Oceanography: waves

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Oceanography waves

• Adam F. Sprague

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Parts of a wave

• The section of the wave that rises above the
  undisturbed position is called the crest.
• That section which lies below the undisturbed
  position is called the trough

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Parts of a wave

• the amplitude is the maximum positive
  displacement from the undisturbed position of the
  medium to the top of a crest.

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Parts of a wave

• the wavelength exists between any point on a wave
  and the corresponding point on the next wave in
  the wave train

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Waves and wind

• What determines wave height?
• Fetch, Speed

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fully developed sea

• When the wind blows sufficiently long from the
  same direction, the waves it creates, reach
  maximum size, speed and period beyond a certain
  distance (fetch) from the shore.
• Because the waves travel at speeds close to that
  of the wind, the wind is no longer able to
  transfer energy to them and the sea state has
  reached its maximum.

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Big Waves

• The biggest waves on the planet are found where
  strong winds consistently blow in a constant
  direction. Such a place is found south of the
  Indian Ocean, at latitudes of -40º to -60º, as
  shown by the yellow and red colors on this
  satellite map. Waves here average 7m, with the
  occasional waves twice that height!

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Wave Speed

• Waves speed is related to its wavelength and
  period.

• SpeedLength of wave/wave period or
• CL/T

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Deep water wave

• Deepwater waveD1/2WL

• Lg/2?(T)squared
• Or L1.56m?secsquared (T)squared
• CL/T or
• C1.56T or
• Csquared1.56L

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Wave steepness

• SteepnessHeight/Length or SH/L
• If Height to Length exceeds 17 the wave becomes
  to steep and starts to break.

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Waves entering shallow water

• As waves enter shallow water, they slow down,
  grow taller and change shape. At a depth of half
  its wave length, the rounded waves start to rise
  and their crests become shorter while their
  troughs lengthen. Although their period
  (frequency) stays the same, the waves slow down
  and their overall wave length shortens. The
  'bumps' gradually steepen and finally break in
  the surf.

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Interaction with sea floor

• As a trough advances the particles slow and start
  to drop
• Water particles slow their backward speed and
  start to rise
• The motion of rising and falling creates
  orbitals
• Orbitals are what interact with approaching wave
  to shore causing it to break

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Shallow Water waves

• Lwavelength, D Depth, C Speed, G gravity,
  Ttime
• D
• CvgD or C3.13vD or L3.13vDT

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Waves entering shallow water

• The photo shows waves entering shallow water at
  Piha, New Zealand. Notice how the wave crests
  rise from an almost invisible swell in the far
  distance. As they enter shallow water, they also
  change shape and are no longer sinus-like.
  Although their period remain the same, their
  distance between crests and their speed,
  diminish.

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Surf breaker types

• Spilling breakers result from waves of low
  steepness (long period swell) over gentle slopes.
  They cause rows of breakers, rolling towards the
  beach. Such breakers gradually transport water
  towards the beach during groups of high waves.
  Rips running back to sea, transport this water
  away from the beach during groups of low waves.

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Surf breaker types

• Plunging breakers result from steeper waves over
  moderate slopes. The slope of a beach is not
  constant but may change with the tide. Some
  beaches are steep toward high tide, others toward
  low tide. A plunging breaker is dangerous for
  swimmers because its intensity is greatly
  augmented by backwash from its predecessor.

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Surf breaker types

• Surging breakers occur where the beach slope
  exceeds wave steepness. The wave does not really
  curl and break but runs up against the shore
  while producing foam and large surges of water

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Tsunami and seismic sea waves

• Tsunami are caused by rapid displacement of ocean
  water usually from
• earthquakes (called seismic sea waves)
• underwater volcanoes
• calving (breaking off) of icebergs
• landslides (underwater)

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Tsunami

• Tsunami are shallow water waves, because their
  wavelength is so long (up to 200km), ½ wavelength
  would be deeper than the ocean. Because they are
  shallow water waves, they are affected by the
  contour of the bottom and can be refracted in
  unexpected ways.

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Tsunami

• Tsunami waves in the open ocean are pretty much
  unnoticeable. They reach a height of only 1-2
  feet in the open ocean. Once the waves reach
  shore, however, they are very destructive.
• Tsunami waves travel very fast (speeds up to 470
  miles per hour),

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Indonesia
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