Koz 
SoG = (Af) ) Ej (7a) 
j=Ki¢t1 
Significant height is estimated from the total energy assigned to each peak by 
an equation similar to equation (4). 
3. Example Problem. 
This problem illustrates a method for estimating peak frequencies, periods, 
and significant heights for a spectrum with two major peaks. 
GIVEN: Wave spectrum from Huntington Beach, California, for which the signifi- 
cant height is Hy = 5.7 feet (175 centimeters) (Fig. 8 and Table). Energy 
density is expressed as a percent of the sum of energy densities for all f; 
listed in Table. H, = 5.7 feet is based on 100 percent of the energy in the 
spectrum. 
FIND: Estimate a separate significant wave height and peak period for each 
wave train indicated by the spectrum. 
SOLUTION: To identify major spectral peaks, the difference in energy density 
between peak 2 and the valley between peaks 1 and 2 is estimated along the 
vertical axis in Figure 8 or from Table (about 4.5 percent). Since this is 
greater than 3 percent, peaks 1 and 2 are accepted as major peaks. Several 
other peaks appear at frequencies higher than 0.15 hertz, but there is no 
other combination of peak and valley for which the difference in energy den- 
sity exceeds 3 percent. Therefore, peaks 1 and 2 are the only major peaks. 
Frequencies for peaks 1 and 2 are estimated along horizontal axis or from 
Table: 
fp = fg = 0.081 Hz 
fp2 = £13 = 0.135 Hz 
Since fp2 is not integral multiple of fp, assume that peaks 1 and 2 repre- 
sent independent wave trains. The reciprocal of each peak frequency gives 
peak period. 
1 
Tn, = ~—=12.3 5s 
Pp fp 
1 
T = S 
p2 fo 
Compute total energy density, E, in the full spectrum by combining equations 
(3a) and (4) and rearranging to get 
Se yee ey a 
te J Na Af 
52 
_ (Sof 23% i 
saan Gel 0.01074 Hz 
192 ft2/Hz 
17 
