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.
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First,
when the distance over which the wind is blowing is limited or when the
fetch is limited.
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Second,
when the wind has not been in contact with the sea for a sufficient length
of time, or when the duration time is limited. Now let’s look at each
situation.
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FETCH
LIMITED SEA.— When the fetch length is too short, the wind is not in contact
with the waves over a distance sufficient to impart the maximum energy to the
waves. The ranges of frequencies and wave heights are therefore limited, and the
wave heights are less than those of a fully developed sea. The process of wave
generation is cut off before the maximum energy has been imparted to the waves
and the fetch is in a steady state. This leads to the conclusion that for every
wind speed, a minimum fetch distance is required for the waves to become fully
developed, and that if this minimum fetch requirement is not met, the sea is
fetch limited.
DURATION
TIME LIMITED SEA.— When the wind has not been in contact with the waves long
enough, it has had insufficient time to impart the maximum energy to the waves,
and the growth of the frequency range and wave heights ceases before the fully
developed state of the sea has commenced. Such a situation is known as a
duration time limited sea. This leads to
the conclusion that for every wind speed, a minimum duration time is required
for the waves to become fully developed; and that if this minimum
duration time requirement is not met, the sea is duration time limited. The
state of the sea, then, is one of three conditions: fully
developed, fetch limited, or duration time limited.
Table
6-1 shows the various wind speeds, fetch lengths and minimum wind duration times
needed to generate a fully developed state of the sea. When conditions do not
meet these minimum requirements, the properties of the waves must be determined
by means of graphs and formulas.
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