Transcript Waves – Chapter 8

Waves
http://www.youtube.com/watch?v=nd2jtwviyC8
Waves
• When undisturbed by wind (or some other
factor such as an earthquake), the sea surface
is naturally smooth!
• Waves are moving energy and begin as a
disturbance
– Wind blowing across the surface of the
ocean generates most waves
– Tides, turbidity currents, coastal landslides,
calving icebergs, and sea floor movement
can also cause waves
Waves
• In an ocean wave, energy is moving at the
speed of the wave, but water is not!
• Waves move energy, with very little
movement of particles (including water
particles!)
• The water ‘associated’ with a wave does not
move continuously across the sea surface!
• Imagine a seagull resting
on the ocean surface
• The bird moves in circles –
up and forward as the tops
of the waves move toward
its position, and down and
backward as tops of the
waves move past
• Energy in the waves flows
past the bird, but the gull
and it’s patch of water
move only a short distance
Each
circle is
equal in
diameter
to the
wave
height
Orbital Waves
• The bigger the wave, the larger the size of the
orbit
• The diameter of the orbit diminishes rapidly
with depth
Wave motion is negligible
when orbits reach a
diameter that is 1/23 of
those at the surface
Wave motion is negligible
below a depth of one
half of the wavelength
Components of a Wave
• Ocean waves has distinct parts:
– Wave crest: highest part of the wave above
average water level
– Wave trough: lowest part of the wave below
average water level
– Wave height: the vertical distance between a
wave crest and its trough
– Wavelength: the horizontal distance between 2
successive crests, or troughs
Making Waves
• Ocean waves are classified by
– the disturbing force that creates them
– the extent to which the disturbing force
continues to influence the waves once they
are formed
– The restoring force that works to flatten
them
– Their wavelength
Making Waves
• Energy that causes waves to form is called a
disturbing force
• Wind blowing across the ocean surface
provides the disturbing force to generate
capillary waves (waves <1.73 cm) and wind
waves
• Landslides and tectonic processes (volcanic
eruptions, faulting of the sea floor) are the
disturbing forces for seismic sea waves, or
tsunamis
Making Waves
• The restoring force seeks to return the water
to flatness after a wave has formed in it;
gravity provides the restoring force on all
waves >1.73cm
Wavelength is the most useful
measure of wave size
Deep vs. shallow water waves
• Waves moving through water deeper than ½
their wavelength are deep water waves
– Example: A wind wave with a 20m wavelength is
considered to be a deep water wave so long as it is
passing through water >10m deep
• Waves in water shallower than ½ their
wavelength are shallow water waves
– Example: A wave with a 20m wavelength will act
as a shallow-water wave if the water is <10m deep
Shallow water waves
• When a wave approaches the shore, its
proximity to the bottom flattens out the orbits
of water molecules
• Causes the water at the bottom to move back
and forth; no longer in a circular pattern
Ocean Motion
• In the ocean, only capillary and wind waves
can be deep water waves
• Why???
• Remember, deep water waves occur when
moving through water deeper than half their
wavelength…
Gee, that’s just swell…
• Generally speaking, the longer the
wavelength, the faster the wave
• When waves move away from their area of
origination, wind speeds diminish and they
eventually move faster than the wind
• Mature waves from a storm sort themselves
into groups of waves with similar wavelengths
and speeds as they outrun their smaller
‘relatives’
Gee, that’s just swell…
• This results in swells; uniform, symmetrical
wind waves that have traveled out of their
area of origination
When waves meet
• Because longer waves will outrun shorter
waves, wind waves from different storm
systems can interfere with one another
• When waves meet, they add to or subtract
from one another
• Such interaction is called interference
– Constructive: additive
– Destructive: subtractive (cancellation)
Dude, constructive and destructive
waves rule
• Surfers depend on constructive and
destructive waves to generate their ‘wave
sets’
• Constructive interference between waves of
different wavelengths create the sought-after
big waves
• Destructive interference diminishes the waves
and makes it easier for the surfer to swim back
out
• Constructive: crests of waves coincide
• Destructive: crest and trough of waves coincide
Going Rogue
• Occasionally, wind waves of many
wavelengths can approach a single point/spot
from different directions
• A huge wave crest develops suddenly from the
constructive interference, generated a rogue
wave
• Rogue waves are much larger than
surrounding waves and can be extremely
hazardous
http://www.youtube.com/watch?v=l_8hOai9hGQ
Just how big can waves be?
Just how big can waves be?
