Wind, waves, and Swell

Wind, waves, and Swell

By: María Suarez Bilbao *, Coastal Engineering Department.

WHAT IS THE SWELL? Do you remember that last May 2nd Acapulco coast was affected by a phenomenon called “SWELL”? The news was a nationwide boom invading social networks, of course many videos like this were posted on YouTube. But … do you know what the SWELL is? how and why is it generated? Here we explain it to you.
In the ocean, there are many types of waves that propagate energy along the surface of the sea. The mechanisms that contribute this energy are of different nature: wind and meteorological disturbances (waves), storms and earthquakes (tsunamis), the attraction of stars and rotation of the earth (tidal waves), etc. Each of these waves have characteristic periods and lengths.
For the problems of interest in both coastal protection and engineering, the most important waves are the waves generated by the wind with periods between 3 and 30 seconds, also called gravitational waves, whose most common examples are the waves called in coastal engineering as “Sea” (wind waves) and “Swell”.
The waves are mainly originated by the wind friction on the sea surface, therefore the characteristics of these waves will depend on the force of that wind, the duration blowing in the generation zone and the affected oceanic area
The wave generated by the wind in a certain area of the ocean has characteristics (height, period, direction) that vary randomly. Therefore, the waves observed in the generation zone are chaotic, more irregular, disordered, with little correlation between heights and successive periods and with variable propagation directions around generating wind direction. These “local” waves are called wind waves or SEA.
Once it is formed, the wave propagates at the expense of its own energy outside the generation zone (outside the area where the wind that generated it blows). And as it moves away from it, a series of phenomena occur which “arrange it” in wave trains with similar periods and directions, losing the chaotic and disorderly appearance it had when generated. This less irregular, more ordered waves, are generated by the propagation of the wind waves traveling great distances from the generation zone and are called SWELL.
The SWELL can travel very large distances. As an example, in the Canary Islands (Spain) exist records of SWELLS from waves generated by storms occurring in the South Atlantic Ocean (more than 7000 km away). A dominant part of the wave energy that reaches the Spanish Atlantic coasts is in the SWELL form, the same happens in the Mexican Pacific coasts. On the other hand, In the Mediterranean, where the distances are smaller, the energy of the dominant waves is a consequence of SEA waves, as in the Mexican Caribbean and the Gulf of Mexico.
Moreover, it is important to note that waves generated in a wind area, continue propagating by their own energy, entering areas with different meteorological characteristics, so that, the ocean waves at a certain point happen to be a mixture of locally generated wind waves (SEA) and the waves propagated from other areas (SWELL). Therefore, continuously in one place may be wind waves, swell waves or combined waves. Generally, the wave energy spectrum and its analysis will be indicative of the observed sea state.
Now you know what the SWELL, so is, so if you are asked what happened in Acapulco on May 2nd, 2015, you can explain it well and even be the life of the party!
Please leave us your comments, and if you have any questions, CONTACT US.
We will read each other soon!

This post is also available in: Spanish