Waves

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Waves

• Chap 10

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• The energy of the wind is transferred to the
  water.
• The wind blows over the water and piles it up
  into waves.
• The stronger (faster) the wind, the bigger the
  waves.
• The longer distance over which the wind can blow
  ('fetch'), the bigger the waves.
• The more days a given wind blows (time), the
  bigger the waves.

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Waves have identifiable parts

• crest (p.222, f.10.2-3)
• trough
• wavelength (L)
• wave height (also called amplitude) (H)
• period
• celerity

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• In deep water, longer wavelengths and longer
  periods make for faster waves (greater celerity).
  
• Waves generally move in sets, rather than as
  single waves.
• These sets of waves are called wave trains.

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How water moves in a wave?

• Waves don't actually move water from one place to
  another.
• Water in a wave moves up and down.
• The water moves up as a crest passes, and moves
  down as a trough passes.
•  Each molecule of water draws a circle as one
  complete wave (crest to crest) passes.

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• waves are moving from left to right (p.233,
  f.10-1,3)
• As the wave crest approaches the seagull, the
  bird moves UP.
• As the trough approaches, the bird moves DOWN.
• There is a small amount of side-to-side motion
  (more with longer wavelengths), but mainly the
  water motion is up and down.
• This is called orbital motion.

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• The circle (orbit) of water motion at the
  surface sets in motion another orbit of water
  beneath it.
• However, some of the energy from the first orbit
  is lost, so the next orbit is somewhat smaller
• Eventually the orbit of motion gets so small, it
  disappears altogether, and there is no motion.

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• This water depth is called wave base and is
  usually 1/2 the wavelength.
• This means that waves in deep water do not touch
  bottom!!! They do not touch bottom unless the
  water depth is less than 1/2 the wave's
  wavelength.
• Waves start out in the open ocean, and eventually
  move toward land, across the continental shelf
  and up onto the beach.

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• As waves approach the beach, water depth
  decreases (it gets shallow!), and
• waves start to touch bottom where water depth is
  around 1/2 the wavelength.
• When waves begin to touch the seafloor, friction
  starts to distort the perfect circles of the
  orbits.
• The orbits change from circles to ellipses
  (oval-shaped)

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