What is a Wave?brosenhe/Oceanography/15_Ocean_Waves.pdfWave Speed • Speed is equal to wavelength...
Transcript of What is a Wave?brosenhe/Oceanography/15_Ocean_Waves.pdfWave Speed • Speed is equal to wavelength...
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Ocean Waves
What is a Wave?
• A response to a generating force (in this case a pebble thrown into a pond)
• System returns to normal through restoring force (in this case, surface tension of the water)
Ripple on Great Pond, Eastham, MassachusettsLaura Perkins
Where’re the waves?!
• No good waves in Louisiana• Great waves in California• Winter waves are best in
some regions• Waves are better at certain
parts of the beach than at others
• Why?
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Waves
• Large waves in certain regions– http://www.youtube.com/watch?v=LhKFTqxn6
qs• Seasonal waves in other places
– http://www.youtube.com/watch?v=zfJrINTs7wI&feature=related
• Louisiana surfing:– http://www.youtube.com/watch?v=Yd3RlVRZ
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Wave Formation
Anatomy of a Wave
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Wind Waves• Dimensions are astonishingly alike,
no matter the size of the storm
Wave Speed• Speed is equal to wavelength divided by
period– C = L/T
» “C” stands for celerity, another term for wave speed
• A wave’s period always remains the same– If wavelength becomes shorter, it slows down– If it speeds up, the wavelength has to increase
Wave Formation
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Wave Travel
• Storm waves are generated by winds and pressure differences within the storm area
• Within the storm, waves are a jumbled mess of different wavelengths and heights
• Away from the storm, waves traveling at different speeds separate from the others
• This results in distinct groups of waves having similar size characteristics
Wave Travel
Group Speed• Group speed is half of the individual wave
speed– V = C/2
Ripple on Great Pond, Eastham, MassachusettsLaura Perkins
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Interference
• Constructive• Destructive• After point of
interference, waves remain unchanged
Interference
Movement of Waves and Groups
Carleton College Oceanography Course
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Wave Motion
• Orbital Path• No net movement
Wave Motion
• Motion diminishes with depth
• Depth of wave is equal to half of the wave length– D = L/2
Wave Height
• Wind speed• Wind duration• Fetch distance
– Not enough fetch distance in Miami for big, consistent waves
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How big can Southern LA’s waves get?• Assume:
– A tropical storm force wind (58mph)– 60 miles between here and the Bahamas
• Then:
How big can Southern LA’s waves get?• Assume:
– A tropical storm force wind (58mph)– 800 miles between here and the Bahamas
• Then:
Shallow vs. Deep Waves
• Wave height, steepness, and speed change as a wave comes to shore
• Interaction with the bottom causes these changes
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Deep Waves
• Depth of wave (D = L/2) is less than the depth of the water
• Speed (C) and period (P) are defined by the storm
– Wind Strength
–– Wind DurationWind Duration–– Fetch DistanceFetch Distance
• Deep Wave does not “feel bottom.”
Shallow vs. Deep Waves
• Depth of wave is equal to half of the wave length– D = L/2
• If the depth of the water < L/2 of the wave, it is no longer a deep wave.
Shallow Waves
• Waves “feel bottom.”– D < L/2
• Wave slows down (Cs < Cd)• Wave height increases (hs > hd)• Wavelength decreases (Ls < Ld)• Speed solely dependent on water depth
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Shallow Wave Group Movement
Carleton College Oceanography Course
Shallow Waves
Wave Steepness
• Steepness is the wave’s height divided by the wavelength– S = H/L
• Steepness is what causes a wave to break
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When does a wave break?
• When the height to wavelength ratio is greater than one to seven– H/L > 1/7
Waves Coming to Shore
• Waves can approach coasts at an angle
• However, breaking waves almost always seem to be parallel with the coast
Curving of Wave Trains
• The part of the wave that “feels bottom”first is slowed.
• The entire wave begins to curve around this slow point.
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Curving of Wave Trains
• Energy is concentrated on land points
Non-Wind Waves
• Tides– Generated and restored by force of gravity
• Internal waves– In case of ENSO, cause by relaxation of wind-
driven Ekman mixing of the surface layer (Kelvin waves)
• Tsunamis– Generated by seismic events (earthquakes,
landslides, etc.) and restored by gravity
Tsunamis
• Carry huge amounts of energy• As deep water waves, H is usually lower
than waves generated by storms– Hard to detect in the open ocean
• Large amounts of energy result in a tremendous increase in wave height (H) as the tsunami “feels bottom”
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Chilean Landslide
Chilean Landslide
Chilean Landslide
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Chilean Landslide
Chilean Landslide
Chilean Landslide
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Chilean Landslide
Chilean Landslide
Chilean Landslide
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Chilean Landslide
Chilean Landslide
Chilean Landslide
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Chilean Landslide
Chilean Landslide
Chilean Landslide
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Are tsunami’s surfable?
Japan Tsunami
Japan Tsunami
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Japan Tsunami
Japan Tsunami
Hawaiian Tsunami
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Hawaiian TsunamiHawaiian Tsunami
Hawaiian Tsunami
Hawaiian Tsunami
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Hawaiian Tsunami
Summary
• Causes of Waves– Wind– Gravity– Surface Tension
• Anatomy of Waves– H, L, D, C, v, S
Summary
• Group Travel• Interference• Matter Transport or Energy Transport?• Deep Waves vs. Shallow Waves• Wave Height
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Summary
• Different types of waves– Storm (Wind)– Internal– Tides– Tsunamis