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# Ocean Waves

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## Clio Macrakis

on 25 February 2014

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#### Transcript of Ocean Waves

Ocean Waves
Generating Force
- a pulse of energy that disturbs the water
(e.x. Wind, Seismic Disturbances, Displacement)
Restoring Force
- the force that restores water to its undisturbed state
(e.x. Gravity, Surface Tension)
The Basics (vocab)
Gravity Waves
Waves that are big enough that gravity is a bigger restoring force than surface tension
Ripples (Capillary Waves)
Made by slight gusts of wind
Patches can be seen moving as the wind moves (sailors call this
cat's-paws
)
As the surface of the water gets rougher, the wind affects it more
A Wave
Wavelength
Crest
Amplitude
Trough
Undisturbed water level
(equilibrium surface)
Period - the time it takes to get from one crest to the next
Wave Speed
speed = (length of wave)/(wave period)
The speed of a formed wave may change, but the period does not change throughout the life of the wave
C = L /T
C is the celerity, or speed of the wave
Deep-Water Waves
Deep-water waves occur when the water is deeper than 1/2 the wave's length
Storm Centers
Progressive Wind Waves
L = (g/2π) T^2
Because of this equation, we
can determine wave speed from period or wavelength
This means that the
orbits in the water do not extend to the ocean floor.

A wave that has a wavelength of
15 m has to occur in water deeper than 7.5 m for it to be considered a deep water
wave.
On Earth, the wavelength L is 1.56T ^2
(because g=9.81 on Earth)
C = L/T = (g/2π) T
C = 1.56T
C^2 = L^2/T^2 = (g/2π) L
C^2 = 1.56L
(normal waves, started by wind and flattened by gravity)
Some waves are generated in storm centers
Waves move outward from a storm center
When storms follow waves, waves get higher
Irregular wave patterns occur in storm centers - this is called a "sea"
Forced waves are waves that continue to grow while they are being generated. Once away from the storm, wind-driven forced waves become free waves
Long waves moving away from a storm center move faster than the short waves
Dispersion
Sorting, or dispersion,
is the separation of different-lengthed waves.
Wave Trains
are groups of smaller waves with approx. the same period and speed.
Storm center
Intersecting waves
pass through each other

then continue on as they were before.
Waves coming in at right angles create a checkerboard pattern
Waves moving directly at each other combine to create higher waves or somewhat flatten the waves
Wave Height
Wave Steepness
Wave Energy
Episodic Waves
Potential Energy
Kinetic Energy
-- Change in elevation at the water's surface
-- Motion of the wave orbits under the surface
~ equal amounts of potential
and kinetic energy
Affected by wind speed, duration, and fetch
Fetch: the distance that wind blows in the same direction
High waves need long periods of fast wind going in the same direction
Significant wave height: The average wave height of the tallest third
(Typical fetch in a storm is about 920 km, or 500 nautical miles)
Abnormally high waves
Occur because of intersecting waves, changing depths, and currents
Generally very short
Not much known about them, no witnesses
Height divided by length
Waves break if the height exceeds 1/7 their length
White caps - waves with short wavelengths and wind speeds 8-9 m/s
Shallow Water Waves
(Remember C=L/T, meaning period never changes)
C = 3.13*sqrt(D) L = 3.13*T*sqrt(D)
Refraction
Reflection
Diffraction
Shallow water waves move slower than deep water waves.

Wave rays show the direction the waves are going.
The barrier must be in a place where the waves will not break before for the waves to reflect.
A focus point can damage anything around it.
Interfering diffraction patterns can intersect and form checkerboard patterns.
The Surf Zone
Rapidly slowing waves
Steeper and breaking waves
Breakers
Orbital movement at the crest is not slowed as much as at the base of the wave.

This is a
plunger
. The other type of breaker is the more usual
spiller
.
Tsunamis
(Seismic Sea Waves)
In Japanese, tsunami means harbor wave
A sudden rise in the Earth's crust (convergence) causes displacement as a wave
A sudden dip in the Earth's crust (subduction) causes water to rush into the resulting dip in the ocean surface, making a wave
In both cases, the waves have wavelengths long enough for the orbits to touch the bottom of the ocean. The speed depends on the depth of the water (C = 3.13*sqrt(D)) and the magnitude of the displacement
A tsunami presents little to no danger on the open ocean
If a trough leads the tsunami, water at the shoreline may receed quickly
The Pacific Ocean is a common starting point for tsunamis
On April 1, 1946, a series of tsunamis originating by Alaska heavily damaged Hilo, Hawaii. The tsunamis managed to swing around the islands and hit the other side as well because of refraction and diffraction.
Internal Waves
Slower than surface waves
Periods usually longer
Deeper water can sometimes break the surface
C ^2 = g/(2*pi)*L*((p-p')/ (p+p')
p = lower layer density
p' = higher layer density
P
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Standing waves
Waves that reflect on themselves and appear to be consecutive crests and troughs
Nodes are where the wave height never changes
Maximum water flow when surface is flat
Seiches are standing waves that occur in basins. (Seiching is also a verb)
May be started by earthquakes
Clio Macrakis
hello
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