Chapter 9: Waves

What is wave motion?

In deep water, each water molecule within a progressive wave moves in a circular path and only energy, not water, is transported forward

difference between a "sea," "calm," and "swell"

Because winds are always variable in time and space, waves of various heights, wavelengths, and directions of travel are formed in windy areas. This confused sea surface state is called a "sea," as opposed to a "calm" (when there are no waves), and a "swell" when the waves are generally smooth and mostly of the same wavelength

wave energy

In the wave motion, energy is continuously converted between kinetic energy and potential energy. The total energy per unit of area of a progressive wave is proportional to the wave height squared. When wave height is doubled, wave energy is quadrupled.

progressive waves

The wave form of progressive waves moves across the surface of the water. These waves can be characterized by their wavelength, period, frequency, wave speed, wave height, and wave steepness, which are related by simple formulae

are wave heights increasing?

There is evidence that wave heights have increased, at least in some areas, in recent decades. It is not known whether this is due to a natural cycle or to greenhouse-induced global climate change

effects of currents on wave height

Waves become steeper when they flow into an area where the current flows in the opposite direction to the wave direction. Large, steep sided waves in such areas can cause even large ships to break apart and sink

what is the principal restoring force on most ocean waves?

gravity

where do areas of persistent wave activity occur?

in the trade wind and westerly wind zones

what does displacement of the ocean surface cause? how does this move the wave's energy?

it causes a horizontal pressure gradient to be formed between points under the wave crest and points both back and forward toward the trough. Thus, half of the wave's energy is transported forward and half is transported backward

what happens to most wave energy when the wave breaks on a shoreline? can this cause any significant changes in the water? why or why not?

it is dissipated as heat. the amount of heat released is not sufficient to cause a significant temperature change in the water because of water's very high heat capacity

what is the principal restoring force on capillary waves (on very short waves)?

surface tension

what is the principal restoring force on very long waves?

the Coriolis effect

wave frequency

the number of rests that pass per unit of time; the inverse of wave period

what happens when waves reach a height of about 1/7 of their wavelength in deep waters?

they become oversteepened and unstable and break to form whitecaps

do capillary waves lose energy slowly or quickly? why?

they lose energy very quickly due to the action of surface tension

wave period

time between successive crests; the inverse of wave frequency

capillary waves

very short waves

what does wave speed depend on in deep water (where depth is greater than half the wavelength)

wavelength

wave speed equals...

wavelength divided by period OR frequency times wavelength

what does the speed of a deep water wave depend on?

wavelength. it is equal to 1.25 times the square root of the wavelength, or 1.5625 times the period squared

factors affecting maximum wave heights

wind speed, duration, and fetch (distance over which the wind blows). Max heights are only reached when winds blow long enough. If the winds continue to blow beyond this period of time, wave heights do not increase further and energy is lost by breaking waves at the same rate as it is added by the wind. This is a fully developed sea.

forces that create waves

Most ocean waves are created by the action of winds on the sea surface, and these waves generally have periods up to about 30 seconds. Waves with longer periods (up to about two hours) can be caused by earthquakes, seafloor slumps, and turbidity currents. These are called tsunamis. Tides are waves with periods of about 12 or 24 hours that are caused the gravitational attraction between the Earth, moon, and sun

restoring forces

Once the water surface is displaced from its mean level, a restoring force tends to return the surface to level and this causes the wave form to move

storm waves

Storm waves normally all travel away from the storm in the same general direction. However, they tend to spread somewhat laterally. Total energy is not changed but the lengthening of the crests as the wave spreads reduces wave height and the energy per unit length of wave crest

motion in deep water waves

The diameter of the circle in which water within the wave moves is equal to the wave height at the surface and decreases with depth below the wave. At a depth of one-ninth of the wavelength, the circle is reduced to one half of the diameter at the surface. Wave motion is essentially reduced to zero at a depth below the surface equal to one half the wavelength; this is called the depth of no motion

wind waves

Winds blowing across the ocean surface can create or build waves primarily through the shear stress (friction) between the moving air and the water surface. Initially very short wavelength capillary waves are formed. These have rounded crests and V-shaped troughs Capillary waves die out very quickly due to surface tension unless the winds continue to blow. Once capillary waves have been formed, winds can build the waves by pushing on the elevated windward side of the wave and by creating a low pressure area on the side of the wave away from the direction that the wind blows. This mechanism is similar to the lift created by airflow over an airplane wing

sea development and wave height

Winds create and build waves into a sea that has a spectrum of waves of different heights and wavelengths. The waves are initially small in both wavelength and height but progressively build to greater heights and longer wavelengths. As the waves build, their shape changes progressively until they become trochoidal—pointed crests and rounded troughs.

highest wind wave reliably reported?

about 34 m (40 m and higher have been detected indirectly under major hurricanes)

why do most waves lost energy very slowly? what does this allow them to do?

because of internal friction, which allows them to travel very large distances with little loss of energy

why are the highest waves in the Pacific Ocean generally higher than those in other oceans?

because of the large fetch within the east-west wind belts

why do high waves develop in the westerly wind belt around Antarctica?

because the fetch is not limited by landmasses

why do waves seen at the shore often appear to arrive in complex patterns of varying height and period?

because the observed waves are the sum of a number of simple waves of different wavelengths

wavelength

distance between two crests

wave amplitude

equals twice the wave height; equals the vertical distance between crest and trough