Surface waves are deformations of the water surface that transmit energy across the ocean, often traveling thousands of miles from their origin. This energy, generated primarily by wind, travels without transporting the mass of water itself. The energy remains contained until the wave reaches the shore, where interaction with the decreasing depth of the seabed causes a physical process that terminates the wave in a cascading collapse.
How Waves Move in Deep Water
In the open ocean, where the water depth is greater than half of the wave’s wavelength, the wave is classified as a deep-water wave. Water particles do not travel forward with the wave form; instead, they move in near-perfect circular orbits. As the wave passes, water rotates up and forward, then down and backward, returning almost exactly to its original position. This orbital motion is strongest at the surface and diminishes rapidly with depth, becoming negligible below the wave base (half the wavelength). The wave’s speed in deep water is determined by its wavelength, meaning longer waves move faster and can travel immense distances with minimal energy loss.
The Transformation of Waves in Shallow Water
The wave’s behavior changes when it enters shallow water, defined as a depth less than half of its wavelength. This transition, called shoaling, occurs when the wave begins to “feel” the ocean floor. Friction with the seabed deforms the circular orbits of water particles at the base of the wave into flattened ellipses. This drag on the lower portion significantly reduces the wave’s speed, which is now governed by the water depth rather than the wavelength. Since the wave period remains constant, the decrease in speed forces the wavelength to shorten, bunching the waves closer together, which causes the wave height to increase dramatically as it approaches the shore.
The Instability Threshold and Collapse
As the wave continues to shoal and its height increases, it eventually reaches a point of instability that forces the collapse. Stability is lost when the wave height is approximately 0.78 times the water depth. The underlying mechanism for the final break is a velocity differential that develops across the wave’s profile. The wave’s base, experiencing maximum friction with the seabed, slows significantly. Conversely, the crest, moving unimpeded near the surface, maintains a higher forward velocity. This disparity causes the wave profile to steepen sharply, creating a crest that travels faster than the water supporting it. When the angle of the wave face becomes too vertical, the crest section becomes gravitationally unsupported, resulting in the water mass tumbling forward and down, dissipating the stored wave energy as turbulence and foam.
Breaking Wave Forms and Beach Slope
The way a wave breaks is determined by the steepness of the nearshore seabed, which dictates the rate of wave height gain relative to water depth loss.
On a gently sloping beach, the wave breaks as a spilling breaker. The crest gradually rolls down the front face in a cascade of white water, releasing energy slowly over a considerable distance.
A moderately steep beach slope produces a plunging breaker. This is the classic surfing wave where the crest curls over the base and falls with force, creating a hollow tube.
A very steep beach or coastline leads to a surging breaker. Here, the wave does not fully break but rushes up the beach face with little foam or turbulence before retreating. This happens when the wave runs out of supporting water depth too quickly for the crest to fully overturn.