Environmental Engineering Reference
In-Depth Information
Fig. 2.25 Modi ed Pierson-Moskowitz spectrum
Comparing the coefficients results in
1 10 3
g 2
74 g 4
U 4
a ¼ 8
:
and
b ¼ 0
:
The following two-parameter (biased) spectrum is mainly used in practice:
S z ðÞ¼ 124 H s
T 4
0
5
exp 496 T 4
0
4
v
v
This format is known as the modified Pierson-Moskowitz spectrum and is often used in its
dimensionless form (Figure 2.25):
"
#
5
4
S z ðÞ
H s
1
8 p
v T 0
2 p
1
p
v T 0
2 p
T 0 ¼
2
exp
h
i
Þ 5
Þ 4
¼ 124 v T 0
ð
exp 496 v T 0
ð
It can be used to describe both fully developed and developing sea states, that is old and
young sea states. One of the pairs of values [H s ;T 0 ]or[H s ; T] is needed for the
unambiguous short-term description of a developing long-crested sea state. A third
parameter, the direction angle
m H , is needed as well for short-crested sea states.
The designer is recommended to use the JONSWAP spectrum for sea areas with limited
fetch, or rather developing sea states (see Figure 2.26). This spectrum is based on
comprehensive measurements carried out off the German North Sea coast [23]. It is
suitable for shallower water and extreme sea state relationships and is mostly written in
the following form (see also [11]):
h
i
"
#
2
ð
Þ
4
exp vv p
5
4
v
v p
S z ðÞ¼a g 2
5
2
p
v
exp
g
2 s
v
(
s a
for
v v p
where s ¼
s b
for
v > v p
Search WWH ::




Custom Search