9.13: Chapter Summary

Complexity of Ocean Waves. 

Waves usually seem to come in haphazard sequences of heights and periods. The reason is wave interference, the combination of waves of several different periods and from different directions.

Wave Motion. 

Water within a progressive wave moves in almost circular orbits whose diameters decrease with depth and are almost zero at a depth of half the wavelength. There is a very small net forward motion of water with the wave. Waves are created when the water surface is displaced. Once started, the wave motion is sustained by a restoring force that tends to return the sea surface to a flat, level state. Gravity is the principal restoring force for most ocean waves, and surface tension is important for very short-period capillary waves. The Coriolis effect affects very long wavelength waves, such as tides and tsunamis.

Tides are created by gravitational attractions of the sun and moon. Some ocean waves are caused by earthquakes or the passage of vessels. Most ocean waves are created by winds. Winds blowing over a calm water surface create tiny capillary waves that increase sea surface roughness. The wind increases the wave height and causes capillary waves to be combined into waves with longer wavelengths.

Capillary waves are dissipated quickly as energy is lost through water viscosity. Most ocean waves are gravity waves and lose little energy until they reach the shore, break, and dissipate much of their energy as heat.

Wave Breaking and Wave Height. 

Winds are usually variable, causing seas with waves of many different wavelengths and heights. If winds are strong and blow long enough, waves begin to break when their height is one-seventh of their wavelength. Maximum wave heights are determined by the wind strength and duration and by the fetch. Wave heights up to 34 m or more have been reported. Wave period and wave steepness increase when waves travel directly into a current.

Wave Trains and Wave Dispersion. 

Progressive waves travel in groups called “trains” that move at half the speed of the individual waves. The leading wave in the train is continuously destroyed and a new one created at the rear. Wave trains having waves of different wavelengths combine to produce the confused sea state that is characteristic of storms. The speed of deep-water waves increases with wavelength, so waves of different wavelengths move away from a storm at different speeds. Thus, waves are sorted by wavelength dispersion as they move away from a storm. As wavelength dispersion occurs, wave steepness declines and a swell is formed.

Waves in Shallow Water. 

When waves enter water shallower than half their wavelength, the water's orbital motion encounters the seafloor. As a result, wave orbits are flattened and both wave speed and wavelength are reduced, while period remains unchanged. Waves continue to slow as they enter progressively shallower water until the water depth is less than one-twentieth of the wavelength, after which the speed is determined only by depth. Waves are refracted in shallow water because any part of the wave in shallower water is slowed more than any part in deeper water. Refraction tends to turn waves to align parallel to the shore, focuses wave energy on headlands, and spreads energy within bays. Waves are reflected off barriers such as seawalls, and they are diffracted when part of the wave passes a barrier that blocks the rest of the wave.

As waves enter shallow water, wave height slowly decreases until water depth is about one-tenth of the wavelength and then increases rapidly as depth decreases further. Waves break when they become oversteep. There are four types of breakers: spilling breakers, in which the crest tumbles down the front of the wave; plunging breakers, in which the crest outruns the bottom of the wave and curls over before crashing down; collapsing breakers, in which the front face of the wave collapses; and surging breakers, in which the water simply surges up and down the shoreface.

Waves transport water forward onto the beach. Water returns offshore through narrow rip currents at intervals along the beach. Rip currents are a major cause of drownings, but even weak swimmers can survive them if they are aware that rip currents are narrow.

Tsunamis and Storm Surges. 

Tsunamis are trains of very long-wavelength (100 to 200 km) waves with periods of 10 to 30 min created by earthquakes or other such events that abruptly displace a section of seafloor. Tsunamis travel at speeds in excess of 700 km•h–1 but behave as shallow-water waves because their wavelength far exceeds the ocean depths. Tsunamis are rarely more than a meter or two in height until they enter shallow coastal waters, where their height builds rapidly. Tsunamis can do tremendous damage because they pour onshore for 10 min or more before the wave crest passes.

Hurricanes and other major storms can cause storm surges in which the water level ahead of the storm is elevated. Storm surge can reach far inland of the normal high water line.

Rossby and Kelvin Waves. 

Rossby waves are very low-amplitude waves that move slowly from east to west, have wavelengths of tens or hundreds of kilometers, and are responsible for periodic but irregular variations in weather along their track. Kelvin waves flow along the coastlines of the ocean basins—counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere, and directly west to east at the equator.

Internal Waves. 

Internal waves form on pycnoclines. They have long periods and wavelengths, have large heights in comparison with surface waves, and tend to break near the edges of continental shelves, where they enhance the vertical mixing of water.

Standing Waves. 

Standing waves are formed in basins where the ends of the basin prevent progressive waves from passing. At the node of a standing wave, the water surface does not move vertically but there are horizontal reversing currents. At the antinodes, water motion is vertical and there are no horizontal currents.