EM 1110-2-1100 (Part II)
30 Apr 02
II-7-2. Wave Diffraction
a. Definition of diffraction.
(1) Consider a long-crested wave that has a variable height along its crest. As this wave propagates
forward, there will be a lateral transfer of wave energy along the crest (perpendicular to the direction of wave
propagation). The energy transfer will be from points of greater to lesser wave height. This process is known
as wave diffraction.
(2) Nearshore wave refraction will cause concentrations of wave energy at points where wave
orthogonals converge. Diffraction will lessen this refraction-induced energy concentration by causing wave
energy to transfer across the orthogonals away from the region of concentration. Consequently, wave
diffraction can have a small effect on the resulting heights of waves that approach harbor entrances.
(3) Diffraction has a particularly significant effect on wave conditions inside a harbor. When waves
propagate past the end of a breakwater, diffraction causes the wave crests to spread into the shadow zone in
the lee of the breakwater. The wave crest orientations and wave heights in the shadow zone are significantly
altered.
b. Diffraction analysis.
(1) Much of the material developed for wave diffraction analysis employs monochromatic waves.
Ideally, an analysis should employ the directional spectral conditions. But, for a preliminary design analysis,
one or a set of monochromatic wave diffraction analyses is often used to represent the more complex result
that occurs when a directional spectrum of waves diffracts at a harbor.
(2) In the material presented below, monochromatic results are presented first. Then, some of the
available results for diffraction of irregular waves are presented. These results are based on the superposition
of several monochromatic waves having a range of representative frequencies and directions. This type of
analysis requires significant effort, but it can be carried out where the situation so requires. Also, physical
model tests employing a directional wave spectrum can be used for a harbor diffraction analysis.
c. Diffraction at a harbor entrance. A major concern in the planning and design of coastal harbors is
the analysis of wave conditions (height and direction) that occur inside the harbor for selected incident design
waves. These waves may shoal and refract after they pass through the harbor entrance; but, the dominant
process affecting interior wave conditions is usually wave diffraction. Two generic types of conditions are
most commonly encountered: wave diffraction past the tip of a single long breakwater and wave diffraction
through a relatively small gap in a breakwater.
(1) Waves passing a single structure.
(a) Figure II-7-2 shows a long-crested monochromatic wave approaching a semi-infinite breakwater in
a region where the water depth is constant (i.e. no wave refraction or shoaling). A portion of the wave will
hit the breakwater where it will be partially dissipated and partially reflected. The portion of the wave that
passes the breakwater tip will diffract into the breakwater lee. The diffracted wave crests will essentially form
concentric circular arcs with the wave height decreasing along the crest of each wave. The region where
wave heights are affected by diffraction will extend out to the dashed line in Figure II-7-2.
Harbor Hydrodynamics
II-7-3
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