Ocean waves, often generated by winds far across the sea, travel vast distances before encountering coastlines. As they near the shore, they undergo a transformation, culminating in wave breaking. A breaking wave occurs when its crest can no longer sustain itself and overturns, transforming its organized energy into turbulent motion. This natural process shapes coastlines and is a dynamic aspect of the ocean environment.
Water Depth and Wave Breaking
The primary reason most ocean waves break as they approach land is a process called shoaling. As waves move from deep ocean waters into progressively shallower areas, the decreasing water depth significantly alters their behavior. When the water depth becomes less than about half of the wave’s wavelength, the bottom of the wave begins to interact with the seabed. This interaction creates friction, causing the lower part of the wave to slow down.
While the base of the wave experiences this deceleration, the upper part of the wave continues to move at its original speed. This difference in speed causes the wave to “pile up” as the faster crest overtakes the slower base. Consequently, the wave’s height increases, and its wavelength, the distance between successive crests, decreases.
This compression of energy into a smaller water column further amplifies the wave’s height. The wave becomes increasingly steep and unstable, leading to breaking.
The Steepness Threshold
While shoaling explains why waves break near shore, a wave will ultimately break when it becomes too steep to support itself. This intrinsic characteristic is defined by its steepness, which is the ratio of its wave height to its wavelength. As a wave approaches the critical steepness, the water particles at the crest begin to move faster than the wave’s overall progression, causing the crest to become unstable and topple forward.
A general guideline indicates that a wave will typically break when its steepness ratio exceeds approximately 1:7. This means if a wave’s height is more than one-seventh of its wavelength, it becomes unstable. For example, a wave 2 feet high with a 14-foot wavelength would be at this threshold.
Visualizing Wave Breaks
The appearance of a breaking wave varies depending on conditions, primarily the slope of the seabed. These visual differences allow for the classification of breaking waves into three main types: spilling, plunging, and surging.
Spilling breakers occur on gently sloping seabeds. As the wave approaches the shore, the crest gradually becomes unstable, and turbulent whitewater spills down the front face of the wave. This break dissipates energy slowly over a considerable distance, creating a gentle, long-lasting break.
Plunging breakers form where the seabed has a moderately steep slope or sudden depth changes, such as over a reef or sandbar. The wave’s crest curls over and crashes downward, often forming a hollow tube or barrel shape. This break releases significant energy in a powerful, often noisy impact.
Surging breakers occur on very steep shorelines with deep water close to the beach. These waves build up but often do not fully break or curl; instead, they surge rapidly up the beach face with little foam or spray. Surging waves retain much of their energy, making them powerful as they rush onto the shore.
Additional Factors in Wave Breaking
While water depth and wave steepness are the primary determinants, other environmental factors can also influence when and how a wave breaks. Strong winds, particularly those blowing against the direction of wave propagation (offshore winds), can affect wave steepness. Offshore winds can help to hold up the wave crest, contributing to the formation of more hollow, plunging breaks. Conversely, onshore winds can cause waves to break prematurely and become more spilling in nature.
Ocean currents can also play a role. If a wave encounters an opposing current, its height can increase while its wavelength decreases, effectively steepening the wave and making it more prone to breaking.
The complex topography of the seabed can significantly alter wave breaking. Features like submerged reefs, sandbars, or submarine canyons create localized areas of shallowing or wave focusing. This can cause waves to break in unexpected locations or with altered characteristics.