do Instituto Hidrográfico

72 ANAIS DO INSTITUTO HIDROGRÁFICO N.º 15
their variation), as they are delivered instantaneously by
the corresponding commands. For the case of the point,
it is made a comparison with the maximum value of the
respective variable in the field. It is also available the wave
data evolution along the isolines. The direct connection
with the pre-processing stage permits a further extension
from a two-point line to a path generated by many
lines as the one in figure 8 and also to define the data
distribution along any isoline without imposing these as
an ‘a priori’ output request to the model.
4.3 Spectral analysis
The harmonic or spectral analysis is a technique
that unravels the waves generated by different storms
and describes the complete distribution of wave energies
and periods. The 1D spectrum presents variance
density per unit frequency interval for each frequency.
The variance units are m 2 per frequency interval
Ds, which can be converted into true energy through
multiplication by rg/8, where r is the water density, and
g is the acceleration of gravity. The total variance for
each point can be obtained by summing the variance
densities. The most complete description of waves is
provided by a directional wave spectrum, which includes
the direction of wave approach as well as the wave variance
at a specific frequency. Such a 2D spectrum is
given in the three-axis plot in figure 17. As it can be seen
also in the command panel of figure 17, ‘TOTAL WAVE’
provides both the 1D and 2D spectra and can be followed
the evolution of the spectral shape towards to the shore.
Moreover could be performed comparative analyzes
between the spectral shapes in various places.
4.4 The surf zone conditions
The processes and the phenomena associated with
the depth-induced wave breaking are particularly
important in the coastal engineering applications. This
because a great amount of the wave energy is finally
dissipated in the nearshore regions determining the
geometry and composition of the beaches and influencing
the coastal structures and works. A rapid and
adequate evaluation of the wave conditions in these
areas being therefore of essential importance. For estimating
the process of wave breaking SWAN, uses the
spectral version of [Eldeberky and Battjes (1996)],
expanded to include directions. In terms of energy dissipation
the breaking process is associated with a significant
increase. Once the breaking line is identified can be
evaluated the wave data along its points. In figure 18 are
presented some characteristics of the breaking as the
variation in relationship with the shore of the distance
where is initiated the wave breaking and the distributions
of the significant wave height and the depth on the
breaking line. The commands available for this module
could be seen also in figure 18 and they are: identifying
the location of the breaking line, determining the variation
of the breaking ratio along it, evaluation of the
breaking type, assessing the wave direction just before
breaking, computation of the number of fronts in the
surf and finally displaying the dissipation variation
towards to the shore. The breaker-type prediction used
the deep-water form of the Iribarren number (j ‘),
which combine the beach slope with the wave steepness.
In reference to this parameter the breaking-type classification
is the following [Komar (1998)]:
Point data Line data
Fig. 15 – Global data evaluation Fig. 16 – Local data assessment