Waves

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Waves
Laird Hamilton riding the Wave (Riding Giants)
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Waves

• When undisturbed by wind (or some other factor
  such as an earthquake), the sea surface is
  naturally smooth!
• Waves are moving energy and begin as a
  disturbance
• Wind blowing across the surface of the ocean
  generates most waves
• Tides, turbidity currents, coastal landslides,
  calving icebergs, and sea floor movement can also
  cause waves

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Waves

• In an ocean wave, energy is moving at the speed
  of the wave, but water is not!
• Waves move energy, with very little movement of
  particles (including water particles!)
• The water associated with a wave does not move
  continuously across the sea surface!

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• Imagine a seagull resting on the ocean surface
• The bird moves in circles up and forward as the
  tops of the waves move toward its position, and
  down and backward as tops of the waves move past
• Energy in the waves flows past the bird, but the
  gull and its patch of water move only a short
  distance

Each circle is equal in diameter to the wave
height
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Orbital Waves

• As a wave travels, the water passes the energy
  along by moving in a circular path, called an
  orbit
• An wave in which water particles move in closed
  circles is called an orbital wave
• Because the wave form moves forward, orbital
  waves are a type of progressive wave

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Orbital Waves

• The bigger the wave, the larger the size of the
  orbit
• The diameter of the orbit diminishes rapidly with
  depth

Wave motion is negligible when orbits reach a
diameter that is 1/23 of those at the surface
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Wave motion is negligible below a depth of one
half of the wavelength
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Components of a Wave

• Ocean waves has distinct parts
• Wave crest highest part of the wave above
  average water level
• Wave trough lowest part of the wave below
  average water level
• Wave height the vertical distance between a wave
  crest and its trough
• Wavelength the horizontal distance between 2
  successive crests, or troughs

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Making Waves

• Ocean waves are classified by
• the disturbing force that creates them
• the extent to which the disturbing force
  continues to influence the waves once they are
  formed
• The restoring force that works to flatten them
• Their wavelength

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Making Waves

• Energy that causes waves to form is called a
  disturbing force
• Wind blowing across the ocean surface provides
  the disturbing force to generate capillary waves
  (waves lt1.73 cm) and wind waves
• Landslides and tectonic processes (volcanic
  eruptions, faulting of the sea floor) are the
  disturbing forces for seismic sea waves, or
  tsunamis

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Making Waves

• The restoring force seeks to return the water to
  flatness after a wave has formed in it gravity
  provides the restoring force on all waves gt1.73cm

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Wavelength is the most useful measure of wave size
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Deep vs. shallow water waves

• Waves moving through water deeper than ½ their
  wavelength are deep water waves
• Example A wind wave with a 20m wavelength is
  considered to be a deep water wave so long as it
  is passing through water gt10m deep
• Waves in water shallower than 1/20 their
  wavelength are shallow water waves
• Example A wave with a 20m wavelength will act as
  a shallow-water wave if the water is lt1m deep

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Shallow water waves

• When a wave approaches the shore, its proximity
  to the bottom flattens out the orbits of water
  molecules
• Causes the water at the bottom to move back and
  forth no longer in a circular pattern

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Ocean Motion

• In the ocean, only capillary and wind waves can
  be deep water waves
• Why???
• Remember, deep water waves occur when moving
  through water deeper than half their wavelength

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Gee, thats just swell

• Generally speaking, the longer the wavelength,
  the faster the wave
• When waves move away from their area of
  origination, wind speeds diminish and they
  eventually move faster than the wind
• Mature waves from a storm sort themselves into
  groups of waves with similar wavelengths and
  speeds as they outrun their smaller relatives

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Gee, thats just swell

• This results in swells uniform, symmetrical wind
  waves that have traveled out of their area of
  origination

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When waves meet

• Because longer waves will outrun shorter waves,
  wind waves from different storm systems can
  interfere with one another
• When waves meet, they add to or subtract from one
  another
• Such interaction is called interference
• Constructive additive
• Destructive subtractive (cancellation)

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Dude, constructive and destructive waves rule

• Surfers depend on constructive and destructive
  waves to generate their wave sets
• Constructive interference between waves of
  different wavelengths create the sought-after big
  waves
• Destructive interference diminishes the waves and
  makes it easier for the surfer to swim back out

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• Constructive crests of waves coincide
• Destructive crest and trough of waves coincide

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Going Rogue

• Occasionally, wind waves of many wavelengths can
  approach a single point/spot from different
  directions
• A huge wave crest develops suddenly from the
  constructive interference, generated a rogue wave
• Rogue waves are much larger than surrounding
  waves and can be extremely hazardous

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Just how big can waves be?
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Just how big can waves be?

