When a wave travels across the ocean, the water itself does not travel along with it for great distances. The fundamental nature of a surface water wave is a transfer of energy, not a massive movement of fluid across the sea. If waves transported mass, water from the middle of the Atlantic would pile up on the shores of Europe or North America. Instead, the wave form moves horizontally, while the water particles perform a localized dance.
The Primary Movement: Circular Orbits
In deep water, where the ocean floor does not influence the wave, a water particle moves in a circular orbital motion. As the wave crest approaches, the particle moves up and slightly forward, rising with the peak. Conversely, as the wave trough passes, the particle moves down and slightly backward.
This continuous cycle of up-forward and down-backward motion creates a nearly closed circle, allowing the wave’s energy to propagate horizontally across the surface. An object floating on the water, such as a seabird or a cork, illustrates this principle by bobbing up and down and oscillating slightly back and forth. The diameter of this circular orbit at the water’s surface equals the height of the passing wave.
The net displacement of the water particle is minimal; it returns to almost its original position after the wave has passed. This orbital movement is the mechanism by which the wave transfers its energy from one particle to the next.
Depth and Energy Dissipation
The circular orbital motion that defines wave movement is not uniform throughout the entire water column. The size of the orbital path decreases rapidly with increasing depth below the surface. This reduction in motion is exponential, meaning the deeper the water, the less the wave affects the particles.
The depth at which wave-induced motion becomes negligible is called the wave base, approximately half the wave’s wavelength (\(L/2\)). At this depth, the orbital motion is reduced to less than four percent of its size at the surface. Below the wave base, the water is essentially undisturbed by surface activity.
This explains why submarines or large offshore structures can avoid severe surface storms by submerging below this depth. Since most open-ocean waves have wavelengths of a few hundred meters or less, the deeper parts of the ocean are unaffected by the passing surface energy.
Transformation Near the Shore
As a wave approaches the coastline and the water depth decreases, the physics of water movement fundamentally changes in a process called shoaling. Shoaling begins when the water depth becomes less than the wave base, or less than half the wavelength (\(\)d < L/2[/latex]). The bottom of the wave begins to "feel" the seabed, which introduces friction. This friction with the ocean floor slows down the bottom of the wave, which causes the circular orbits to flatten and become elliptical, with a more pronounced horizontal axis. Since the wave's period must remain constant, the slowing of the wave speed (celerity) forces the wavelength to decrease, effectively compressing the wave. To conserve the energy that is still being transferred, the wave height must increase dramatically as the wave length shortens. This energy concentration causes the wave to become steeper and more unstable. The wave finally breaks when the water particle velocity at the wave crest exceeds the forward speed of the wave form itself. This results in the visible plunging or spilling action, where the water at the crest is thrown forward. This is the only point where significant mass transport occurs, as the fluid is forced out of its orbital path and surges toward the shore. The depth at which a wave breaks is typically when the water depth is about 1.3 times the wave height.