Waves

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Chapter 10

• Waves

2
Wave Formation

• Generating Force
• Restoring Force
• Surface tension
• Gravity

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Wave Formation

• Gravity waves
• Generating Force
• Wind
• Ripples
• Capillary waves
• Cats-paws

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Anatomy of a Wave

• Crest
• Trough
• Wavelength (L)
• Wave height (H)
• Amplitude 1/2H

• Period (T)
• Seconds/cycle
• Frequency
• Cycles/second

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Wave Motion

• As waves pass, wave form and wave energy move
  rapidly forward, not the water!
• Two basic motions associated with waves
• Forward motion
• Orbital motion

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Orbital Motion

• As wave approaches, a water particle will trace a
  circular path
• Diameter of orbit at surface wave height

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Wave Base

• Wave particle orbits are not detectable below
  1/2 wavelength

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Wave Speed

• Celerity (C)
• C wavelength/period
• C L/T
• Period remains the same

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Wave Measurement

• The simplest technique is to examine the
  oscillation of waves against a pole fixed marked
  in meters
• Oceanographers will employ pressure sensors
  anchored to the sea floor

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Wave Measurement

• Satellite Imagery

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Deep-water Waves

• Waves that propagate in water than is one-half
  their wavelength
• Wave base is above the sea floor

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Deep-water Waves

• L (g/2p)T2
• L (1.56 m/sec2)T2
• L/T (1.56 m/sec2)T

• C 1.56T
• C2 1.56L

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Storm Centers

• Progressive wind waves (PWWs)
• Wind
• Gravity
• Propagate in particular direction

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Storm Centers

• Progressive wind waves (PWWs)
• Confused seas
• Forced waves

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Dispersion

• Free waves
• No longer wind-driven waves
• Period does not change

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Group Speed

• Wave Train
• Group Speed (V)
• One-half the wave velocity for deep-water waves
• V C/2

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Constructive Wave Interference

• Crests troughs coincide
• Build up of crest deepening of trough

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Destructive Wave Interference

• Crest of one wave coincides with trough of
  another wave (and vice versa)
• Dampening of wave

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Wave Height

• The variety and size of wave generated depends on
  3 factors
• Speed of the wind
• How long the wind blows
• Size of the fetch of the wind

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Wave Height

• 40-50o S
• Ideal wave production
• No landmasses to limit fetch
• Westerlies
• Typical maximum fetch
• 920 km (500 miles)

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Wave Height

• Larger waves will be produced on the side of the
  storm where the winds are blowing in the same
  direction as the storm is moving
• 10-15 m (33-49 ft)
• 100-200 m (330-660 ft) long

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Wave Height

• Episodic waves
• Produced as a result of constructive interference
• Occur most often near edge of continental shelf
• 20-30 m
• 50 knots