Class 29 -- The Oceans

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Class 29 -- The Oceans

• WAVES ALONG COASTS
• Wind-generated waves
• Shallow water waves (more)
• Changes as waves move onto shore
• Longshore transport
• Tsunamis
• Seiches (omitted 2011)

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Changes in Wind-Generated Waves As They Move
Onshore

• 1. Changes in wave characteristics --
• Waves "feel bottom" in shallow water (D lt L/2).
• "Deep-Water waves --gt "Shallow Water" waves
• Shallower water --gt Slower, closer spacing
• T remains constant
• H increases (wave energy compressed into smaller
  area)

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Changes in Waves As They Move Onshore

• Wave Theory The shallow water wave model says
• When D lt L/20, S L controlled only by depth
• S vgD 3.13 vD
• Top of wave can crash forward as the bottom of
  the wave slows down
• If H/L gt 1/7 (D H) --gt "breakers," "surf"

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Changes in Wind-Generated Waves As They Move
Onshore
Changes in wave direction
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• (2) Changes in wave direction -- refraction
• If waves approach at an oblique angle...
• Bending of wave "fronts" (crests)
• Bending of wave movement direction

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Why does refraction occur?

• a. Wave loses speed as water gets shallower
• b. The part of wave that is in shallow water
  moves more slowly than the part in deep water
• Result
• Wave movement direction turns toward shallow
  water
• Waves crests become closer to parallel with shore

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Beach
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• Refraction irregular shapes of coastlines
  causes
• a) Focussing of wave energy onto headlands or
  shallower parts of beaches

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• Refraction irregular shapes of coastlines
  causes
• b) Quieter conditions on recessed beaches
• c) Erosion in headlands and deposition of
  sediment in recessed areas

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• Tsunamis video

http//www.youtube.com/watch?vPBZGH3yieLc model
http//www.youtube.com/watch?v5-zfCBCq-8I sea
wall
http//www.nytimes.com/2011/03/13/weekinreview/13w
ater.html?refscience weight of water
http//www.nytimes.com/interactive/2011/03/11/worl
d/asia/maps-of-earthquake-and-tsunami-damage-in-ja
pan.html?refscience Maps and block diagrams
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Tsunami Very long wavelength waves caused by
earthquakes, etc.
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Tsunami Very long wavelength waves caused by
earthquakes, etc.

• Example Earthquake Sea floor drops 6m
• T 10 -20 min (600-1,200 s)
• L 100 - 200 km (100,000 - 200,000 m)
• H 1 - 2 m when water is deep -- not noticeable

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Tsunami Very long wavelength

• L 100 - 200 km (100,000 - 200,000 m)
• How deep is the ocean- average???
• The Shallow-water wave model applies
• The waves are REFRACTED

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2011 Honshu Tsunami Simulation- NOAA
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Tsunami Very long wavelength waves caused by
earthquakes, etc.

• Example Earthquake Sea floor drops 6m
• T 10 -20 min (600-1,200 s)
• L 100 - 200 km (100,000 - 200,000 m)
• H 1 - 2 m -- not noticeable in open ocean!
• Behave like Shallow-water waves L gtgt depth of
  ocean
• Thus, speed controlled by depth only (wave
  theory)
• S vgD 200 m/s (400 mph)

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Tsunami changes as it moves into shallower water
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http//www.youtube.com/watch?v3q9gC8mQNhk (Japane
se Coast Guard vessel rides tsunami)
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TSUNAMIS in coastal areas

• H 1-2 m in open ocean, long L
• But as they move onshore
• S decreases
• L decreases (compresses waves)
• 2. H increases to perhaps 20 m, but wavelength
  is long, so these waves act like floods, not
  breakers
• 3. Energy focused by bottom topography and
  man-made barriers

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Damage in Crescent City, CA, March 2011 Tsunami
from Japan
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• Prediction of tsunamis
• Earthquake event
• Large vertical displacement required
• Many large quakes --gt no tsunami
• Can predict wave migration
• Plenty of time to warn people

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• Prediction of tsunamis
• http//nctr.pmel.noaa.gov/animate.html

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SEICHES (Omitted 2011)

• Standing waves- they dont move
• Sloshing back and forth- bathtub
• Occur in harbors, bays
• Natural resonance period (T) of a basin
• Depends on depth and dimensions
• If entering wave has same T, it resonates" in
  basin
• Water moves back and forth, up and down
• Winds, tides, and waves can cause seiches

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