3.5 Wind-Generated Waves
H,
-+
Time
95
Fig. 3.10: A typical wave record of sea surface variations at a given location
points have been marked in Fig. 3.10 with closed circles, and they separate
wave periods of successive waves. The corresponding wave periods, Ti , and
wave heights, Hi, are noted. It should be noted that the definition of an
individual wave height entirely depends on the choice of the trough occurring
before or after the crest. In Fig. 3.10, a wave height is defined as zero-upcrossing wave height and the wave crest occurs before the wave trough. Other
possible definitions of the wave height are discussed by Massel (1996a).
Because of the large variation of waves in a wind-induced wave train, there
is a need to define some statistical measures which characterize the wave train.
An obvious candidate for such characteristics is the mean wave height, fI.
It is simply an arithmetic mean of all wave heights in a given record, i. e.:
1 i=N
fI = - L Hi·
N i=1
(3.14)
Usually, a record of about 20 minutes duration provides good representation
of the sea state; such records generally contain more than 100 waves (N > 100).
In addition to the mean wave height, H, the significant wave height, H s , is
frequently used in oceanographic and engineering practice. The concept of the
significant wave height was introduced by Sverdrup and Munk (1947). They
defined significant wave height, H s , as the average of the highest one-third of
wave heights. This wave height is close to the mean wave height estimated by
H,
-+
Time
95
Fig. 3.10: A typical wave record of sea surface variations at a given location
points have been marked in Fig. 3.10 with closed circles, and they separate
wave periods of successive waves. The corresponding wave periods, Ti , and
wave heights, Hi, are noted. It should be noted that the definition of an
individual wave height entirely depends on the choice of the trough occurring
before or after the crest. In Fig. 3.10, a wave height is defined as zero-upcrossing wave height and the wave crest occurs before the wave trough. Other
possible definitions of the wave height are discussed by Massel (1996a).
Because of the large variation of waves in a wind-induced wave train, there
is a need to define some statistical measures which characterize the wave train.
An obvious candidate for such characteristics is the mean wave height, fI.
It is simply an arithmetic mean of all wave heights in a given record, i. e.:
1 i=N
fI = - L Hi·
N i=1
(3.14)
Usually, a record of about 20 minutes duration provides good representation
of the sea state; such records generally contain more than 100 waves (N > 100).
In addition to the mean wave height, H, the significant wave height, H s , is
frequently used in oceanographic and engineering practice. The concept of the
significant wave height was introduced by Sverdrup and Munk (1947). They
defined significant wave height, H s , as the average of the highest one-third of
wave heights. This wave height is close to the mean wave height estimated by