BASIC
PRINCIPLES OF OCEAN WAVES
Ocean waves are advancing crests and
troughs of water propagated by the force of the wind. When winds start to blow,
the frictional effect of the wind on the water creates ripples that form more or
less regular arcs of long radii. As the wind continues to blow, the ripples
increase in height and become waves.
A wave is visible evidence of energy moving
in an undulating motion through a medium, such as water. As the energy moves
through the water, there is little mass motion of the water in the direction of
travel of the wave. This can best be illustrated by tying one end of a rope to a
pole or other stationary object. When the free end of the rope is whipped up and
down, a series of waves moves along the rope toward the stationary end. There is
no mass motion of the rope toward the stationary end, only the energy traveling
through the medium, in this case the rope.
A sine
wave is a true rhythmic progression. The curve along the
centerline can be inverted and superimposed upon the curve below the centerline.
The amplitude of the crest is equal to the amplitude of the trough, and the
height is twice that of the amplitude. Sine waves are a theoretical concept
seldom observed in reality. They are used primarily in theoretical groundwork so
that other properties of sine waves may be applied to other types of waves such
as ocean waves. Principles of other types of waves are modified according to the
extent of deviation of their properties from those of sine waves.
Waves that have been created by the local
wind are known as sea
waves. These waves are still under the influence of the
Sea
surface forecasting
local wind and are still in the generating
area. They are composed of an infinite number of sine waves superimposed on each
other, and for this reason they have a large spectrum, or range of frequencies.
Sea waves are very irregular in appearance. This irregularity applies to almost
all their properties. The reason for this is twofold: first, the wind in the
generating area (fetch) is irregular both in direction and speed; second, the
many different frequencies of waves generated have different speeds. Figure 6-1
is a typical illustration of sea waves. The waves found in this aerial
photograph are irregular in direction, wave length, and speed.
As the waves leave the generating area (fetch)
and no longer come under the influence of the generating winds they become swell
waves. Because swell waves are no longer receiving energy from
the wind, their spectrum of frequencies is smaller than that of sea waves. Swell
waves are also smoother and more regular in appearance than sea waves. Figure
6-2 illustrates typical swell wave conditions.
Figure 6-1.-Aerial photo of sea waves.
|
