Wave refraction is the bending of waves as they approach the shoreline, a phenomenon that shapes coastlines around the world. This change in wave direction occurs because the wave’s speed is altered when it moves from deep water into shallower areas. The effect is similar to how a car slows and turns when one wheel encounters mud while the other remains on pavement. Understanding this physical process requires focusing on the relationship between water depth and wave velocity.
How Changing Water Depth Governs Wave Speed
The speed at which an ocean wave travels is determined by its wavelength and the depth of the water it is moving through. In deep water, where the depth is greater than half the wave’s wavelength, the speed is independent of the seafloor and governed by the wave’s length. These are called deep-water waves, and their energy travels without interference from the bottom.
As the wave approaches the coast, the water depth decreases to less than half the wave’s wavelength. The wave transitions into a shallow-water wave, and its speed becomes directly proportional to the square root of the water depth. Consequently, the wave slows down as the depth lessens. This dependence on depth is the cause of wave refraction, as different parts of the same wave crest travel at different speeds.
The Physical Mechanism of Shoaling
The process of a wave changing characteristics as it enters shallow water is known as shoaling. When a wave moves into depths less than half its length, the orbital motion of water particles near the bottom interacts with the seafloor. This contact introduces a frictional drag force that slows the wave’s forward velocity. This slowing effect is not uniform across the entire wave front, which leads to the bending action.
If a wave crest approaches the shore at an angle, the portion reaching shallower water first experiences this frictional slowing. The rest of the wave crest, still in deeper water, continues to move at its original, faster speed. This speed differential causes the wave crest to pivot or bend toward the area of lower speed. Refraction continues until the crest is nearly parallel to the underwater depth contours and the shoreline. As the wave slows, its wavelength decreases and its height increases significantly to conserve the energy flux.
Coastal Impacts of Wave Bending
Wave refraction systematically redistributes wave energy along a coastline, which is a major factor in shaping coastal features. When waves encounter an irregular coastline featuring headlands and bays, the bending mechanism focuses the wave energy. The shallow water surrounding protruding headlands causes wave crests to slow down and converge on the landform. This convergence concentrates high wave energy on the headlands, leading to intense erosion.
Conversely, wave energy diverges within the recessed areas of the bays. As wave crests enter the deeper water of the bay, the energy is distributed over a wider area. This divergence results in significantly lower wave energy within the bay, creating calmer waters. Sediment deposition is favored in these calmer areas. Over long geological timescales, this consistent pattern of energy concentration on headlands and dissipation in bays works to straighten the irregular coastline.