Environmental Engineering Reference
In-Depth Information
influences of time and fetch (x) can be taken into account using dimensionless notation
as follows:
u ¼
g
time
=
u
u
2
j ¼ g x
=
where
u wind speed at 10m above the surface of the water, averaged over 1 h (u
¼
V
1h
, see
Table 2.9)
The dimensionless peak frequency can be calculated as follows:
n ¼
v
p
2
p
u
g
¼
Max
0
:
3
3
=
7
:
0
:
16
;
2
:
84
j
;
16
:
8
u
From this it follows that the peak period (T
p
)is
1
n
u
g
T
p
¼
The following format can be used for the significant wave height (H
s
):
u
2
g
5
=
3
H
s
;
JONSWAP
;
wind
¼
0
:
0094
n
Three JONSWAP spectra for various wind speeds are plotted in Figure 2.26.
The constant parameters are the fetch (x
¼
600 km),
the time (
¼
12 h) and the
characteristic dimensions for the width of the spectrum (
s
a
¼
0.07 and
s
b
¼
0.09).
Table 2.11 lists the associated parameters.
The TMA spectrum is a modified form of the JONSWAP spectrum and depends on
the depth of water. It can be used for the wind-generated sea state in a finite depth
of water:
s
d
g
S
z;
TMA
ðÞ¼
S
z;
JONSWAP
ðÞF
k
vðÞ
and
v
d
¼ v
Table 2.11
JONSWAP spectra for three wind speeds
U
10 m
[m/s]
v
p
[s
1
]
p
[s
1
]
p
[s] m
0
[m
2
]
a
g
s
[m] H
s
[m]
25
0.012429
2.734
0.638
0.102
9.9
2.03
1.42
5.7
20
0.012020
2.533
0.725
0.115
8.7
1.14
1.07
4.3
15
0.011505
2.292
0.854
0.136
7.4
0.54
0.74
2.9
