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ABSTRACT
The attractive force of the moon and the action of the wind have a significant impact on the sea. This can be observed in phenomena lasting for half a day or one day i.e. tides, and under the form of shorter lasting phenomena such as swell and seiches. Such agitations have a significant impact on the coast line and water bodies. They shape seabeds and coasts. In order to build constructions on areas subjected to such phenomena, these phenomena must be accurately predicted; their observation and a sound understanding of their principle is thus required. This article offers an extensive description of tides, swell, seiches or even tsunamis by detailing the physcical models that rule them, the theoretical statistics which allow for their prediction and the means in order to address them.
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Read the articleAUTHOR
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René BONNEFILLE: Professor of Maritime Hydraulics at the École Nationale Supérieure des Techniques Avancées and the École Nationale des Ponts et Chaussées - Doctor of Science - Engineer from the École Nationale Supérieure d'Électrotechnique et d'Hydraulique in Toulouse, France
INTRODUCTION
The marine environment is always agitated; this agitation manifests itself mainly in the form of long-period phenomena of the order of magnitude of half a day or a day: the tides, and in the form of short-period phenomena: the swell and the seiches. Tides are astronomical in origin. The swell is generated by the wind. Seiches are the oscillations that affect certain harbour basins or gulfs; they have a period of the order of a minute, and may be due to variations in meteorological conditions.
Currents accompany these movements: tidal currents, relatively weak far from the coast, can reach speeds of several meters per second near the shore. They affect practically all water depths with the same intensity. As they vary slowly over time, they can be considered constant on an hourly scale.
Swell currents are alternating and very strong during storms (several meters per second). They mainly affect the water near the surface (5 to 10 m thick), and diminish with depth.
Lastly, there are slow-moving currents (5 to 30 cm per second) due to the entrainment of surface water by the wind, or to differences in water density linked to variations in salinity or temperature.
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