APPENDIX E 

 DISCUSSION OF STORM-GENERATED CURRENTS 



Severe storms and hurricanes can generate intense temporary current fields in the 

 vicinity of the storm. The potential magnitude of these currents is of concern to OFEF for 

 engineering design and overall survivabihty of the farm structure and kelp plants. Actual 

 measurements of currents during severe storms are rare, principally due to the difficulty of 

 making them under extremely adverse conditions. Nearly all literature references on storm- 

 driven currents deal with theoretical computation of currents associated with storm surge 

 (Ref. 50). Until very recently, these computational methods employed highly simphfied 

 equations that could at best only produce rough estimates of current conditions. 



The response of the ocean surface-to-wind stresses is realized in several ways. The 

 most apparent responses, and the most important in terms of OFEF structural dynamics, are 

 surface waves and horizontal water transport. The total flow field to which a structure is 

 subjected during storm-induced periods consists of wave-associated particle velocities plus 

 the more slowly varying current component. Two papers which consider the effect of cur- 

 rents and waves on moored systems, and are applicable to Phase 1 , 2, and possibly early 

 Phase 3 OFEF's are Refs. 51 and 52. 



The only actual cuiTent measurement ever made during a severe cyclonic storm of 

 hurricane intensity was taken from NCAA Buoy EB-10 during Hurricane Eloise, September 

 1975 (Ref. 53). This hurricane reached the coast of Florida with 1 10-knot (57-m/sec) 

 winds at 1200, 23 September 1975. The wind magnitude, current speed, and wave height 

 chronology are given in Figure E.l. The eye of the hurricane passed within 10 miles (16 

 kilometers) of buoy EB-10 at 0200, 23 September, approximately 150 miles (240 kilome- 

 ters) south of Mississippi (Position (a) on Fig. E.l). The maximum measured wind speed by 

 aircraft was 95 knots (49 m/sec), the wind speed at the buoy was 68 knots (35 m/sec) and 

 occurred at 0100, 23 September, just prior to the passage of the eye. Wave response paral- 

 lels wind speed precisely showing a maximum height of 8.8 meters during the same hour as 

 the maximum wind speed. Currents at 50 meters, however, showed a delay of nearly 24 

 hours between the build up of the wind and the increase in current speed. Average current 

 speed prior to the storm passage was 0.6 knot (0.3 m/sec) increasing to 1 .8 knots (0.9 m/sec) 

 over a 1 2-hour period (about the same length of time as the increase in wind speed to maxi- 

 mum magnitude). Shortly after the storm, oscillations (26- to 27-hour periods) in current 

 speed with decreasing magnitude and direction were observed; these were detectable for two 

 weeks after passage of the hurricane. Although the data for Figure E. 1 were taken in the 

 Gulf of Mexico, the general relationships probably are similar for cyclonic storms in the 

 eastern Pacific. Surface currents were not measured from EB-10 and would be extremely 

 difficult to measure in a storm due to wave and surface chop action. 



Surface currents can be measured indirectly by measuring ship drift. One such 

 measurement was made shortly after the passage of a typhoon in the northwest Pacific. A 

 Japanese research vessel showed an anticyclonic drift pattern, 48 hours after the passage of a 



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