| Summary: | Ocean wave trains. Constitution -Propagation. Methods of prediction. When the sea is being blown up by the wind the agitation consists of a 3 — dimensional ondulating movement. The swell thus produced then becomes more regular, all the while moving forward in the decay area until it takes on an almost cylindrical form. An oceanic perturbation of this nature is made up of rather complex wave trains, which nevertheless are limited in size. This note briefly describes their principal features, including their statistical properties and then gives some simple synthetic formulae by means of which a general picture of the movement and of its evolution can be obtained. These asymptotic formulae are accurate enough, at least in the decay area. The controversial point : the slow ine rease in wave période, as related to the action of passive resistances, is mentionned. A rapid survey is made of two ways in which swell is currently predicted in the United States 1. The Sverdrup-Munk-Bretschneider method based on a simplified analysis but taking passive damping forces into account 2. The Pierson-Neumann method based on a more rigourous analysis but not including energy dissipation 3. Indications are given of another method also obtained from an exact analysis but condensed in a synthetic form and allowing including the effect of passive damping forces. The three methods are compared for average swell conditions in the North Atlantic. Methods 2 and 3 appear to be very comparable within the limits of method 2 i. e. up to medium travel distances. Nearly identical results are afforded for medium and large travel distances by methods 1 and 3, the last of which taking energy dissipation into consideration. L'agitation de la mer se présente comme un mouvement ondulatoire à trois dimensions durant son stade de génération par le vent. La houle produite se régularise ensuite, tout en progressant dans la zone d'épanouissement, et prend, pour finir, un caractère presque cylindrique. Une perturbation océanique de cette nature ...
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