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Deep
water. Water that is greater in depth than one-half the wave
length.
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Shallow
water. Water that is less in depth than one-half the wave length.
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Fetch
(F). An area of the sea surface over which a wind with a constant direction
and speed is blowing, and generating sea waves. The fetch length is measured in
nautical miles and has definite boundaries.
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Duration
time (t). The time that the wind has been in contact with the
waves within a fetch.
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Fully
developed state of the sea. The state the sea reaches when the
wind has imparted the maximum energy to the waves.
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Nonfully
developed state of the sea. The state of the sea reached when the
fetch or duration time has limited the amount of energy imparted to the waves by the
wind.
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Steady
state. The state reached when the fetch length has limited the
growth of the waves. Once a steady state has been reached, the frequency range produced will
not change regardless of the wind.
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Wind
field. A term that refers to the fetch dimensions, wind duration,
and wind speed, collectively.
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Effective
duration time. The duration time that has been modified to
account for the waves already present in the fetch or to account for waves generated by a
rapidly changing wind.
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E
value. E is equal to the sum of the squares of the individual
amplitudes of the individual sine waves that make up the actual waves. Since
it is proportional to the total energy accumulated in these waves, it is
used to describe the energy present in them and in several formulas
involving wave energy.
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Co-cumulative
spectra. The co-cumulative spectra are graphs in which the total
accumulated energy is plotted against frequency for a given wind speed. The
co-cumulative spectra have been devised for two situations: a fetch limited
wind and a duration time limited wind.
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Upper limit of frequencies (fu). The upper
limit of frequencies represents the lowest valued frequencies produced by a
fetch or that are present at a forecast point. This term gets its name from
the fact that the period associated with this frequency is the period with
the highest value. The waves associated with this frequency are the largest
waves.
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Lower
limit of frequencies (fL,). The lower limit of frequencies represents the
highest value frequencies produced by a
fetch or that are present at a forecast
point. This term gets its name from the fact that the period associated with
this frequency is the period with the lowest value. The waves associated
with this frequency are the smallest waves.
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Filter
area. That area between the fetch
and the forecast point through which swell waves propagate. This area is so
termed because it filters the frequencies and permits only certain ones to
arrive at a forecast point at a forecast time.
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Significant frequency
range. The
significant frequency range is the range of frequencies between the upper
limit of frequencies and the lower limit of frequencies. The term
significant range is used because those low-valued frequencies whose E
values are less than 5 percent of the total E value and those high-valued
frequencies whose E values are less than 3 percent of the total E value are
eliminated because of insignificance.
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