The rhythmic pulse of the ocean, observed as a sequence of large waves followed by a period of calm, is a predictable physical phenomenon known as wave sets. These sets manifest as a burst of high-energy waves, followed by a period of smaller waves or a “lull” before the pattern repeats. This consistent cycling is governed by the physics of how multiple waves interact and propagate across the ocean basin. Understanding this pattern requires looking closely at how waves are created and how their individual speeds combine into a larger, traveling energy packet.
How Individual Ocean Waves Are Formed
Ocean waves are created by the transfer of energy from wind to the water’s surface. As wind blows over the water, frictional drag disturbs the surface, generating ripples that grow into substantial waves. The size and energy of these developing waves are determined by three factors: wind speed, duration, and fetch.
Wind speed must be greater than the speed of the wave crests for continuous energy transfer. Duration is the length of time the wind blows, and fetch is the uninterrupted distance the wind travels in a consistent direction. Large waves, such as deep-ocean swell, only form when all three factors—high wind speed, long duration, and extensive fetch—are maximized.
The Role of Superposition and Interference
The appearance of a wave “set” is rooted in the principle of superposition: when two or more waves meet, their individual displacements are added together. Waves generated by a distant storm travel at slightly different speeds, wavelengths, and periods, causing them to constantly overlap and interact. This continuous overlap results in wave interference.
When the crests of multiple waves align, they combine energy to produce a single, much taller wave through constructive interference—this forms the high-energy wave set. Conversely, when the crest of one wave meets the trough of another, they partially or completely cancel each other out, resulting in a momentarily flat or very small wave through destructive interference—this creates the brief lull.
Why Waves Travel in Groups (Group Velocity)
The rhythmic pattern of sets is dictated by wave dispersion, where waves with different wavelengths travel at different speeds. In deep water, longer wavelengths travel faster than shorter ones, causing the waves to sort themselves out as they move away from their origin. The speed of an individual wave crest is called phase velocity, but the speed of the overall energy packet—the wave set—is the group velocity.
For deep-water surface gravity waves, the group velocity is half the phase velocity, meaning the energy travels at half the speed of the individual crests. This difference in speed is why the wave set appears to be a slow-moving envelope of energy through which individual waves pass. As a wave crest approaches the back of the energy group, it grows in height due to constructive interference, travels through the peak of the set, and then shrinks and vanishes as it moves past the front into a region of destructive interference. This constant process of individual waves forming, moving through, and dissolving within the larger, slower-moving energy packet creates the observable, repetitive cycling of wave sets.
How Wave Sets Break Near the Shore
As the traveling wave set approaches the coast, the deep-water physics transition into shallow-water dynamics, a process known as shoaling. When the water depth decreases to less than half the wave’s wavelength, the wave begins to “feel” the bottom. Friction with the seafloor slows the wave’s progression, forcing the waves to bunch up, which shortens their wavelength and increases their height to conserve the energy being transported.
The energy of the wave set becomes concentrated vertically, amplifying the constructive interference that defines the set. This transformation makes the high-energy set more visible right before it breaks. Breaking occurs when the wave becomes so steep and unstable that the velocity of the water particles at the crest exceeds the speed of the wave form itself, causing the crest to spill or plunge forward. This critical point is reached when the water depth is approximately 1.3 times the height of the wave.