7. PRESENCE OF MULTI-DIRECTIONAL WAVE CONDITONS AND NEARSHORE WAVE
TRANSFORMATION
As noted previously, one of the interests in this work was on the relative contributions of northern and southern
swells to the wave climate along the Chilean coast. Comprehensive directional wave measurements for Chile are
not yet available, thus, the wave model provides an initial indication of the importance of swell to coastal
infrastructure design.
As a first step towards developing an understanding of swell impacts, a procedure for isolating the individual wave
systems from the wave model directional spectra was developed based on the work of Hanson and Phillips (2001).
Basically, the procedure breaks down each individual spectrum into subsets representing each of the sea and swell
wave components. The following steps were carried out in the process:
1.
Each peak in the computed spectra is isolated by means of spectral partitioning using a steepest ascent
algorithm ("queen's move" ascent).
2.
The peaks associated with wind seas are identified and combined based on a wave age criterion.
3.
Adjacent swell peaks were evaluated to see if they derived from the same wave system based on criteria
established regarding directional spreading, peak separation and minimum energy between peaks.
In this study, low-energy wave components were not considered; only those spectral partitions with an equivalent of
characteristic wave height of greater than 0.5 m were included. An analysis was then performed to determine the
frequency that waves arrive from two or more directions with an angular separation of 45 degrees or greater.
Figure 7 shows the percentage of time that multi-directional waves occur for locations along the Chilean coastline
going from north (left on x-axis) to south (right). The very high percentage of time that more than one wave system
influences the wave climate in Chile may be seen. There is also an increasing trend in multi-directionality from
north to south. Part of the sharp change in multi-directionality in the north end of the country may be associated
with the northern sheltering created by the western "bulge" of South America.
A more detailed examination of the various spectral peaks isolated in the data set showed that not all of the multi-
directionality was associated with north and south swell systems, particularly in the southern part of the coastline.
In the south, the local seas typically are generated by western winds, but swell waves can arrive from a more
southerly direction.
It is also interesting to examine the seasonal variation of the multi-directional wave conditions, as shown in Figure 8
for three selected locations. It may be noted that wave conditions are less complex in the Southern Hemisphere
winter months when Northern Pacific wave conditions are less severe. The seasonal effect is smaller in the south
than in the north.
Clearly, part of the multi-directionality may be attributed to issues of simulation of northern swell and a possible
over-estimation of the magnitude of these swells. Nevertheless, the complexity of the wave climate in Chile is
apparent, and this has many implications with respect to how nearshore wave transformation should be performed.
It is important that the multi-modal nature of the waves be considered in shallow water modeling procedures, and
that summary parameters, such as the Mean Wave Direction, are not meaningful in this context.
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