b. Wave Conditions on Bay Side of Island . 



(1) Use conditions with 100-year recurrence interval: 

 Hg = 3.59 m 

 Tg = 7.78 s 



c. Wave Conditions on Ocean Side of Island . From hindcast statistics 

 (wave height exceeded 0.1 percent of the time in shallow water), use 



H„ = 6.0 m 



o 



Note that the reciprocal of an exceedance probability associated with 

 a particular wave according to the present hindcast statistics is not the 

 return period of this wave. For structural design purposes, a statistical 

 analysis of extreme wave events is recommended. 



2. Revetment Design: Ocean Side of Island . 



The ocean side of the island will be protected by a revetment using 

 concrete armor units. 



a. Type of Wave Action . The depth at the site required to initiate 

 breaking to the 6.0-meter design wave is as follows for a slope in front of 

 the structure where m = zero (see Ch. 7, Sec. 1): 



H, = 0.78 d, 

 b b 



or 



. _ "& _ 6.0 ^ ^ 

 ^b - 0778 "0778 = ^-^ " 



where H^ is the breaker height and dj, is the water depth at the 

 breaking wave. 



Since the depth at the structure (d « 12.0 m) is greater than the 

 computed breaking depth (7.7 m) , the structure will be subjected to non- 

 breaking waves . 



b. Selection Between Alternative Designs . The choice of one cross 

 section and/or armor unit type over another is primarily an economic design 

 requiring evaluation of the costs of various alternatives. A comparison of 

 several alternatives follows: 



Type of Armor Unit: Tribars vs Tetrapods 



Structure Slope: 1:1.5, 1:2, 1:2.5, and 1:3 



Concrete Unit Weight: 23.56 kN/m^ , 25.13 kN/m^ , 26.70 kN/m^ 



The use of concrete armor units will depend on the availability of 

 suitable quarrystone and on the economics of using concrete as opposed to 

 stone. 



8-47 



