Square waves are formed when two wave systems intersect, creating a sort of wavy pattern on the surface of the water. This phenomenon, which is also known as “cross sea“, can be dangerous because it creates rip currents that put boats at risk, increasing the likelihood of accidents.
In Italy it is very difficult to see this type of phenomenon, while it is very common inisland of Rè (Nouvelle-Aquitaine, France), since in that area two seas intersect with different weather patterns. It is also often seen on the beaches that bathe Tel-Aviv (Israel) and in the great american lakes (especially in Lake Michigan).
When and Why Cross Sea Occurs
The phenomenon – easier to see in the shallow waters of some coastal areas bathed by ocean waters – occurs when two or more wave systems travelling in mutually perpendicular directions (or travelling along directions that form an angle between the 60° and 120° (between them) collide, giving shape to these square waves. The colliding waves could be powered by winds (wind waves) with waves no longer supported by winds and therefore called “dead waves” (swell). If, however, only dead waves cross, we speak of “cross swell“. In any case, the phenomenon occurs when two wave systems that initially had different directions intersect.
Square Waves and Refraction and Diffraction
But not only that: the cross sea can also be observed following phenomena of refraction and/or diffraction of the waves.
What exactly is the refraction? Refraction is the phenomenon by which a wave, passing from one medium to another (for example from deep water to shallow water), changes direction. This happens because the speed of the wave varies depending on the depth of the water. But how does this affect the formation of square waves? As a wave approaches the shore, the part closest to the shore slows down due to shallower water, while the part farther away continues to move faster. This causes the wave to curve and change direction. If two wave systems approach the shore from slightly different directions, refraction can cause their crests to cross each other, forming a checkerboard pattern.
In the diffraction Instead, a wave, encountering an obstacle or an opening, tends to “spread” or “bend” around it. This happens because waves do not always propagate in a straight line, but can “curve” around obstacles. How does it affect the formation of square waves? In two ways:
- Interacting with obstacles: When waves encounter obstacles such as reefs, jetties, or sandbars, diffraction causes them to bend and change direction. This can contribute to complex patterns on the surface of the water, including checkerboard patterns (cross-seas).
- Interference between scattered waves: Waves scattered by different obstacles can interfere with each other, adding to or canceling each other. This interference can accentuate or attenuate certain patterns, such as square waves.
Sometimes refraction and diffraction work together. For example, a wave approaching a shoreline with varying depths will experience both refraction (due to the change in depth) and diffraction (due to the presence of reefs or headlands). The interaction of these two phenomena can create very complex and fascinating wave patterns.
So both refraction and diffraction are important factors in the formation of square waves, but it is essential to emphasize that the formation of these patterns is the result of a combination of factorsincluding wind conditions, seabed topography and storm surge characteristics.
Why Square Waves Can Be Dangerous
Although observing square waves on the beach (in shallow water where the currents are not strong) does not present a danger, it is necessary to keep in mind that the force of rip currents generated by the collision of two different wave systems and the rapid changes in tides represent a risk to both swimmers and boats. In fact, over the years there have been several shipwrecks And maritime accidents where this phenomenon was also among the causes. Clearly, if you are in the water and you see that this type of wave is forming, you should move away as soon as possible to avoid being dragged from different angles and not resurfacing.
