SURFS UP

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SURFS UP!-- Waves in the ocean
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WHY do waves occur?
Waves form at the interface between two layers of
different densities in response to forces
applied by one layer to the other.
Wind waves - waves at the surface caused by wind
blowing across the water.
Tsunami - waves caused by displacement of the
ocean bottom (earthquake, landslide, volcano .)
Tidal waves - (next chapter)
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Energy is transported in waves, NOT water mass.
Notice this surfer. He just bobs up and
down, but gets no closer to the shore.
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HOW is energy transmitted, especially when you
just said that there is no mass transfer?
Gravitational potential energy is stored in an
object when it is raised above its resting
height gravitational force change in height
Example Why does dropping an egg from 1mm not
break the egg and dropping it from 30 cm does?
!!more grav. potential energy from 30 cm
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Waves transfer gravitational potential energy
between adjacent water particles
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Water particles have an orbital motion
and waves are felt to a depth of 1/2 of the
distance between crests (wavelength)
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Wave terminology
height size of wave
t, s
cycle -- one repetition of wave
pattern
wavelength (L) -- distance between adjacent crests
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The two pictures in the previous slides are taken
an one instant in time. Consider what happens at
one place over time (watch the sitting surfer bob
up and down)
period (T) time it takes for adjacent crests to
pass one point in space (one up-down-up cycle)
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Waves exist in both TIME and SPACE
Wavelength distance between two adjacent peaks
at one instant in time
L
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The physics quantity that connects time and
distance is speed
CL/T
speed (meters/sec) wavelength(meters)/period(sec
)
I expect students to be able to solve for one
quantity in this equation, given the other two
(Guess what will be on the test?)
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Wind waves
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Wind waves get energy from the wind
Big waves start as really tiny waves
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If the wind continues to blow, the waves will
grow. The maximum height of the wave (i.e., how
much gravitational potential energy has
been acquired by the water from the wind) depends
on
1) speed of the wind
2) how long it blows
3) how steady is the direction
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Waves behave differently, depending on whether
they are in deep water or shallow water
Shallow water
transitional waves
Deep water
(depth gt L/2)
(depth lt L/20)
but C L/T ALWAYS!
Longer wavelengths travel faster
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How big can waves get?
Stable waves have a maximum height of 1/7 of the
wavelength (but they can be smaller)
but waves rarely reach their maximum heights
possible
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Most wind waves have periods of 0.5-20 s, so
wavelengths are between 0.4 m and 500 m and
speeds are between 0.8 m/s and 28 m/s (longer
wavelengths travel faster and have longer periods)
The LONGEST wavelength observed was 829 m.
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To get REALLY big waves, you need interference
Interference is the combination of two or more
waves into a single resulting one.
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Rogue waves are rare, exceptionally large waves
that result from constructive interference (wave
crests add)
Oh, sh--!
Rogue wave
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Rogue waves can also affect the shore
This is a class observing waves in
BritishColumbia, sitting on rocks 25 m above the
ocean.
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When waves encounter the shore (depth lt L/2),
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Wave energy is dissipated through turbulence and
heating of the water at the shore, but the
temperature of the water does not rise
perceptibly.
WHY?
High heat capacity of water!!
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Steeply sloping bottoms can lead to really
large waves
Uh-oh!
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Seiches shallow water wave in confined area
(lake, harbor, glass of water, bucket, etc.)
Triggered by wind, change in atmospheric pressure,
earthquakes.
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Usually not dangerous on coasts because wave
height is small, but
This storm-triggered seiche in 1938 on Lake
Michigan killed 10 people on the breakwater.
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The last type of wave for today Tsunami
Wave triggered by movement of ocean bottom
(earthquake usually, but landslides, volcanos
also)
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To generate a tsunami, the ocean floor must
be offset
The San Andreas fault is a strike slip fault, so?
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Tsunami have periods of 1,000 s and
wavelengths of 200 km(200,000 m). Are these
shallow or deep water waves?
What is L/2? Are there any ocean basins deeper
than this?
What is L/20? Are there any ocean basins deeper
than this?
Tsunami are ALWAYS shallow water waves!
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and
Thus,
C 3.1?4600 m 210 m/s (470 mph)
This is FAST, but a ship sitting at sea would
simply rise up and fall back down in about 9
minutes not really noticeable because wave
height might be only 0.5 m for large wave.
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Dec 28, 2004 M9.1 earthquake in Indonesia
Notice the time elapsed
and how the tsunami is higher close to the
earthquake
Wave height in the open ocean was 1-2 m
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But as the tsunami neared shore, the situation
changed.
Wave height increased up to 24 m
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Here is a sequence of photos showing the 2004
tsunami on a Thai beach
What can we learn from this?
Not one wave, but multiple ones
Wave period of 15 minutes
Water may recede offshore
Wave does not break and form surf.
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What to do?
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Do NOT go to the beach to watch!
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50 of the time, tsunami are preceded by their
wave troughs and water recedes first
DONT GO SHELL COLLECTING!
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Summary
Waves are generated at boundaries between layers
of different densities by the action of a force
from one layer to the next. Wind and ocean
bottom movement are the forces.
Waves transfer energy, not water mass.
Energy flow Wind motion ?Gravitiational
potential energy in wave ? Heat and turbulence
at shore ? Transport w/o loss ? ?
Wave speed, period, and length are related CL/T
Deep water wave speed depends on wavelength
(C1.25?L)
Shallow water wave speed depends on water
depth(C3.1 ?d