Chapter 9: Waves and Water Dynamics

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Chapter 9 Waves and Water Dynamics
Fig. 9-10
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WAVE CHARACTERISTICS
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Waves are moving energy

• Forces cause waves to move along air/water
  interface or within water
• Wind (most surface ocean waves)
• Movement of fluids with different densities
• Internal waves often larger than surface waves
• Mass movement into ocean(e.g. land slide) Splash
  waves

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• Seafloor movement
• Tsunami or seismic sea wave
• Gravitational attraction Earth, Moon, Sun
• Tides
• Human activities
• Wakes of ships
• Explosions

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Progressive waves

• Longitudinal
• Push-pull
• Transverse
• Side-to-side or up-and-down
• Orbital
• Circular orbit
• Ocean surface waves

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Types of waves
Fig. 9-3a
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Wave characteristics

• Crest, trough
• Wave height is proportional to energy
• Wave length
• Wave height/wave length wave steepness
• Waves break when H/L is 1/7
• Wave period, frequency

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Wave characteristics

• Wave base is 1/2 wave length
• Negligible water movement due to waves below this
  depth

Fig.9-6a
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Deep-water wave

• Depth of water is greater than 1/2 wavelength
• Speed of wave form (celerity) is proportional to
  wavelength

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Shallow-water wave

• Water depth is less than 1/20 wavelength
• Friction with seafloor retards speed
• Wave speed (celerity) is proportional to depth of
  water
• Orbital motion is flattened

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Transitional waves

• Water depth is 1/2 to 1/20 of wavelength
• Characteristics of deep and shallow-water waves
• Wave speed (celerity) is proportional to both
  wavelength and depth of water

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Three types of waves
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Wave equations

• Wave speed wavelength/period
• S L/T
• Frequency 1/period
• F 1/T
• Wave speed (m/s) 1.56 x period
• S 1.56 x T

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Surface ocean waves

• Most wind-driven
• Small wind-driven waves
• Capillary waves
• Larger wind-driven waves
• Gravity waves

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Sea

• Storm at sea creates waves
• Wave energy depends on
• Wind speed
• Fetch
• Duration
• Chaotic mixture of different wavelengths and wave
  heights

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Wave dispersion

• Longer wavelength waves outdistance shorter
  wavelength waves
• Waves travel in groups or trains with similar
  characteristics
• Swell made up of waves of similar wavelength and
  period

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Wave interference

• Constructive
• Wave heights increase
• Destructive
• Wave heights decrease
• Mixed
• Wave heights vary in wave train (surf beat)

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Interference illustrated
Fig. 9-14
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Rogue waves
Fig. 9-16

• Unusually large waves
• Constructive interference
• Waves meet strong ocean current

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Shoaling waves

• Waves reach surf zone
• Wave speed decreases
• Wave length decreases
• Wave height increases
• Wave steepness 1/7, wave breaks
• Surface tension no longer able to hold wave
  together

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Breakers

• Spilling
• Gentle beach slope
• Plunging
• Moderately steep slope
• Surging
• Abrupt slope

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Wave refraction

• Shoaling waves bend so wave fronts approach a
  shore nearly parallel

Fig. 9-19a
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• Wave energy focused on headland
• Wave energy dispersed over bay

Fig. 9-19b
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Wave diffraction

• Wave energy transferred around or behind barriers

Fig. 9-20
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Wave reflection

• Waves bounce back from steep slopes or seawalls
• Reflected wave may constructively interfere with
  other waves

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Standing waves

• Two waves with same wavelength moving in opposite
  directions
• Node no vertical movement
• Greatest horizontal movement
• Antinode greatest vertical movement

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Fig. 9-22
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Tsunami or seismic sea wave

• Caused by sudden changes in volume of ocean basin
• Mainly submarine faults
• Volcanic eruptions
• Submarine landslides

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Fig. 9-23a
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Tsunami

• Very long wavelength
• Travels fast
• Raises sea level as crest shoals
• Trough causes sea level to fall
• Disastrous for infrastructure at coasts
• Possibly much loss of life

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Tsunami warning system

• Monitor seismic activity
• Monitor changes in unusual wave activity
• Warning
• People evacuate