Ocean waves are a fundamental characteristic of the ocean, representing the constant transfer of energy across the water surface. An ocean wave is a periodic disturbance that travels through the water, moving energy from one location to another, though the water itself does not travel over long distances. Understanding these characteristics is foundational for maritime activities, coastal engineering, and the study of ocean dynamics. The measurable properties of waves determine their behavior, which dictates everything from navigation safety to coastal erosion.
The Basic Anatomy of a Wave
The basic anatomy of an ocean wave includes several measurable characteristics. The highest point of the wave is called the crest, while the lowest point is known as the trough. Wave height is defined as the vertical separation between the crest and the preceding trough.
The horizontal distance separating two successive crests or troughs is termed the wavelength. This length measures the wave’s size, ranging from centimeters in small ripples to hundreds of kilometers in large tsunami waves. A wave’s speed and behavior are directly tied to its wavelength.
The wave period is the time required for two consecutive crests to pass a fixed point. This period is measured in seconds and remains constant for a wave as it travels across the ocean. The wave period is a stable measurement used by oceanographers to classify different types of waves.
How Ocean Waves Are Generated
The majority of surface ocean waves are generated by wind friction acting upon the water surface. The size a wind-generated wave can achieve is determined by three interacting factors: wind speed, duration, and fetch. Faster wind transfers more energy to the water, creating larger waves.
A strong wind must blow for a sufficient duration to allow waves time to grow to their maximum potential size. Fetch is the uninterrupted distance over open water that the wind blows in a single direction. Large, powerful waves develop when high wind speed, long duration, and extensive fetch are present.
While wind is the dominant force, other disturbances also generate significant waves. Seismic events, such as underwater earthquakes or volcanic eruptions, displace the seafloor and create tsunamis. These are extremely long waves traveling at great speeds. Landslides or large icebergs falling into the water can also generate localized, high-energy splash waves.
Wave Movement and Energy Transfer
Waves travel across the ocean by transferring energy through the water. In the open ocean, water particles move in a circular or orbital motion as the wave passes beneath them. A floating object on the surface will bob up and down, tracing a circular path rather than being carried forward by the wave.
The diameter of these orbital paths decreases rapidly with depth, becoming negligible at a depth equal to about half the wave’s wavelength. Waves traveling over water deeper than this threshold are classified as deep-water waves. The speed of a deep-water wave depends solely on its wavelength.
Waves enter the shallow-water classification when the water depth is less than one-twentieth of their wavelength. Here, the orbital motion of the water particles flattens into ellipses as they interact with the seafloor. The speed of a shallow-water wave is determined by the water depth, causing the wave to slow down as the water becomes shallower.
The Transformation of Waves Near Shore
As a wave moves from deep water into the shallow zone near the coast, it undergoes shoaling. When the wave begins to “feel the bottom,” friction with the seabed causes its speed to decrease. Because the wave period remains constant, the slowing wave decreases its wavelength, causing the waves to bunch up.
The reduced speed forces the wave energy to compress into a smaller space, resulting in an increase in wave height. The wave crest becomes peaked while the trough flattens, increasing the wave steepness. This transformation continues until the wave becomes unstable and the crest collapses forward, a moment known as breaking.
Wave breaking occurs when the wave steepness, defined as the ratio of wave height to wavelength, exceeds 1:7. The slope of the seabed dictates the type of breaker that forms. Spilling breakers occur on gently sloping beaches, where the crest tumbles down the face of the wave. Plunging breakers, which form a distinct curling tube, happen on steeper beaches. Surging breakers are found on very steep coasts where the wave rushes up the beach face without fully collapsing.