Waves genesis

Résumé du document

Sea waves have a large variety of forms, dimensions and propagation speeds. Their behavior can be modeled with simple undulatory forms, analyzed in terms of period, speeds and gap (wavelength). A wave is in fact a mechanical force which is spread on the surface of contact between two fluids. Waves correspond to periodic deformations of an interface. In oceanography, the waves on the surface are materialized by a deformation of the surface of the sea, i.e. of the interface between the atmosphere and the ocean. During the propagation of a wave from one point towards another, there is no transport of material (the waves do not make advance the boat). The wave transports only energy; a point reached by a wave reproduces the state of the source with a less amplitude and a delay due to the time needed for the wave to traverse the distance which separates it from the source. There are also transverse waves, where the direction of the movement of the water particles is orthogonal with respect to the direction of propagation.

Extraits

[...] To develop them, the waves need time and space. Influence of the duration of the gust of wind: A wind of force 7 (=30knots) blowing in open sea, form waves such as: Influence of the fetch: Still for a wind of force This last table shows well that the larger the zone of fetch is, the more the formed waves will be high. What explains why in the Mediterranean, where the zone of fetch is small, the waves are less consequent than those striking regularly Hawaii, located in the middle of the Pacific and receiving swells reaching 20 metres sometimes? [...]

[...] In lower part, tiny ripples can appear, but disappear when the wind stop. These oscillations are not due to the friction of the air on the surface of water but to turbulences which accompany the air volume displacements (local variations of pressure). This superficial agitation is then activated by the push of the wind on the back of the undulations Growth of waves : The waves are growing as long as their speed remains lower than the speed of the wind. [...]

[...] So we understand why there is such a difference of power between a breaking wave on the island of Hawaii, where sea-beds plunge in an abrupt way to a few hundreds metres of the shore and, a wave reaching the Breton coast, braked in its progression by a continental shelf of several tens of kilometres, soft inclined. These same reasons explain the power of the waves in Aquitaine, the “golfe de Gascogne” plunging deeply close to the coasts, in particular on the level of Capbreton where the waves strike the shore with an impressive force. So the waves powerful depend mainly on the increase of the bottom (sudden or progressive). Bibliographic references Dixon P Le guide complet du surf, éd. Atlantica Le Carrer O La croisière hauturière et côtière, éd. [...]

[...] If the wind goes in the same direction that the current, the swell remains constant; but if both are opposed the sea curves itself and the waves can break. The white color of foam is due to the particles of air locked up by the peak of vagueness at the time or it breaks down. The power of the surge is all the more important as the increase of the fast continental shelf. Indeed, before reaching the coasts the swell meets with broad the continental shelf which slows down its progression, thus decreasing the speed and the power of the waves. [...]

[...] Waves genesis Sea waves have a large variety of forms, dimensions and propagation speeds. Their behaviour can be modelled with simple undulatory forms, analyzed in term of period, speeds and gap (wavelength). A wave is in fact a mechanical wave which is spread on the surface of contact between two fluids. The waves correspond to periodic deformations of an interface. In oceanography, the waves of surface are materialized by a deformation of the surface of the sea, i.e. of the interface between the atmosphere and the ocean. [...]