Ocean waves are disturbances that transmit energy through water, sculpting coastlines and influencing global weather patterns. The wave itself is the movement of a form across the surface, not a mass transport of water across the ocean basin. This energy transmission is governed by scientific principles that classify the mechanics of their generation and eventual dissipation at the shore.
The Scientific Classification of Ocean Waves
Ocean waves are classified as a type of surface gravity wave, specifically known as orbital waves because of the unique way water particles move within them. Unlike longitudinal or transverse waves, the water particles in an ocean wave follow a circular or nearly circular path as the energy passes. This orbital motion transmits energy horizontally while the water itself returns to its original position.
The diameter of these orbits is largest at the surface and decreases rapidly with depth. This motion effectively ceases at the wave base, a depth equal to half the wavelength (the distance between two successive wave crests). Waves traveling in water deeper than this measurement are considered deep-water waves.
Waves that begin to interact with the seafloor are classified as shallow-water waves. This interaction starts when the depth falls below half the wavelength. Deep-water wave speed is determined by the wavelength, making them dispersive because longer waves travel faster. Shallow-water wave speed, however, is governed solely by the water depth, which is important for understanding the behavior of tsunamis and tides.
How Ocean Waves Are Generated
The vast majority of ocean waves are wind-generated waves, created by friction between moving air and the water surface. The process begins with tiny ripples, or capillary waves, which grow into larger gravity waves as the wind pushes on the crests. The size of these wind waves is determined by three factors: the speed of the wind, the duration it blows, and the fetch, which is the uninterrupted distance over which the wind blows.
Other forces generate waves with much longer wavelengths and periods, which are classified as shallow-water waves regardless of ocean depth. Tides result from the gravitational pull exerted primarily by the Moon and the Sun. These astronomical forces create extremely long-period waves that cause the rhythmic rise and fall of sea level along the coasts.
Tsunamis, often mistakenly called tidal waves, are generated by sudden, massive displacements of water, typically from tectonic events like submarine earthquakes, landslides, or volcanic eruptions. Because their immense wavelengths can span hundreds of kilometers, tsunamis behave as extremely fast shallow-water waves even in the deepest ocean. This geological force moves the entire water column, unlike wind waves that only affect the surface layer.
The Dynamics of Waves Approaching the Shore
As a deep-water wave approaches a coastline, it undergoes a transformation process known as shoaling. Shoaling begins when the wave’s orbital motion interacts with the seabed, which occurs when the water depth becomes less than half of the wave’s wavelength. Friction with the bottom causes the wave to slow down, and because the wave’s period remains constant, its wavelength must decrease, causing the waves to “bunch up.”
To conserve energy flux, the wave’s height must increase as the speed and wavelength decrease, making the wave crests steeper. The water particles’ orbital paths, which were circular in deep water, flatten into ellipses as they are constrained by the seafloor. This steepening continues until the wave reaches a point of instability, typically when the wave height is about one-seventh of the wavelength.
The wave then breaks when the crest’s velocity exceeds the speed of the underlying wave form, causing the crest to outrun the base and collapse. The type of breaking wave depends largely on the slope of the seabed. Spilling breakers occur on gently sloping beaches, where the crest gradually spills down the face of the wave. On steeper beaches, plunging breakers form a hollow tunnel as the crest curls over and crashes violently. Surging breakers occur on very steep slopes, where the wave barely breaks, surging up the beach face.