The dominant agents of erosion in coastal environments are waves. Driven by wind and tidal action, waves continuously erode, transport, and deposit sediments along ocean coastlines. The sand is also continuously moved parallel to the beach by longshore currents and is frequently deposited in harbors, where it must be periodically dredged to keep the harbor open for commercial shipping.
Waves move because the surface of the water gains energy from winds that blow over it. Short waves tend to be produced by local storms; long rolling swells are generated by large, distant storm systems up to thousands of miles away. The surf is that zone where waves break against the shoreline. The energy of the waves then sorts the sand and moves it along the beach. Beaches expand or dwindle according to changing coastal conditions. The restless ocean and its waves make many coastal landforms fragile and short lived. Wave energy is dependent on weather conditions, the length of the waves, wind speed, the duration of the wind, and the distance the wind travels over open water (fetch).
Wave height. The tops of waves are called crests and are separated by the lowest points, called troughs. The most powerful waves have the greatest wave height. Wave height is the vertical distance between the crest and the trough. Normal waves can be nearly 5 meters (15 feet) high; severe tropical storms can generate waves up to 15 meters (50 feet) high.
In rare cases, wave energy is derived from a submarine earthquake. Called a tidal wave, seismic sea wave, or tsunami, these gigantic walls of water can be as high as 90 meters (300 feet) and do tremendous damage to coastlines and cities.
Wavelength. The wavelength is the horizontal distance between two adjacent crests or two adjacent troughs (Figure 1). Typical wavelengths vary from about 30 to 300 meters (100−1,000 feet), and waves move at speeds up to 50 miles per hour. The depth of the wave motion is about half the wavelength; for example, if the wavelength is about 150 meters, 75 meters below the wave crest the water is calm.
Figure 1
Waves
Wave movement. Individual molecules of water are not physically transported with the waves as they move across the surface. The energy of the wave passes through the water molecules and does not carry them along. At the surface of the wave, a water particle moves in a roughly circular, vertical orbit; the radius of the orbit is equal to about half the wave height. During the passing of the wave, the water particle follows a circular path and returns to its original position after the wave has passed. The deeper the water particle is from the surface of the ocean, the smaller is its orbit. Water particles at depths greater than half the wavelength have essentially no motion generated by surface waves. Waves in the open sea are called waves of oscillation because of this orbital motion.
The circular orbits of water molecules are flattened into oval patterns as the wave approaches the shallow water near the shore. Friction with the bottom begins to slow the wave down, and the upward slope of the bottom pushes the water upward to form higher waves. A high wave in which the crest falls forward in front of the main body of the wave is called a breaker. At this point, the waves have become waves of translation. As the water crashes onto shore, its motion is controlled by the back and forth energies in the surf zone. The still‐turbulent sheet of water that sweeps up the slope of the beach is called the swash; the lower‐energy water that flows back down the beach into the surf zone is called the backwash.
Waves generally approach the shore at an angle. The end of the angled wave closest to shore reacts to the decreasing depth by slowing, while the other end of the wave continues at full speed. Consequently, as the depth decreases, the wave crest bends to become more parallel with the shore. This process is called wave refraction.
Longshore currents and rip currents. Wave action continuously moves sand across or along the beach in the surf zone. Even after refraction most waves are still not exactly parallel to the shore. The push of these waves creates longshore currents, which carry sand parallel to the coastline and roll pebbles and gravel along the bottom. Longshore currents are usually quite strong and transport most of the sand in the shoreline environment. Generally, 1 to 2 million tons of sand are moved along a single beach environment every year.
A rip current is water that flows straight back out to sea after its waves have broken on the beach. These currents are most prominent immediately after a large set of waves has broken and tend to develop where wave heights are lower. They flow quickly back through the surf zone and dissipate in the open ocean. Rip currents look like fingers of discolored, muddy water that extend through the surf zone. Being caught in a rip current can frighten even the most experienced swimmers. Because the currents tend to be narrow, a person can swim out of one by swimming parallel to the shore across the current, not toward the shore against it.