fullu arisen sea. The assumption of a universal form for the fully arisen
sea pennits the computation of other wave characteristics such as total
wave energy, significant wave height, and period of maximum energy. The
equilibrium State between wind and waves rarely occurs in the océan, and
may never occur for higher wind speeds.
A more general model may be constructed by assuming that the sea is
calm when the wind begins to blow. Intégration of the équations governing
wave growth then permits the considération of changes in the shape of the
spectrum with increasing fetch and duration. If enough wave and wind
records are available, empirical data may be analyzed to provide similar
information. Pierson, Neumann, and James (1955) introduced this type of
wave prédiction scheme based almost entirely on empirical data. Inoue
(1966, 1967) repeated this exercise in a manner more consistent with the
Miles-Phillips theory using a differential équation for wave growth. Inoue
was a member of Pierson’s group when this work was carried out, and his
prédiction scheme may be regarded as a replacement for the earlier
Pierson-Neuman-James (PNJ) wave prédiction model. The topic has been
extended by Silvester and Vongvisessomjai (1971) and others.
These simplified wave prédiction schemes are based on the implicit
assumption that the waves being considered are due entirely to a wind
blowing at constant speed and direction for an overwater distance called
the fetch and for a time period called the duration.
In principle it would be possible to consider some effects of variable
wind velocity oy tracing each wave train. Once waves leave a generating
area and become swell, the wave energy is then propagated according to the
group velocity. The total energy at a point, and the square of the significant wave height could be obtained by adding contributions from individual
wave trains. Without a computer, this procedure is too laborious, and
theoretically inaccurate.
A more practical procedure is to relax the restrictions implied by
dérivation of these schemes. Thus wind direction may be considered constant if it varies from the mean by less than some finite value, say 30°.
Wind speed may be considered constant if it varies from the mean by less
than t 5 knots (2.5 meters/second) or % barb on the weather map. This
assumption is not much greater than the uncertainty inhérent in wind
reports from ships. In this procedure, average values are used and are
assumed constant over the fetch area, and for a particular duration.
Fhe theoretical spectra for the partial ly arisen sea can be used
to develop formulas for such wave parameters as total energy, significant
wave height and period of maximum energy density.
Similar formulas can also be developed empirically from wind and wave
observations. A quasi-empirical - quasi-theoretical procedure was used by
Sverdrup and Munk (1947) to construct the first widely used wave prédiction System. The Sverdrup-Munk prédiction curves were revised by
Bretschneider (1952a, 1958) with additional empirical data. Thus, this
pre iction System is often called the Sverdrup-Munk-Bretschneider (SMB)
t 1S ^6 most convenient wave prédiction System to use when a
limited amount of data and time are available.
3-34
sea pennits the computation of other wave characteristics such as total
wave energy, significant wave height, and period of maximum energy. The
equilibrium State between wind and waves rarely occurs in the océan, and
may never occur for higher wind speeds.
A more general model may be constructed by assuming that the sea is
calm when the wind begins to blow. Intégration of the équations governing
wave growth then permits the considération of changes in the shape of the
spectrum with increasing fetch and duration. If enough wave and wind
records are available, empirical data may be analyzed to provide similar
information. Pierson, Neumann, and James (1955) introduced this type of
wave prédiction scheme based almost entirely on empirical data. Inoue
(1966, 1967) repeated this exercise in a manner more consistent with the
Miles-Phillips theory using a differential équation for wave growth. Inoue
was a member of Pierson’s group when this work was carried out, and his
prédiction scheme may be regarded as a replacement for the earlier
Pierson-Neuman-James (PNJ) wave prédiction model. The topic has been
extended by Silvester and Vongvisessomjai (1971) and others.
These simplified wave prédiction schemes are based on the implicit
assumption that the waves being considered are due entirely to a wind
blowing at constant speed and direction for an overwater distance called
the fetch and for a time period called the duration.
In principle it would be possible to consider some effects of variable
wind velocity oy tracing each wave train. Once waves leave a generating
area and become swell, the wave energy is then propagated according to the
group velocity. The total energy at a point, and the square of the significant wave height could be obtained by adding contributions from individual
wave trains. Without a computer, this procedure is too laborious, and
theoretically inaccurate.
A more practical procedure is to relax the restrictions implied by
dérivation of these schemes. Thus wind direction may be considered constant if it varies from the mean by less than some finite value, say 30°.
Wind speed may be considered constant if it varies from the mean by less
than t 5 knots (2.5 meters/second) or % barb on the weather map. This
assumption is not much greater than the uncertainty inhérent in wind
reports from ships. In this procedure, average values are used and are
assumed constant over the fetch area, and for a particular duration.
Fhe theoretical spectra for the partial ly arisen sea can be used
to develop formulas for such wave parameters as total energy, significant
wave height and period of maximum energy density.
Similar formulas can also be developed empirically from wind and wave
observations. A quasi-empirical - quasi-theoretical procedure was used by
Sverdrup and Munk (1947) to construct the first widely used wave prédiction System. The Sverdrup-Munk prédiction curves were revised by
Bretschneider (1952a, 1958) with additional empirical data. Thus, this
pre iction System is often called the Sverdrup-Munk-Bretschneider (SMB)
t 1S ^6 most convenient wave prédiction System to use when a
limited amount of data and time are available.
3-34