CONCEPT OF THE SIGNIFICANT WAVE
147
It is important that the mathematical model used to represent the sea surface (the time history of wave height) should conform as closely as possible to
the actual waves. In practice, there are two ways to accomplis!) this. First,
a purely deterministic method can be used in which the sea surface records
are fitted to a harmonie or Fourier sériés. This représentation is unaffected by
nonlinear wave effects. Second, a statistical method can be used in which the
sea surface records are fitted to a spectral fonction in the frequency domain.
It is emphasized that the second method is based on the concept of superposition and is adéquate only for linear seas. Varions efforts hâve been made to
include nonlinear phenomena in spectral descriptions but these methods are
not widely utilized. For purposes of structural design, however, linear models
usually provide adéquate descriptions of the sea surface.
For further discussions of offshore waves as random processes, the reader is
referred to the works of Longuet-Higgins (1963), Munk (1950), Kinsman (1965),
Pierson and Neumann (1966), and the overview of Young (1999).
6.2
CONCEPT OF THE SIGNIFICANT WAVE
The concept of the significant wave was first published by Sverdrup and Munk
(1947), and later discussed by Wiegel (1949), Bretschneider (1959), and Kinsman (1965). The significant wave height is a useful parameter for describing an
irregular sea surface and its wave spectra.
What is the significant wave height? The significant wave height is not an
identifiable wave form that propagates like a physical wave as in the classical
wave théories. The significant wave height, Hs, is defined as the arithmetic
average of the highest one-third of the waves in a wave record. Its associated
statistical parameter, the significant period Ta, is defined as the average period
of the highest one-third of the waves in this wave record. It is noted that the
symbols .Hj/3 and T^/^ are also commonly used to dénoté H, and Ta, respectively.
The significant wave height is an important parameter in statistical analysis
of wave mechanics for a number of reasons. The statistical distribution of wave
heights and most energy spectrum analyses are related to the significant height.
In fact, the major portion of the wave energy of the spectrum surrounds the
significant wave height. Thus, it has been the practice historically to report sea
conditions in the form of significant wave heights. Also, the effects that irregular
seas hâve on many types of fixed and floating objects and on shore processes
such as littoral transport, hâve been related to significant wave heights, with an
accuracy sufficient for these engineering applications.
In summary, the irregular sea can be described in an abbreviated format by
two parameters: the significant wave height and the significant period. However,
designers of offshore Systems sometimes need to know other statistical features
of the sea, such as the maximum wave height or the maximum water surface
élévation. This led to a number of studies that attempted to establish further
statistical corrélations for offshore waves. A brief summary of these corrélât ions
is now presented.
147
It is important that the mathematical model used to represent the sea surface (the time history of wave height) should conform as closely as possible to
the actual waves. In practice, there are two ways to accomplis!) this. First,
a purely deterministic method can be used in which the sea surface records
are fitted to a harmonie or Fourier sériés. This représentation is unaffected by
nonlinear wave effects. Second, a statistical method can be used in which the
sea surface records are fitted to a spectral fonction in the frequency domain.
It is emphasized that the second method is based on the concept of superposition and is adéquate only for linear seas. Varions efforts hâve been made to
include nonlinear phenomena in spectral descriptions but these methods are
not widely utilized. For purposes of structural design, however, linear models
usually provide adéquate descriptions of the sea surface.
For further discussions of offshore waves as random processes, the reader is
referred to the works of Longuet-Higgins (1963), Munk (1950), Kinsman (1965),
Pierson and Neumann (1966), and the overview of Young (1999).
6.2
CONCEPT OF THE SIGNIFICANT WAVE
The concept of the significant wave was first published by Sverdrup and Munk
(1947), and later discussed by Wiegel (1949), Bretschneider (1959), and Kinsman (1965). The significant wave height is a useful parameter for describing an
irregular sea surface and its wave spectra.
What is the significant wave height? The significant wave height is not an
identifiable wave form that propagates like a physical wave as in the classical
wave théories. The significant wave height, Hs, is defined as the arithmetic
average of the highest one-third of the waves in a wave record. Its associated
statistical parameter, the significant period Ta, is defined as the average period
of the highest one-third of the waves in this wave record. It is noted that the
symbols .Hj/3 and T^/^ are also commonly used to dénoté H, and Ta, respectively.
The significant wave height is an important parameter in statistical analysis
of wave mechanics for a number of reasons. The statistical distribution of wave
heights and most energy spectrum analyses are related to the significant height.
In fact, the major portion of the wave energy of the spectrum surrounds the
significant wave height. Thus, it has been the practice historically to report sea
conditions in the form of significant wave heights. Also, the effects that irregular
seas hâve on many types of fixed and floating objects and on shore processes
such as littoral transport, hâve been related to significant wave heights, with an
accuracy sufficient for these engineering applications.
In summary, the irregular sea can be described in an abbreviated format by
two parameters: the significant wave height and the significant period. However,
designers of offshore Systems sometimes need to know other statistical features
of the sea, such as the maximum wave height or the maximum water surface
élévation. This led to a number of studies that attempted to establish further
statistical corrélations for offshore waves. A brief summary of these corrélât ions
is now presented.