the example problem below. Figure 4-35 can be used to determine the 

 longshore component of wave energy flux when deepwater wave height, H^, 

 period, T, and deepwater wave angle, a^, are known. 



************** EXAIIPLE PROBLEM 



*************** 



GIVEN : A wave in deep water has a height, H^ = 5 feet and a period, 

 T = 7 seconds. While in deep water, the wave crest makes an angle. 



a^ = 25 with the shoreline. 



FIND: 



(a) The longshore component of wave energy flux. 



(b) The angle the wave makes with the shoreline when it breaks [assum- 

 ing refraction is by straight, parallel bottom contours). 



SOLUTION: Calculate, 



Ho 



5.0 



= 0.0032 



gT^ 32.2 (7.0) 



Enter Figure 4-35 with a^, = 25° and H^/gT^ = 0.0032, read P^/pg^H^ T = 

 1.5 X 10"^. This corresponds with a breaker angle a^, = 9.5° which is 

 obtained by interpolation between the dashed curves of constant aj,. 

 Tlierefore, 



Pj = (0.0015)pg^H2T , 



Pj = (0.0015) (2) (32.2)2 (5)2 (7 q) , 



Pj = 544.3 , say 540 ft.-lb./ft.-sec. , 

 and 



afc = 9.5° . 

 ************************************** 



Equations 4-25 and 4-28 are valid only if there is a single wave 

 train with one period and one height. However, most ocean wave condi- 

 tions are characterized by a variety of heights with a distribution usu- 

 ally described by a Rayleigh distribution. (See Section 3.22.) For a 

 Rayleigh distribution, the correct height to use in Equation 4-28 or in 

 the formulas shown in Table 4-8 is the root-mean-square amplitude. How- 

 ever, most wave data are available as significant heights, and coastal 

 engineers are used to dealing with significant heights. 



Significant height is implied in all equations for P^g. The value 

 of Pj^g computed using significant height is approximately twice the 



4-95 



