Pierson (1951), Longuet-Higgins (1957), and Kinsman (1965), hâve suggested a solution to the ocean-wave refraction problem. The sea surface
waves in deep water become a number of component monochromatic waves, each
with a distinct frequency and direction of propagation. The energy spectrum for each component may then be found and the conventional refraction
analysis techniques applied. Near the shore, the wave energy propagated
in a particular direction is approximated as the linear sum of the spectra
of wave components of ail frequencies refracted in the given direction from
ail of the deepwater directional components.
The work required for this analysis, even for a small number of individual components, is laborious and time consuming. More recent research
by Borgman (1969) and Fan and Borgman (1970), has used the idea of directional spectra which may provide a technique for solving complex refraction
problems more rapidly.
2.4 WAVE DIFFRACTION
2.41 INTRODUCTION
Diffraction of water waves is a phenomenon in which energy is transferred laterally along a wave crest. It is most noticeable where an otherwise regular train of waves is interrupted by a barrier such as a breakwater
or an islet. If the latéral transfer of wave energy along a wave crest and
across orthogonals did not occur, straight, long-crested waves passing the
tip of a structure would leave a région of perfect calm in the lee of the
barrier, while beyond the edge of the structure the waves would pass unchanged in form and height. The line separating two régions would be a
discontinuity. A portion of the area in front of the barrier would, however, be disturbed by both the incident waves and by those waves reflected
by the barrier. The three régions are shown in Figure 2-26a for the hypothetical case if diffraction did not occur, and in Figure 2-26b for the
actual phenomenon as observed. The direction of the latéral energy transfer is also shown in Figure 2-26a. Energy flow across the discontinuity
is from Région II into Région I. In Région III, the superposition of
incident and reflected waves results in the appearance- of short-crested
waves if the incident waves approach the breakwater obliquely. A partial
standing wave will occur in Région III if the waves approach perpendicular
to the breakwater.
This process is also similar to that for other types of waves, such
as light or Sound waves.
Calculation of diffraction effects is important for several reasons.
Wave height distribution in a harbor or sheltered bay is determined to
some degree by the diffraction characteristics of both the natural and
manmade structures affording protection from incident waves. Therefore,
a knowledge of the diffraction process is essential in planning such
facilities. Proper design and location of harbor entrances to reduce
such problems as silting and harbor résonance also require a knowledge
of the effects of wave diffraction. The prédiction of wave heights near
2-79
waves in deep water become a number of component monochromatic waves, each
with a distinct frequency and direction of propagation. The energy spectrum for each component may then be found and the conventional refraction
analysis techniques applied. Near the shore, the wave energy propagated
in a particular direction is approximated as the linear sum of the spectra
of wave components of ail frequencies refracted in the given direction from
ail of the deepwater directional components.
The work required for this analysis, even for a small number of individual components, is laborious and time consuming. More recent research
by Borgman (1969) and Fan and Borgman (1970), has used the idea of directional spectra which may provide a technique for solving complex refraction
problems more rapidly.
2.4 WAVE DIFFRACTION
2.41 INTRODUCTION
Diffraction of water waves is a phenomenon in which energy is transferred laterally along a wave crest. It is most noticeable where an otherwise regular train of waves is interrupted by a barrier such as a breakwater
or an islet. If the latéral transfer of wave energy along a wave crest and
across orthogonals did not occur, straight, long-crested waves passing the
tip of a structure would leave a région of perfect calm in the lee of the
barrier, while beyond the edge of the structure the waves would pass unchanged in form and height. The line separating two régions would be a
discontinuity. A portion of the area in front of the barrier would, however, be disturbed by both the incident waves and by those waves reflected
by the barrier. The three régions are shown in Figure 2-26a for the hypothetical case if diffraction did not occur, and in Figure 2-26b for the
actual phenomenon as observed. The direction of the latéral energy transfer is also shown in Figure 2-26a. Energy flow across the discontinuity
is from Région II into Région I. In Région III, the superposition of
incident and reflected waves results in the appearance- of short-crested
waves if the incident waves approach the breakwater obliquely. A partial
standing wave will occur in Région III if the waves approach perpendicular
to the breakwater.
This process is also similar to that for other types of waves, such
as light or Sound waves.
Calculation of diffraction effects is important for several reasons.
Wave height distribution in a harbor or sheltered bay is determined to
some degree by the diffraction characteristics of both the natural and
manmade structures affording protection from incident waves. Therefore,
a knowledge of the diffraction process is essential in planning such
facilities. Proper design and location of harbor entrances to reduce
such problems as silting and harbor résonance also require a knowledge
of the effects of wave diffraction. The prédiction of wave heights near
2-79