equivalent waves in the record is obtained by dividing the duration of the 

 record by this significant period. The highest waves are then ranked in 

 order, with the highest wave ranked 1. The height of the wave ranked 0.135 

 times the total number of waves is taken as the significant wave height. The 

 derivation of this technique is based on the assumption that the Rayleigh dis- 

 tribution law is exact. Harris (1970) and Thompson (1977) showed that this 

 procedure agrees closely with values obtained by more rigorous procedures 

 which require the use of a computer. These procedures are described in 

 Chapter 3, Section 11,3 (Energy Spectra of Waves). 



The following problem illustrates the use of the theoretical wave height 

 distribution curves given in Figure 3-5. 



*************** EXAMPLE PROBLEM 1*************** 



GIVEN ; Based on an analysis of wave records at a coastal location, the 

 significant wave height Hg was estimated to be 3 meters (9.84 feet). 



FIND ; 



(a) Hj^Q (average of the highest 10 percent of all waves). 



(b) Hj^ (average of the highest 1 percent of all waves). 

 SOLUTION : H = Hg = 3 meters 



Using equation (3-10) 



or 



^'ms=T:^ = -dl6- 2.12 m (6.95 ft) 



(a) From Figure 3-5, curve b , it is seen that for P = 0.1 (10 percent) 



^ 10 



-~^ « 1.80; H^Q = 1.80 H^g= 1.80 (2.12) = 3.82 m (12.53 ft) 



(b) Similarly, for P = 0.01 (1 percent) 

 «1 



„ 2.36; H, = 2.36 H = 2.36 (2.12) = 5.0 m (16.41 ft) 

 rms ^ ^^^ 



Note that 



J^ = ll|2 ^^ H,„ = 1.27 H 

 H 3 10 s 

 s 



3-10 



