DESCRIPTIONS OF WAVE ENERGY SPECTRA
155
The source of the latter relation is the International Towing Tank Conférence
(ITTC) (Price and Bishop, 1974), and the resulting spectrum is known as the
ITTC spectra. Shown in Table 6.2 are the ITTC recommended values for
significant wave height and average wave period for several wind speeds.
Table 6.2 ITTC Recommended Data for Equations (6.21)-(6.23)
V
Wind Speed,
m/s
T, Average
Wave Period,
s
Ht, Significant
Wave Height,
m
5.14
2.7
__
10.3
5.3
3.1
15.4
8.0
5.1
20.6
10.7
8.1
25.7
13.4
11.0
Another form of the wave height-period spectra, developed by Ochi and
Hubble (1976) from the work of Bretschneider (1959), is
S„(w) =
(6.24)
'
4 ws
in which
is the frequency at the maximum of the spectrum, where the corresponding period is Tm = 27r/wtn. Listed in Table 6.3 are empirical équations
for u;m in terms of the significant wave height H„ and its probability of occurrence. Also listed in that table are values for u>m and Tm for the spécial case of
H, = 12.2 m, a case considered later in Example Problem 6.1.
It is emphasized that the spectra expressed by équations (6.21) through
(6.24) are amplitude half-spectra and that the square root of the area under each
spectral diagram must be multiplied by 4 in order to obtain the corresponding
significant wave height. That is, Hs = 4^/(area)2Table 6.3 Spectral Model Data (Bretschneider, 1959 and Ochi, 1978)
Equation for
wm. rad/s
For
= 12.2 m:
Probability
of Occurrence
«m, rad/s ^rn> 3
0.048 (8.75- In Ht)
0.30
20.9
0.0500
0.054 (8.44- In Hs)
0.32
19.6
0.0500
0.061 (8.07- In H,)
0.34
18.5
0.0875
0.069 (7.77—In H,)
0.36
17.3
0.1875
0.079 (7.63-In H,)
0.41
15.5
0.2500
0.099 (6.87-In H,)
0.43
14.5
0.1875
0.111 (6.67-In H,)
0.46
13.6
0.0875
0.119 (6.65— In
0.49
12.7
0.0500
0.134 (6.41-In J/,)
0.52
12.0
0.0500