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| Waves |
Wind creates waves. As an air current (moving stream of air) moves over an undisturbed water surface, friction between air and water creates a series of waves that move across the surface. The size of the waves depends on the wind speed, the duration of the wind, and the distance over which the wind blows. (The distance of open water surface that the wind blows over is called the fetch.) A week-long tropical storm in the Pacific Ocean might produce waves as tall as three-story houses; a ten minute gust blowing across a small lake might make waves that are only a few inches tall.
Waves move away from their point or area of origin in widening circles, like ripples moving away from a pebble dropped into a pool. In an ocean basin (the deep ocean floor), waves from many different wind events are moving across the sea surface at any given moment. When sets of waves meet they interact to form new patterns. By the time they reach the coastline, waves have been affected by many wind events.
Ocean waves may appear that the water is moving forward but in actuality the water is moving in a circle as the water molecules lift and fall. (A molecule is the smallest unit of a substance that has the properties of that substance.) Imagine floating in the ocean in a raft.
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When a wave approaches, you rise and fall as it passes, but you don’t move toward the beach. The same thing happens to the water molecules below you. As a wave arrives, the water particles rise and fall in small circles as the wave passes, but they are not carried forward. The highest point the wave reaches is called the crest.
The lowest point of the wave is called the trough. The wavelength is the distance from one crest to the next. The water molecules closest to the surface move in the largest circles, and deeper water moves less. Molecules below a depth known as wave base are undisturbed by a passing wave. Wave base is equal to half the horizontal distance between wave crests, or one-half a wavelength.
Breaking waves
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| Breaking waves |
Waves change form when they approach a coastline. When the seafloor is shallower than the wave base, it interferes with the circular motion of the water at the bottom of the wave. Waves that were broad, gentle swells in the open ocean grow taller and their crests get closer together. Eventually, the wave grows too tall to support itself and it breaks; the wave crest collapses over the front of the waves.
Spilling breakers that gradually become more steep and then crumble typically form along shallowly sloping shores. Plunging breakers that grow tall and curl sharply generally pound steep coastlines. Big waves start to break farther from shore than smaller waves because they have deeper wave bases.
Water from breaking waves sloshes forward up the beach. It returns in an outgoing current along the seafloor called undertow. The forward and back motion of water in the surf zone (area of rough water next to the land, where ocean waves hit the shore) between the breaking waves and the beach is called swash.
Like water in a washing machine, water in a surf zone endlessly cycles between the breakers and the beach. Beaches are subjected to relentless swashing that breaks down all but the most resistant sediments (sand, gravel, or silt). The mineral quartz is particularly strong, and beach sand is often composed of identical quartz grains that waves have rounded into perfect spheres and sorted by size.
Wave refraction
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| Wave refraction - San juanico baja california sur, Mexico |
Waves bend when they reach coastlines. It is extremely rare for wind to blow exactly toward a perfectly straight coastline, and waves almost always approach shorelines at an angle. Wave bending or refraction occurs because the end of a wave that reaches shallow water first slows down and breaks before the deeper end. Water moving in the surf zone flows sideways along the beach from the direction of the approaching wave, and gravity pulls the returning water directly downhill.
Water and sediment thus move in a zigzag pattern that carries them along the beach. Wave refraction produces longshore currents, which are currents that flow parallel to coastlines in shallow water. If you have ever dropped your towel on the beach and gone for a swim only to discover that you have been carried away from your towel, you have experienced a long-shore current.
Wave refraction also brings the eroding power of waves onto headlands, the jagged, rocky, narrow strips of land that extend into the ocean. Longshore currents carry the eroded sediment away from headlands and deposit it in bays. Waves thus, straighten irregular coastlines by wearing down the headlands and filling the bays. A typical arc-shaped bay with headlands at each end has two longshore currents that flow from the headlands toward each other.
The shallow, strong, outgoing current that forms at the tip of the bay where they meet is called a rip current. Rip currents also form where large waves pile water between a sand bar (a ridge of sand built up by currents) and a beach. Rip currents, can be dangerous to swimmers because they can form or become strong suddenly.
Waves and longshore currents can also mold sand into strings of barrier islands (a long, narrow island parallel to the mainland) formed from sediment deposits. These islands are often called depositional coastlines. In the Gulf of Mexico, waves have washed sand from the Mississippi River Delta.
Longshore currents have deposited the sand in a long streamer of barrier islands along the Louisiana and Texas coastlines. Tidal inlets (inlets maintained by the tidal flow) separate barrier islands from each other and shallow bays called lagoons separate them from the mainland. The barrier island of the United States eastern seaboard, included the Outer Banks of the Carolinas, formed in a similar fashion.
Wave patterns and coastline conditions are constantly changing, and coastline features are continuously remolded. The waves from a large hurricane, for example, can completely destroy a barrier island, beach houses and all, and reshape a new one in a matter of days.