• The size of waves depends on
• Wind strength
• Wind duration
• Fetch (distance over which the wind blows)
• A strong wind must blow continuously in one
  direction for 3 days for the largest waves to
  develop
• The greatest potential for large waves occurs
  beneath the strong and nearly continuous winds
  surrounding Antarctica

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Some personal experience
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Photo taken here (bridge)
50 ft
A photograph of a wave taken from the bridge 50
feet above sea level
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70-foot wave crossing the Southern Ocean
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Encountering The Perfect Storm just north of
Antarctica (Ross Sea, Southern Ocean)
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When ocean waves encounter land

• Deep water waves change to shallow water waves as
  they approach the shore
• Once the wave passes over water whose depth is
  less than one half its wavelength, the wave
  feels the bottom
• When this happens, the circular motion of the
  water molecules in the wave is interrupted
• Orbits flatten to ellipses near the bottom

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When ocean waves encounter land

• The waves energy must now be packed into less
  water depth, and so the wave crests become
  peaked, rather than rounded
• Interaction with the bottom slows the incoming
  wave, but waves behind it continue toward shore
  at their original speed
• This results in a bunching up of the waves,
  which decreases their wavelength, but increases
  their height

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Surfs Up!

• When the wave steepness reaches the 17 ratio
  (waves height is 7x it wavelength), the wave
  will break as surf

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How to score an epic wave

• Waves break along the shore in different ways
• Waves are influenced by
• The bottom slope (the steeper the slope, the more
  violent and toppling the wave)
• Contour and composition of the bottom (gradually
  shoaling bottoms sap waves of their strength as
  the wave loses energy interacting with the
  bottom)
• Localized winds and fetch

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Why is surfing so much better along the west
coast of the U.S. than the east? (no offense)
NOT Long Island
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Waves refract when they approach a shore at an
angle

• Waves usually approach the shore at an angle
• Different parts of the wave is at different
  depths, so the wave must bend, or refract as
  parts of the wave reaches shallower water and
  slows
• The slowing and bending of waves in shallow water
  is called wave refraction the waves refract in a
  line nearly parallel to the shore

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Waves refract when they approach a shore at an
angle
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Big Waves Storm Surge
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Big Waves Tsunamis

• The Japanese term for large, often destructive
  waves that occasionally roll into their harbors
  is tsunami (tsu harbor nami wave)
• Tsunamis originate from sudden changes in the sea
  floor caused by tectonic activity (undersea
  volcanic eruptions, faulting, collapse of large
  oceanic volcanoes) and even underwater avalanches
  such as those caused by turbidity currents

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Tsunamis

• The majority of tsunamis are caused by fault
  movement
• Underwater fault movement displaces the earths
  crust, generates earthquakes, and if it ruptures
  the seafloor, produces a sudden change in water
  level at the ocean surface (up or down)
• The wavelength of a tsunami is 125 miles, so it
  is a shallow water wave everywhere in the ocean

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Tsunamis

• In the open ocean, tsunamis travel at speeds gt435
  miles per hour
• Tsunamis in the open ocean have heights of only
  0.5 meters (1.6 feet)!
• However, once they approach the shore, they slow
  in the shallow water and increase in wave height
• Surges ashore mistaken for an extremely high
  tide and so mistakenly called tidal waves

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Abrupt vertical movement along a fault on the sea
floor raises or drops water column creating a
tsunami that travels from deep to shallow water
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Killer Waves

• 86 of all tsunamis occur in the Pacific Ocean
  (Why?)
• On December 26, 2004, an enormous earthquake
  struck off the coast of Sumatra in Indonesia
• Occurred 19 miles beneath the sea floor near the
  Sunda Trench, where the Indian Plate is being
  subducted beneath the Eurasian Plate
• Ruptured 750 miles of sea floor!

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Killer Waves

• This thrusted the seafloor upward, displacing gt30
  feet of water above it
• The resulting tsunami spread across the Indian
  Ocean, literally washing away many coastal
  villages and causing approximartely 300,000 human
  deaths in Indonesia (esp. Thailand) and along
  coastal India and Africa
• Although much smaller, the tsunami was also
  detected in the Atlantic, Pacific and Arctic
  Oceans!

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Indonesian capital of Banda Aceh (before tsunami)
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Indonesian capital of Banda Aceh (after tsunami)
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Sequence of photos of tsunami inundating Chedi
Resort in Phuket, Thailand on December 26, 2004