. 1 gives a list of personnel working in the wind-generated 

 ocean current field for future contact as needed. 



259 



