Waves%20and%20Water%20Dynamics

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Waves and Water Dynamics

• Essentials of Oceanography

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Pebble in Still Water

• What happened when I dropped the pebble into the
  still tank?
• Do the particles at the point of disturbance move
  outward too?http//www.brighthub.com/education/k
  -12/articles/39375.aspxixzz1A5lRdeRe

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Cork in the disturbance

• How does the cork move?

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What Causes Waves?

• Waves are created by releases of energy
  (disturbances) including
• Wind
• Movement of fluids of different densities
• Mass movement into the ocean (splash waves)
• Underwater sea floor movement (tsunami)
• Pull of the moon and sun (tides)
• Human activities

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Most Ocean Waves Are Wind-generated
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Anatomy of a Wave

• Crest- maximum elevation about still water level
• Trough- max depression below still water level
• Wavelength (L)- crest to crest, or trough to
  trough
• Height (H)- vertical distance crest to trough
• Amplitude- H/2, still water to crest or trough
• Period (T) - time for one wavelength to pass a
  point
• Frequency number of wave crests passing a point
  in 1 second

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Anatomy of a Wave
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Wave Characteristics and Terminology (Continued)

• If wave steepness exceeds 1/7, the wave breaks
• Period (T) the time it takes one full waveone
  wavelengthto pass a fixed position

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How do waves move?

• 1. The direction of propagation of wave, that is,
  the direction in which the disturbance travels.
• 2. The disturbance is transferred due to the
  oscillation of the particles of the medium
  involved. The direction of these oscillations is
  the second direction.

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Types of Progressive Waves

• Longitudinal
• Back-and-forth motion
• Transverse
• Side-to-side motion
• Orbital
• Combination

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Circular Orbital Motion

• As a wave travels, the water passes the energy
  along by moving in a circular orbit
• Floating objects also follow circular orbits

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Orbital Motion in Waves

• Orbital size decreases with depth to zero at wave
  base
• Depth of wave base ½ wavelength, measured from
  still water level

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What about Surfing?
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Wave Characteristics

• Period (T) time between wave crests
• Is a constant for any wave
• Function of the disturbing force
• If wind energy decreases
• Speed and wavelength can decrease
• Period does not change
• Wavelength ?
• Size of the orbits
• Water depth relative to wavelength ?
• Shape of the orbits

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Deep- and Shallow-water Waves

• Deep-water waves
• Water depth gt wave base
• Shallow-water waves
• Water depth lt 1/20 of wavelength

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Deep Water Waves

• Orbits dont reach the seafloor
• Water depth gt ½ wavelength
• If L 20 m, depth gt 10 m
• Only wind waves can be deep water waves
• Wavelengths for tsunamis and tides are so long
  (100s 1000s km)
• Seafloor maximum depth 11 km
• Ocean depth would need to be gt50 km for these to
  be deep water waves

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Shallow Water Waves

• Orbits reach and interact with the seafloor
• Water depth lt 1/20 wavelength
• Orbits feel the bottom
• Orbits flatten ? elliptical
• Water at seafloor moves back and forth

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Wave Speed (S)

• General formula
• Deep-water waves
• Wave speed (S) in meters per second 1.56 T in
  seconds
• Wave speed (S) in feet per second 5.12 T in
  seconds
• Shallow-water waves (d water depth)

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Producing Waves

• Disturbing Force energy input
• Wind wind waves
• Landslide/faulting tsunami
• Moon/sun gravitation tides
• Restoring Force returns water to flatness
• Tends to overcompensate leading to oscillations
• Surface tension- for small waves (capillary
  waves)
• Gravity for larger waves- nearly friction-free
  travel long distances

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History of a Wave

• Wind waves wind energy transferred to the water
• Wind blows on the water friction causes
  stretching of the surface
• Ripples (capillary waves) form
• Winds energy transferred to the water to drive
  the wave forward
• Larger surface area more energy transferred
  bigger waves

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Factors Controlling Wave Height

• Wind strength
• Strong wind more energy taller waves
• Wind duration
• How long the wind blows in one direction
• Waves produced by strong winds will dissipate
  when wind stops
• Fetch - distance wind blows in one direction
• Longer continuous input taller waves

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Fully Developed Sea

• Factors that increase wave height
• Increasing wind speed
• Increasing duration (time) of wind
• Increasing fetch (distance)
• A fully developed sea is the maximum height of
  waves produced by conditions of wind speed,
  duration, and fetch

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Largest Wind-generated Waves Authentically
Recorded

• In 1935, the vessel USS Ramapo experienced large
  waves while crossing the Pacific Ocean
• Wave height was measured at 34 meters (112 feet)

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Waves at the Shore
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Waves Undergo Physical Changes in the Surf Zone
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Surf Zone

• As waves approach the shoreline
• Become shallow water waves
• Slow down due to friction
• Become closer together
• Steepen
• Break
• Lose energy