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10 - 2

Anatomy of a Wave

Physics, mathematics, and space science lend themselves to explaining the behavior of waves and tides on Earth.

A wave is the transmission of energy through matter. Key word is “through.” When energy moves through matter as a wave, matter moves, but overall doesn’t shift forward or backward. It transmits the energy to adjacent matter, allowing the energy to continue.

There are three types of progressive waves: 1. Longitudinal waves move through all states of matter and occur

when energy moves in the same direction that the energy travels. 2. Transverse waves only transmit through solids. They occur

when the energy motion is perpendicular to the travel direction. 3. Orbital waves only transmit through fluids. They occur when

the energy moves the fluid in a circular motion as it passes. Crest – highest point above average water level. Trough – lowest point below average water level. Height – vertical measurement from trough to crest. Wavelength – horizontal distance between the identical point of two waves. Period – time it takes for the same spot on two waves to pass a single point. Frequency – the number of waves that pass a fixed point in one second.

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Speed = wavelength / period

This is sometimes abbreviated:

S = L / T

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10 - 3

Wave Causes and Characteristics

Disturbing forces cause waves – wind, gravity, and seismic activity. Restoring forces resist waves – surface

tension, gravity, and Coriolis effect. Deepwater waves occur in water that

is deeper than half their wavelength, the bottom does not affect their orbital motion.

Shallow-water waves occur in water that is shallower than half their wave-length, the bottom creates drag that affects their orbital motion.

Three factors affect maximum wave size: wind speed faster than the wave, wind duration (time wind blows), and fetch (surface area over which wind blows).T

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Wave Causes and Characteristics (continued)

A fully developed sea can have waves largeror smaller than the maximum theoretical size. If waves are in phase, wave energies are constructive

and combine into larger waves. If waves are out ofphase, they can cancel each other out.

Internal Waves Internal waves are found inside the ocean. They can occur within different

density layers. These waves can be as much as 30 meters (100 feet) tall. Internal waves are found below the surface. The wave motion in a deep layer can cause a thermocline

or halocline to slowly rise and fall as the wave passes. Scientists don’t exactly know what causes internal

waves. It is likely they get their energy from wind,gravity, or seismic forces, just like surface waves.

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Surf and Breaking Waves

In deep water, a wave breaks when its height exceeds one-seventh of its wavelength – H:L ratio exceeds 1:7. Due to drag from the seafloor, the bottom of the wave slows so that the top of the

wave is traveling faster than the bottom. This, and exceeding the 1:7 ratio, makes the wave break, toppling the upper part of the wave forward.

There are three basic types of wave breaks: 1. Plunging breakers are characterized by a curl as the top of the wave pitches

through the air before splashing into the bottom. They occur on moderately steep beaches.

2. Spilling breakers are characterized by the top of the wave tumbling and sliding down the front of the wave as it decelerates slowly. They occur on beaches with a gentle slope.

3. Surging breakers occur on very steep beaches that are almost like walls.Because they do not slow, they surge virtually unbroken and can be very destructive.T

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Surf and Breaking Waves (continued)

Waves rarely hit the shore squarely. Refraction, diffraction, and reflection affect wave behavior. On an irregular shoreline, refraction concentrates

wave energy toward protrusions because the sideof the wave nearest to the protrusion slows first,turning the wave toward it.

Wave diffraction occurs when waves hit an obstacle, such as a jetty. Energy shifts within the wave, allowing a new wave pattern toform past the obstacle or through an opening.

Reflection occurs when waves hit an abrupt obstacle that is nearly perpendicular in the water, such as a sea wall. Reflected wave energy can bounce around the sides of an enclosed area, creating complex wave patterns.

Reflection can cause a standing wave. This wave is a vertical oscillation in which water rocks back and forth, rising and falling at the ends, but relatively motionless near the center.

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Destructive Waves

1. Storm surge is a destructive wave that forms when high winds push water against the shore, where it piles up. The shallower the water offshore, and the further it extends offshore, the greater the surge.

2. Seiches form in bodies of water smaller than an ocean as a wave that rocks back and forth. It results from a strong wind that pushes the water level up on one side of a lake or basin.

3. Tsunamis result from sudden water displacement caused by a landslide, iceberg, volcanic eruption, or, more commonly, an earthquake. Tsunamis are always shallow-water waves.

Their wavelength is about 200 kilometers(120 miles), there is no ocean deep enoughto make a tsunami behave as adeepwater wave.

Tsunamis have very long periods and they areso huge that they are nearly imperceptible asthey travel. Boats at sea may rise and fall severalmeters as a tsunami passes, but cannot detectit had passed under them.

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The Cause of Tides

Tides result from the gravitational pull of the moonand, to a lesser degree, the sun.

Isaac Newton proposed the equilibrium theory as an explanation for the tides. His theory assumed that the Earth is perfectly uniform,

that water is very deep, and that there are no landmasses. The problem with this theory was that it was too simplistic.

Pierre-Simon Laplace modified Newton’s model to account for tidal variations. Laplace’s model is called the dynamic theory which shows

that there aren’t only two tidal bulges; rather, there areseveral tidal bulges.

Besides lunar and solar gravity, the imperfectsphere of the Earth, the season, the shape of theocean basin and the Coriolis effect, all influence the tides.

Tides rotate around more than a dozen amphidromicpoints. These are points where the water doesn’t rise and fall with the tides.

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Tidal Patterns and Currents

A diurnal tide is having a single low and high tidedaily (Gulf of Mexico).

A semidiurnal tide is having two roughly equal highand low tides daily (East coast of the US).

A mixed tide is having two unequal high and lowtides daily (West coast of the US).

The daily tides create a current that flows into andout of bays, rivers, harbors, and other restricted areas. The inflow of a tide is the flood current, the outflow is a

slack current, the midpoint is a slack tide.

A tidal bore forms when the incoming tide producesa wave that flows into a river or other narrow area.This is a true tidal wave and can be severalmeters/feet high in places (Amazon River).

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The Sun, Moon, and Types of Tide

When there’s a new moon (no moon visible),both the sun and moon are aligned on thesame side of the Earth.

During a full moon, the sun and moon arealigned on opposite sides of the Earth. Both positions create the highest and lowest

tides, called spring tides.

When the moon is in a quarter moon phase,it and the sun are at a right angle to the Earth. The sun’s gravitation pulls to the side of the

moon’s tidal bulge. This tends to raise the low tide and lower the

high tide. These weaker tides are called neap tides.

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