(a) For Hg = 32.1 feet and U = 100 knots, T^ = 11.8 seconds. 

 fhis corresponds to a fetch length of 39 nautical miles and a 

 minimum duration of 100-knot winds of 2.7 hours. 



(b) For Hg = 32.1 feet and a fetch of 1,000 nautical miles, 

 Tg = 16.8 seconds. This relates to U = 39 knots and a minimum 

 duration of 39-knot winds of 52 hours. 



Therefore, the wave period used to proportion the structure was: 



„ 11.8 + 16.8 ^. _ , 

 Tp = r = 14.3 seconds 



s 2 



In addition, safety factors were evaluated for the 11.8- and 16.8-second 

 conditions to determine the sensitivity of the structure's stability to 

 variations on wave period. 



3. Effect of Perforated Exterior Wall. 



The breakwater is made of concrete caissons of cellular construction 

 with a perforated front wall and wave chamber. 



The seaward perforated vertical wall reduces wave forces exerted on 

 the structure by dissipating a part of the wave energy within the wave 

 chamber and at the perforations. 



The perforated wall decreases reflection and thus lowers the crest 

 of the standing wave (clapotis) . The front wall perforations also reduce 

 scour at the toe of the caisson. 



Since the original concept of a perforated breakwater was developed 

 by Jarlan in 1965, other works have been published on the subject 

 (Coastal Engineering Research Center, 1966; Marks and Jarlan, 1968). 

 The most informative report used in this study of the effect of the wave 

 forces on a perforated breakwater was by Marks and Jarlan (1968) . 



Marks and Jarlan state that the energy dissipated at a vertical per- 

 forated structure is about 65 percent of the total energy of the incident 

 wave. This corresponds to the relationship. 



(Ho) 



(H^)' 



= (1 - 0.65) = 0.35 



where H is the height of the wave approaching the structure, and H^ is 

 the equivalent wave height at the structure. 



Because of the nature of this study and the wave properties being 

 considered, the perforated breakwater may not be as effective as Marks 



12 



