FORECASTING
SWELL WAVES
LEARNING
OBJECTIVES:
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Explain
swell wave
generation and recognize the two fundamental modifications that sea
waves undergo as they leave the fetch
area.
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Define
the terms associated with swell
waves, and explain the five rules used to determine how much of
the swell will reach the forecast point.
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Prepare
an objective swell
wave forecast.
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In the preceding portion of this chapter, we
have discussed the principles of sea
waves and methods of forecasting them. With sea wave forecasting we are
considering the point that we are forecasting to be within the generating area,
with the wind still blowing. This, however, will not be the problem in the
majority of the forecasts that will be required. Normally
the forecast point will be outside the fetch
area; therefore, it will be necessary to determine what effect the distance traveled is going
to have on the waves. In this section we will discuss the basic principles of swell
waves as well as an objective method of determining what changes will take
place in the spectrum of waves as they traverse from the generating area to the
forecast point.
After a sea state has been generated in a
fetch, there are many different wave trains present with different periods, and most
of them are moving out of the fetch in slightly different directions. Because of
these different periods and slight differences in direction, the propagation of
swell waves follows two fundamental processes. These processes are dispersion
and angular spreading.
An accepted fact about wave travel is that the waves
with longer periods move faster than waves with shorter periods. The actual
formula for the speed of the wave train is C=1.515T where C is the speed of the
wave train and T is the wave period in the wave train.
All of the different wave trains (series of
waves all having the same period and direction of movement) in the fetch can be
compared to a group of long distance runners at a track and field meet. At first all of the
runners start out at the starting line at the same time. As they continue on,
however, the faster runners move ahead and the slower runners begin to fall
behind. Thus the field of runners begins to string out along the direction of
travel. The wave trains leaving a fetch do the same thing. The stringing out of
the various groups of waves is called dispersion.
In a swell forecast problem
it is necessary to determine what wave
trains have already passed the forecast point and which have not yet arrived.
After this has been determined, the wave trains that are left are the ones that
are at the forecast point at the time of observation.
As the wave trains leave the fetch, they may
leave at an angle to the main direction of the wind in the fetch. Thus, swell
waves may arrive at a forecast point though it may lie to one side of the
mainline of direction of the wind. This process of angular spreading is depicted
in figure 6-6.
The problem in swell forecasting is to determine how much of the swell will reach the
forecast point after the waves have spread out at angles. This is accomplished
by measuring the angles from the leeward edge of the fetch to the forecast point.
These angles must be measured as accurately as possible, figure 6-7, and are
determined by the following five rules:
1. Draw
the rectangular fetch.
Figure
6-6.-Angu1ar spreading.
Figure
6-7.-Measuremeuts of angles for angular spreading.
2.
Extend the top and bottom edge of the fetch outward parallel to the main
direction of the wind. This is shown as dashed lines in figure 6-7.
3. Draw
lines from the top and bottom edges of the fetch to the forecast point.
4. The
angles to the forecast point are designated Theta 3 (q3) and Theta 4 (q4). Theta
3 is measured from the top edge of the fetch and Theta 4 from the bottom edge.
5. Any
angle that lies above the dashed line is negative while any angle that lies
below the dashed line is positive.
After
the angles Theta 3 and 4 have been measured they are converted to percentages of
the swell that will reach the forecast point. This conversion is made by
entering sea and swell graph 7, figure 6-8, with the positive or negative angles
and reading the corresponding percentages directly. The percentages are then
subtracted ignoring the plus or minus to find the angular spreading.
A
number of terms used in dealing with forecasting sea waves will be used again in
this process; however, a number of new terms will be introduced. Table 6-3 lists
most of these terms with their associated symbol and definition.
As with
objective forecasting of sea waves there are a number of different methods for
forecasting swell waves. Some of the methods are too technical or time consuming
to be of practical use.
When
ship operations are conducted outside a fetch area it becomes necessary to
forecast swell conditions at that location.
Prior
to computing swell conditions the height and period of the significant waves
departing the fetch area must be determined. For more details refer to Sea
and Swell Forecasting, NAVEDTRA 40560.
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