• Wave size, speed, and direction are controlled by winds.
• The wave shape moves but the water within it does not travel.
• Wavelength is the distance between adjacent waves.
• Waves increase height and break along the shore when the depth of water diminishes to less than the wave base.
• Waves are refracted toward headlands.

Wave Motion
horeline erosion is tied to the interaction between waves and the coastline. Wave action erodes, transports, and redistributes sediment along the shoreline. Wave size, speed, and direction are controlled by winds. Water does not travel with waves but simply moves vertically, tracing a circular path as a wave passes. It is the shape of the wave (the waveform) that moves across the ocean surface, not the water itself (Fig. 18). Consider the "wave" performed by a crowd at a sporting event. The wave passes around the stadium as each individual in turn stands up and sits down. The people in the stadium play the role of the water particles in ocean waves.

Figure 18. Animation of wave motion in open water. The waveform moves while water particles follow a circular path and remain in place.

The distance between adjacent wave crests is termed the wavelength. Wave motion only affects the surface waters. Wave motion decreases downward, with increasing distance from the winds, to the wave base. The depth of the wave base is approximately half the wavelength (Fig. 19).

Figure 19. Water particle motion  decreases downward from waves on the ocean surface ending at the wave base.

Waves and Coastlines
Water piles up as waves approach shore because of the effect of friction between the wave and the seafloor above the wave base (Fig. 20). Water in contact with the seafloor is slowed by friction but water on the surface is unaffected and moves forward more rapidly, encroaching on preceding waves and piling up to form taller, steeper waves.

Figure 20. Wave height increases and wavelength decreases as waves approach shore. Steep waves eventually collapse (break) forming surf that surges up the slope of the shore.

The wave eventually collapses (breaks) forming surf that washes up the shore before flowing back down to sea. Turbulent flow in the surf zone, between the line of breaking waves and the shore, can cause erosion by washing sand particles from beaches and breaking rocks from headlands. Some of this material may also be transported along the shore by currents in the surf zone.

Waves approaching a coastline are reoriented to follow the slope of the seafloor; this process is termed wave refraction. The seafloor will shallow more rapidly toward headlands than adjoining bays and waves approaching a rugged coastline will be refracted toward the resistant headlands (Fig. 21). One result of this pattern is that wave erosion is concentrated on headlands while adjoining bays become areas of deposition. Sediment eroded from the headlands are deposited in the relatively quiet waters of the bays to form beaches. The coastline is straightened as erosion wears away the headlands and the bays are filled with sediment (Fig. 21).

Figure 21. Wave erosion is concentrated on headlands and bays become areas of deposition. The coastline is straightened as erosion continues as the headlands are eroded back and the bays are filled with sediment.

Renewed tectonic activity may result in uplift of the coastline and another cycle of erosion. The rugged coastline of Oregon and Washington is constantly revitalized as part of an active plate boundary separating North America and the small Juan de Fuca Plate. The coastline is dominated by rocky headlands separated by small (pocket) beaches.

Wave erosion associated with large storms can remove large sections of beach in a single storm (Fig. 22). Recent hurricanes on the East Coast and El Nino-induced storms along the West Coast were responsible for substantial coastal erosion.

Figure 22. Beach erosion near San Diego, California, following winter storms (1997) associated with the El Nino weather phenomenon. Left image taken in October 1997; right image from April 1998. Images courtesy of USGS Center for Coastal Geology.