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WAVES AND WAVE-INDUCED HYDRODYNAMICS
models can be used across the surf zone. Some examples are Scha¨ffer, Madsen, and
Deigaard (1993) and Kennedy et al. (2000).
If the initial wave field is expanded in terms of slowly varying (in x) Fourier
modes, Boussinesq equations yield a set of coupled evolution equations that predict
the amplitude and phase of the Fourier modes with distance (Freilich andGuza 1984;
Liu, Yoon, and Kirby 1985; and Kaihatu and Kirby 1998). Field applications of the
spectral Boussinesq theory show that the model predictions agree very well with
normally incident ocean waves (Freilich and Guza 1984). Elgar and Guza (1985)
have shown that the model is also able to predict the skewness of the shoaling wave
field, which is important for sediment transport considerations.
The KdV equation (from Korteweg and deVries 1895) results from the Boussi-
nesq theory bymaking the assumption that thewaves can travel in one direction only.
A large body of work exists on the mathematics of this equation and its derivatives
such as the Kadomtsev and Petviashvili (1970) or K–P, equation, for KdV waves
propagating at an angle to the horizontal coordinate system.
In the surf zone and on the beach face, the simpler nonlinear shallow water
equations (also fromAiry) canprovide goodestimates of thewaveformandvelocities
because these equations lead to the formationof bores,which characterize thebroken
waves:
∂u
∂t
+ u ∂u
∂x
= −g ∂η
∂x
(5.13)
∂η
∂t
+ ∂(h + η)u
∂x
= 0 (5.14)
Hibberd and Peregrine (1979) and Packwood (1980) were the early developers of
this approach, andKobayashi and colleagues have produced several workingmodels
(Kobayashi, De Silva, and Watson 1989; Kobayashi and Wurjanto 1992).
5.2.2 WAVE REFRACTION AND SHOALING
As waves propagate toward shore, the wave length decreases as the depth decreases,
which is a consequence of the dispersion relationship (Eq. (5.3)). The wave period
is fixed; the wavelength and hence the wave speed decrease as the wave encounters
shallower water. For a long crested wave traveling over irregular bottom depths, the
change in wave speed along the wave crest implies that the wave changes direction
locally, or it refracts, much in the same way that light refracts as it passes through
media with different indices of refraction.∗ The result is that the wave direction turns
toward regions of shallowwater and away from regions of deepwater. This can create
regions of wave focusing on headlands and shoals.
The simplest representation of wave refraction is the refraction of waves propa-
gating obliquely over straight and parallel offshore bathymetry. In this case, Snell’s
law, developed for optics, is valid. This law relates the wave direction, measured by
an angle to the x-axis (drawn normal to the bottom contour), and the wave speed C
∗ The classic physical example is a pencil standing in a water glass. When viewed from the side, the part
of the pencil above water appears to be oriented in a different direction than the part below water.
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5.2 WATER WAVE MECHANICS 95
in one water depth to that in deep water:
sin θ
C
= sin θ0
C0
= constant, (5.15)
where the subscript 0 denotes deep water.
Wave refraction diagrams for realistic bathymetry provide a picture of howwaves
propagate from theoffshore to the shoreline of interest. Thesediagrams canbedrawn
by hand, if it is assumed that a depth contour is locally straight and that Snell’s law
can be applied there. Typically at the offshore end of a bathymetric chart, wave rays
of a given direction are drawn (where the ray is a vector locally parallel to the wave
direction; following a ray is the same as following a given section of wave crest).
Then each ray is calculated, contour by contour, to the shore line, with each depth
change causing a change inwave direction according to Snell’s law.Nowmost of these
calculations are done with more elaborate computer models or more sophisticated
numerical wave models such as a mild-slope, parabolic, or Boussinesq wave model.
Another effect of the change in wavelength in shallow water is that the wave
height increases. This is a consequence of a conservation of energy argument and the
decrease in group velocity (Eq. (5.8)) in shallow water in concert with the decrease
in C (note that n, however, goes from one-half in deep water to unity in shallow
water but that this increase is dominated by the decrease in C). This increase in wave
height is referred to as shoaling.
A convenient formula that expresses both the effects of wave shoaling and re-
fraction is
H = H0KsKr, (5.16)
where H0 is the deep water wave height, Ks is the shoaling coefficient,
Ks =
√
Cg0
Cg
,
and Kr is the refraction coefficient, which for straight and parallel shoreline contours
can be expressed in terms of the wave angles as follows:
Kr =
√
cos θ0
cos θ
Given the deepwater wave height H0, the group velocity Cg0 , and the wave angle θ0,
the wave height at another depth can be calculated (when it is used in tandem with
Snell’s law above).
Wave diffraction occurs when abrupt changes in wave height occur such as when
waves encounter a surface-piercing object like an offshore breakwater. Behind the
structure, no waves exist and, by analogy to light, a shadow exists in the wave field.
The crest-wise changes inwave height then lead to changes inwave direction, causing
the waves to turn into the shadow zone. The process is illustrated in Figure 5.2, which
shows the diffraction of waves from the tip of a breakwater. Note that the wave
field looks as if there is a point source of waves at the end of the structure. In fact,
diffraction can be explained by a superposition of point wave sources along the crest
(Huygen’s principle).
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96 WAVES AND WAVE-INDUCED HYDRODYNAMICS
Figure 5.2 Diffraction of waves at a breakwater (from Dean and
Dalrymple 1991).
5.2.3 WAVE PROPAGATION MODELS
Historically, wave models used to predict the wave height and direction over large
areas were developed for a wave train with a single frequency, which is referred
to as a monochromatic wave train in analogy to light. Monochromatic models for
wave propagation can be classified by the phenomena that are included in themodel.
Refraction models can be ray-tracing models (e.g., Noda 1974), or grid models (e.g.,
Dalrymple 1988).Refraction–diffractionmodels aremore elaborate, involving either
finite element methods (Berkhoff 1972) or mathematical simplifications (such as in
parabolic models, e.g., REF/DIF by Kirby and Dalrymple 1983).
Spectral models entail bringing the full directional and spectral description of the
waves from offshore to onshore. These models have not evolved as far as monochro-
matic models and are the subject of intense research. Examples of such work are
Brink-Kjaer (1984); Booij, Holthuijsen, and Herbers (1985); Booij and Holthuijsen
(1987); and Mathiesen (1984).
Recent models often include the interactions of wave fields with currents and
bathymetry, the input of wave energy by the wind, and wave breaking. For example,
Holthuijsen, Booij, and Ris (1993) introduced the SWAN model, which predicts
directional spectra, significant wave height, mean period, average wave direction,
radiation stresses, and bottom motions over the model domain. The model includes
nonlinear wave interactions, current blocking, refraction and shoaling, and white
capping and depth-induced breaking.
5.2.4 WAVE BREAKING
In deep water, waves break because of excessive energy input, mostly from the
wind. The limiting wave height is taken as H0/L0 ≈ 0.17, where L0 is the deep water
wavelength.
In shallow water, waves continue to shoal until they become so large that they
become unstable and break. Empirically, Battjes (1974) has shown that the breaking
wave characteristics can be correlated to the surf similarity parameter ζ , which is
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