information refer to the publication Practical Methods
for Observing and Forecasting Ocean Waves (H.O.
Publication 603), which gives the complete range of
fu values and the corresponding periods for wind
speeds, starting from 10 kt, at 2-kt intervals, Notice that
the frequency decreases as the wind speed increases,
This suggests that the higher wind speeds produce
higher ocean waves. The table mentioned above can be
graphed for each wind speed, An example of such a
graph can also be found in H.O. publication 603.
It is difficult to work with actual energy values of
these sine waves; for this reason the square of the wave
amplitude has been substituted for energy. This value is
proportional to wave energy.
The square of the wave amplitude plotted against
frequency for a single value of wind speed constitutes
the spectrum of waves. Thus, a graph of the spectrum
is needed for each wind speed, and the energy associated
with each sine wave can be determined from these
graphs. Each wind speed produces a particular
spectrum; and the higher the wind speed, the larger the
FORECASTING SEA WAVES
LEARNING OBJECTIVES: Describe the
generation and growth of sea waves. Explain
the formation of fully developed seas.
Recognize the factors associated with nonfully
developed seas, and determine and analyze
features associated with sea waves. Define sea
wave terms and describe an objective method
of forecasting sea waves.
Since sea waves are in the generating area,
forecasting of them will be most important when units
are deployed in areas close to storm centers.
Problems encountered in providing these forecasts
will include accurately predicting the storm track and
the intensity of the winds that develop the sea waves.
Now lets look at the generation and growth of sea
GENERATION AND GROWTH
When the wind starts to blow over a relatively calm
stretch of water, the sea surface becomes covered with
tiny ripples. These ripples increase in height and
decrease in frequency value as long as the wind
continues to blow or until a maximum of energy has
been imparted to the water for that particular wind
speed. These tiny waves are being formed over the
entire length and breadth of the fetch. The waves
formed near the windward edge of the fetch move
through the entire fetch and continue to grow in height
and period, so that the waves formed at the leeward edge
of the fetch are superimposed on the waves that have
come from the windward edge and middle of the fetch.
This description illustrates that at the windward edge of
the fetch the wave spectrum is small; at the leeward edge
of the fetch the spectrum is large.
These waves are generated and grow because of the
energy transfer from the wind to the wave. The energy
is transferred to the waves by the pushing and dragging
forces of the wind. Since the speed of the generated
waves is continually increasing, these waves will
eventually be traveling at nearly the speed of the wind.
When this happens the energy transfer from the wind to
the wave ceases, When waves begin to travel faster than
the wind, they meet with resistance and lose energy
because they are then doing work against the wind. This
then explains the limitation of wave height and
frequency that a particular wind speed may create.
Fully Developed Sea
When the wind has imparted its maximum energy
to the waves, the sea is said to be fully developed. The
maximum frequency range for that wind will have been
produced by the fetch, and this maximum frequency
range will be present at the leeward edge of the fetch.
Once the sea is fully developed, no frequency is
produced with a value lower than that of the minimum
frequency value for the wind speed in question, no
matter how long the wind blows. In brief, the waves
cannot grow any higher than the maximum value for that
When the sea is fully developed, the area near the
windward edge is said to be in a steady state, because
the frequency range does not increase any more. If the
wind continues to blow at the same speed and from the
same direction for a considerable period of time, the
major portion of the fetch reaches the steady state.
Nonfully Developed Sea
When the wind is unable to impart its maximum
energy to the waves, the sea is said to be nonfully
developed. This can happen under two circumstances.
First, when the distance over which the wind is blowing
is limited or when the fetch is limited. Second, when
the wind has not been in contact with the sea for a