When observing the ocean, waves often appear to align themselves with the shoreline, even if they were traveling at an angle far out at sea. This phenomenon, where the wave crests change their direction of travel as they move toward land, is known as wave refraction. It is a physical process that causes the incoming waves to bend, ultimately impacting the distribution of their energy along the coast. Understanding this systematic change in direction requires looking beneath the surface at the relationship between the water and the ocean floor.
How Water Depth Controls Wave Speed
In the open ocean, waves are considered deep-water waves, meaning their motion is largely unaffected by the seafloor because the water depth is greater than half of their wavelength. Their speed is primarily influenced by the wave’s length and the pull of gravity. As a wave moves closer to the shore, it eventually enters the transitional zone where the water depth becomes less than half the wavelength. This is the point where the wave begins to “feel the bottom,” and the topography of the seafloor starts to influence its behavior.
Once the wave begins to interact with the seabed, friction acts as a brake, causing the wave to slow down. The velocity of a wave in this shallow water zone becomes directly proportional to the square root of the water depth. This means that as the wave moves into progressively shallower water, its speed continuously decreases. This physical principle—that shallower water leads to slower wave speed—is the foundation for the bending process seen at the shoreline.
Why Uneven Speed Causes Bending
Wave refraction occurs because the entire wave crest rarely hits the shallow area at the exact same moment. Most waves approach the coast at an angle, meaning one segment of the wave crest reaches the shallower water sooner than the rest of the wave that is still in deeper water. The segment of the wave that encounters the decreased depth immediately slows down due to friction with the seabed. The adjacent segment of the wave, which is still in deeper water, continues to travel at its original, faster speed.
This differential speed across the wave front causes the wave to pivot, or bend, toward the slower side. Imagine a line of soldiers marching from a paved road onto a patch of soft sand at an angle; the soldiers who hit the sand first slow down, while those still on the pavement continue at full speed, causing the entire line to wheel toward the sand. The faster part of the wave pivots around the slower part, forcing the wave crest to align itself more parallel to the underwater depth contours. This continuous adjustment ensures that by the time the wave reaches the immediate shoreline, it is traveling nearly perpendicular to the beach, regardless of its initial angle far out at sea.
How Refraction Shapes Coastlines
The bending of waves due to refraction plays a significant part in the shaping of coastlines by controlling where wave energy is delivered. As waves bend, their energy lines either converge or diverge depending on the shape of the shoreline. When waves approach a protruding landform, such as a headland, the refraction process causes the wave energy to become concentrated onto that feature. This concentration of energy leads to intense erosion, causing the headland to be worn away over time.
Conversely, when waves enter an indented area, like a bay, the refraction causes the energy lines to spread out. This dissipation of energy means the waves that reach the inner bay are weaker and less erosive. The reduced wave action in bays allows for the deposition of sand and sediment, which is why beaches are commonly found in these sheltered areas. Wave refraction acts as a natural mechanism that attempts to straighten an irregular coastline by preferentially eroding the exposed headlands and building up the sheltered bays.