12.1: Waves and Wave Processes

Waves are created when wind blows over the surface of the water. Energy is transferred from wind to the water by friction and carried in the upper part of the water by waves. Waves move across the water surface with individual particles of water moving in circles. As the wave crest passes, the water is moving forward. As the wave trough passes, the water is moving backward. This can be demonstrated by watching the movement of a cork or some floating object as a wave passes.

Figure \(\PageIndex{1}\): Particle motion within a wind-blown wave.

Important terms to understand in the operation of waves include: the wave crest is the highest point of the wave; the trough is the lowest point of the wave. Wave height is the vertical distance from the trough to the crest and depends on the amount of energy carried in the wave. Even before reaching shore, wave height increases with increasing wave energy. Wave amplitude is half the wave height, or the distance from either the crest or trough to the still water line. Wavelength is the horizontal distance between consecutive wave crests. Wave velocity is the speed at which a wave crest moves forward, which is related to the energy carried by the wave. Wave period is the time interval it takes for consecutive wave crests to pass a given point.

Figure \(\PageIndex{2}\): Aspects of water waves, labeled.

The circular motion of water particles diminishes with depth and is negligible at about one-half wavelength, an important dimension to remember in connection with waves. Wave base is the vertical depth at which water ceases to be disturbed by waves. In water shallower than wave base, waves will disturb the bottom and ripple shore sand. Wave base is measured at a depth of about one-half wavelength, where the water particles’ circular motion diminishes to zero. If waves approaching a beach have crests at about 6 m (~20 ft) intervals, this wave motion disturbs water to about 3 m (~10 ft) deep. This motion is known as fair-weather wave base. In strong storms such as hurricanes, both wavelength and the maximum depth of water disturbance increase dramatically. The effective depth to which waves can erode sediment is thus called a storm wave base, which is approximately 91 m (300 feet) [1].

Figure \(\PageIndex{3}\): Diagram describing the wave base.

Waves are generated by wind blowing across the ocean surface. The amount of energy imparted to the water depends on the wind velocity and the distance across which the wind is blowing. This distance is called fetch. Waves striking a shore are typically generated hundreds of miles from the coast by storms and may have been traveling across the ocean for days.

Winds blowing in a relatively constant direction generate waves moving in that direction. Such a group of approximately parallel waves traveling together is called a wave train. As wave trains spread from different areas of generation, they may move in different directions and carry different amounts of energy. Interaction of these different wave trains produces the choppy sea surface seen in the open ocean. A wave train coming from one fetch can produce various wavelengths. Longer wavelengths travel at a faster velocity than shorter wavelengths, so they arrive first at a distant shore. Thus, there is a wavelength–sorting process that takes place during the wave train’s travel. This sorting process is called wave dispersion.

Figure \(\PageIndex{4}\): Model of a wave train moving with dispersion.

Behavior of Waves Approaching Shore

On the open sea, waves generally appear choppy because wave trains from many directions are interacting with each other, a process called wave interference. Constructive interference occurs where crests align with other crests. The aligned wave height is the sum of the individual wave heights, a process referred to as wave amplification. Constructive interference also produces hollows where troughs align with other troughs. Destructive interference occurs where crests align with troughs and cancel each other out. As waves approach the shore and begin to make frictional contact with the seafloor (i.e., water depth is a half wavelength or less) they begin to slow down. However, the energy carried by the wave remains the same so they build up higher. Remember that the water moves in a circular motion as the wave passes, with the water that feeds each circle being drawn from the trough in front of the advancing wave. As the wave encounters shallower water at the shore, there is eventually insufficient water in the trough in front of the wave to supply a complete circle, and the crest pours over creating a breaker.

Figure \(\PageIndex{5}\): Types of breakers

A special type of wave is called a tsunami, sometimes incorrectly called a “tidal wave.” Tsunamis are generated by energetic events affecting the seafloor, such as earthquakes, submarine landslides, and volcanic eruptions. In the case of earthquakes, tsunamis can be produced when the moving crustal rocks below the sea abruptly elevate a portion of the seafloor. Water is suddenly lifted creating a bulge at the surface and a wave train spreads out in all directions traveling at tremendous speeds [over 322 km/hr (200 mph)] and carrying enormous energy. Tsunamis may pass unnoticed in the open ocean because they move so fast, the wavelength is very long and the wave height is very low. But as the wave train approaches the shore, each wave begins to interact with the shallow seafloor, friction increases and the wave slows down. Still carrying its enormous energy, wave height builds up and the wave strikes the shore as a wall of water that can be over 30 m (~100 ft) high. The massive wave, called a tsunami runup, may sweep inland well beyond the beach destroying structures far inland. Tsunamis can deliver a catastrophic blow to people at the beach. As the trough water in front of the tsunami wave is drawn back, the seafloor is exposed. Curious and unsuspecting people on the beach may run out to see exposed offshore sea life only to be overwhelmed when the breaking crest hits.

Figure \(\PageIndex{6}\): All waves, like tsunamis, slow down as they reach shallow water. This causes the wave to increase in height.

Reference

1. Rich, J. L. THREE CRITICAL ENVIRONMENTS OF DEPOSITION, AND CRITERIA FOR RECOGNITION OF ROCKS DEPOSITED IN EACH OF THEM. Geol. Soc. Am. Bull. 62, 1–20 (1951).