The windspeed is 90 miles per hour (132 feet or 40.2 meters per second) 

 and the initial wave height at the seaward edge of the area of 6 feet 

 (1.83 meters); the wave period is 4.5 seconds. 



FIND: The decayed wave height at the end of the fetch. 



SOLUTION : From the long dashline in Figure 1, for the windspeed of 90 

 miles per hour and the water depth of 10 feet 



4= ^L±2L^= 0.0185 

 U2 (132)^ 



giving (at the intersection of the above line with the long dashline) 

 ■4 = 0.00 75 



so that the maximum significant wave height 



. 0-0075 U^ ^ 0.0075 (132)^ ^ ^^^ ^^^^ 

 ^^ g 32.2 



From equation (15), 



H„ = 0.78d = 0.78 (10) =7.8 feet 



and from equation (16), the fractional reduction is 



H 



= 7.8 - 6 = 0.486 



tV, - a^rr, 7.8 - 4.1 

 From equation (17), the equivalent initial wave height 



H^g = R^^ Hg;^ = 0.486 X 4.1 = 1.99 feet ; 



from Figure 1, for 



^= 32-2 (1.99) , 0003^8 

 U^ (132)2 



and 



4 = 0.0185 

 U^ 



the fetch is given by 



4= 1.4 

 U^ 



39 



