Waves, Sea

Waves, Sea

 

waves on the surface of seas and oceans. Because of their great mobility, water particles easily come out of equilibrium and oscillate under the influence of various kinds of forces. Waves are caused by the tide-forming forces of the moon and sun, by winds, by fluctuations in atmospheric pressure, by underwater earthquakes, and by deformations of the ocean floor.

According to their cause, sea waves are subdivided into normal tide waves, waves caused by wind, waves caused by fluctuations in atmospheric pressure (seiches), and seismic waves (tsunamis). In most cases, wave motions are characterized by irregular shape. It is necessary to distinguish between the shifting of particles within the wave and the apparent motion of the wave’s shape, which consists in the movement of its profile through space. The particles involved in the wave are moving along closed or almost closed trajectories.

The main characteristics of sea waves are height, which is the vertical distance between the crest and the trough of the wave; length, which is the horizontal distance between two adjacent crests or troughs; the velocity of motion of the wave shape, or phase velocity; and the period. The period of waves caused by wind does not exceed 30 sec; those caused by atmospheric-pressure variations or by earthquakes have periods measured in minutes, dozens of minutes, or hours. The periods of normal tide waves are expressed in hours.

Depending on the prevailing role of the forces acting in the formation of wave motions, waves are subdivided into gravitational and capillary types. Waves that continue to exist after the end of the action of the forces that caused them are called free waves, as distinct from induced waves, which are maintained by an uninterrupted energy input.

Waves caused by wind are of the induced type. They are formed by the energy of the wind through direct pressure of the airflow on the windward sides of the crests and through its friction on the wave’s surface. The development of waves caused by wind begins with the formation of ripples, which are capillary waves. As the capillary waves grow, they change into gravitational waves, which gradually increase in length and height. In their initial stage of development, the waves run in parallel lines, which later disintegrate into individual crests (three-dimensional wave pattern). The surface of the water stirred up by the wind takes on a very complicated shape, which continually changes over time. Waves caused by wind are always present on the surface of the sea, and their dimensions are most varied (sometimes up to 400 m long and 12-13 m high, with speeds of 14-15 m/sec).

In a deep sea the size and pattern of the waves are determined by the wind velocity, the duration of its action, the“wave race” (the distance from the leeward shore to the observation point along the wind direction), and the structure of the wind field and the configuration of the shoreline. In a shallow sea, the depth of the sea and the relief of its bottom also have an effect on the process of wave formation. Shallow seas tend to limit the growth of waves. If the wind that has caused the waves abates, the waves caused by it gradu-ally change into free waves, called swells, whose waves have a more regular shape than those caused by wind and have a greater length between crests. The most frequent case is a mixed wave pattern in which ripples and waves caused by wind occur simultaneously.

The study of sea waves is of great practical interest in connection with numerous problems in navigation, marine hydraulic-engineering construction, and shipbuilding. It requires detailed theoretical and experimental research, using various instruments installed on vessels and on shore.

Among the devices used for observing sea waves are wave-measuring rods or marks, wavemeters, and wave graphs of various systems. Stereophotography makes possible the recording of surface conditions over wide areas. Long-period sea waves—for example, tide waves—are re-corded by devices called tide gauges.

REFERENCES

Snezhinskii, V. A. Prakticheskaia okeanografiia. Leningrad, 1951.
Duvanin, A. I. Volnovye dvizheniia v more. Leningrad, 1968.
Shuleikin, V. V. Fizika moria, 4th ed. Moscow, 1968.

S. S. VOIT

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