Waves represent the movement of energy through water, common in oceans, lakes, and rivers. As they approach a coastline, these dynamic features undergo significant transformations. A “breaking wave” is a natural phenomenon where a wave’s form becomes unstable and collapses, releasing its energy in foam and turbulence. Understanding this process reveals why and where these events occur along our shores.
The Physics of Breaking Waves
As ocean waves travel from deep water towards the shore, they encounter shallower depths, initiating a process known as shoaling. During shoaling, the wave’s interaction with the seabed changes its characteristics. The lower portion of the wave experiences friction with the ocean floor, causing it to slow down. Meanwhile, the upper part continues to move faster, compressing the wave’s length and increasing its height.
This disparity in speed between the top and bottom causes the wave to become progressively steeper. A wave reaches an instability point when its steepness—the ratio of its height to its wavelength—exceeds a threshold, often cited as 1:7. At this point, the crest can no longer be supported by the water underneath and begins to overturn. This overturning is the act of breaking, converting the wave’s organized energy into turbulent energy, creating white water.
Factors Determining the Break Point
The location where a wave breaks, known as the break point, is influenced by several factors: water depth, seabed topography, and the incoming wave’s characteristics. Water depth is a primary determinant; waves generally break when the water depth is approximately 1.3 times the wave’s height. This means taller waves break in deeper water, farther from shore, while smaller waves break closer to the beach.
The slope and contours of the seabed play a role in how and where waves break. A gradual seabed slope allows waves to shoal and steepen more slowly, causing them to break over a longer distance. Conversely, a steep seabed or sudden depth changes, like over sandbars or coral reefs, can cause waves to break abruptly and more forcefully, often closer to shore. The incoming wave’s initial characteristics, including its height, period (time between successive crests), and wavelength, also interact with changing depth to define the breaking point. Longer period waves, often called groundswell, travel faster and can break with more force in deeper water than shorter period waves.
Common Types of Breaking Waves
The interaction between wave characteristics and seabed topography results in different types of breaking waves. Spilling waves occur on gently sloping seabeds. As they approach the shore, their crests gradually become unstable and “spill” down the wave face, creating a continuous line of white water. This gradual breaking process dissipates the wave’s energy over a longer distance, making them softer and more consistent.
Plunging waves form when the seabed slope is moderately steep or features sudden depth changes, such as a reef or sandbar. The crest of a plunging wave becomes vertical, curls over, and crashes down onto the wave’s trough, often trapping air and creating a hollow tube or “barrel.” These waves release most of their energy at once, resulting in a powerful impact.
Surging waves are found on very steep shorelines. These waves do not exhibit a distinct breaking crest; instead, they build up and surge rapidly up the beach face with minimal white water. Their energy is often reflected back into the ocean, and they can be dangerous due to strong backwash and ability to knock people over.