• The size of waves depends on:
– Wind strength
– Wind duration
– Fetch (distance over which the wind blows)
• A strong wind must blow continuously in one
direction for ~3 days for the largest waves to
develop
• The greatest potential for large waves occurs
beneath the strong and nearly continuous
winds surrounding Antarctica
Some personal experience…
Photo taken here (bridge)
50 ft
A photograph of a wave taken from the bridge 50 feet
above sea level
~70-foot wave crossing the Southern Ocean
Encountering ‘The Perfect Storm’ just north of Antarctica
(Ross Sea, Southern Ocean)
When ocean waves encounter land
• Deep water waves change to shallow water
waves as they approach the shore
• Once the wave passes over water whose
depth is less than one half its wavelength, the
wave “feels” the bottom
• When this happens, the circular motion of the
water molecules in the wave is interrupted
• Orbits flatten to ellipses near the bottom
When ocean waves encounter land
• The wave’s energy must now be packed into
less water depth, and so the wave crests
become peaked, rather than rounded
• Interaction with the bottom slows the
incoming wave, but waves behind it continue
toward shore at their original speed
• This results in a “bunching up” of the waves,
which decreases their wavelength, but
increases their height
Surf’s Up!
• When the wave steepness reaches the 1:7
ratio (wave’s height is 7x it wavelength), the
wave will break as surf
How to score an epic wave…
• Waves break along the shore in different ways
• Waves are influenced by:
– The bottom slope (the steeper the slope, the
more violent and toppling the wave)
– Contour and composition of the bottom (gradually
shoaling bottoms sap waves of their strength as
the wave loses energy interacting with the
bottom)
– Localized winds and fetch
Why is surfing
so much
better along
the west
coast of the
U.S. than the
east? (no
offense…)
NOT Long Island
Waves refract when they approach a
shore at an angle
• Waves usually approach the shore at an angle
• Different parts of the wave is at different
depths, so the wave must bend, or refract as
parts of the wave reaches shallower water and
slows
• The slowing and bending of waves in shallow
water is called wave refraction; the waves
refract in a line nearly parallel to the shore
Waves refract when they approach a
shore at an angle
Big Waves: Tsunamis
• The Japanese term for large, often destructive
waves that occasionally roll into their harbors
is tsunami (tsu = harbor; nami = wave)
• Tsunamis originate from sudden changes in
the sea floor caused by tectonic activity
(undersea volcanic eruptions, faulting,
collapse of large oceanic volcanoes) and even
underwater avalanches such as those caused
by turbidity currents
Tsunamis
• The majority of tsunamis are caused by fault
movement
• Underwater fault movement displaces the
earth’s crust, generates earthquakes, and if it
ruptures the seafloor, produces a sudden
change in water level at the ocean surface (up
or down)
• The wavelength of a tsunami is ~125 miles, so
it is a shallow water wave everywhere in the
ocean
Tsunamis
• In the open ocean, tsunamis travel at speeds
>435 miles per hour
• Tsunamis in the open ocean have heights of
only ~0.5 meters (~1.6 feet)!
• However, once they approach the shore, they
slow in the shallow water and increase in
wave height
• Surges ashore; mistaken for an extremely high
tide and so mistakenly called tidal waves
Abrupt vertical movement
along a fault on the sea floor
raises or drops water column
creating a tsunami that travels
from deep to shallow water
Killer Waves
• 86% of all tsunamis occur in the Pacific Ocean
(Why?)
• On December 26, 2004, an enormous
earthquake struck off the coast of Sumatra in
Indonesia
• Occurred ~19 miles beneath the sea floor near
the Sunda Trench, where the Indian Plate is
being subducted beneath the Eurasian Plate
• Ruptured ~750 miles of sea floor!
Killer Waves
• This thrusted the seafloor upward, displacing
>30 feet of water above it
• The resulting tsunami spread across the Indian
Ocean, literally washing away many coastal
villages and causing approximartely 300,000
human deaths in Indonesia (esp. Thailand)
and along coastal India and Africa
• Although much smaller, the tsunami was also
detected in the Atlantic, Pacific and Arctic
Oceans!
Indonesian capital of Banda Aceh
(before tsunami)
Indonesian capital of Banda Aceh
(after tsunami)
Sequence of photos of tsunami inundating Chedi
Resort in Phuket, Thailand on December 26, 2004
Japanese Tsunami:
• On March 11, 2011, a 9.0 magnitude
earthquake struck Japan at 2:46PM
• 16-25 feet of sea floor were displaced upward
• The resulting tsunami reached a height of 133
feet and traveled inland up to six miles
• 15,844 confirmed dead
• 3,451 still missing
http://www.infowars.com/swallowed-by-the-tsunami-horrifying-new-footageshows-race-to-outrun-giant-wave/
Energy map of 03.11.11 tsunami
http://news.nationalgeographic.com/news/2011/03/pictures/110315-nuclear-reactor-japan-tsunami-earthquake-world-photos-meltdown/#
http://news.nationalgeographic.com/news/2011/03/pictures/110315-nuclear-reactor-japan-tsunami-earthquake-world-photos-meltdown/#
http://news.nationalgeographic.com/news/2011/03/pictures/110315-nuclear-reactor-japan-tsunami-earthquake-world-photos-meltdown/#