Transcript Chapter 8
CHAPTER 8
Waves and Water Dynamics
Waves are visual proof of the
transmission of energy across the
ocean
Origin of waves
Most waves are wind-driven
Moving energy along ocean/air interface
Wind main disturbing force
Boundary between and within fluids with different
densities
○ Air/ocean interface (ocean waves)
○ Air/air interface (atmospheric waves)
○ Water/water interface (internal waves) –
movement of water of different densities
Atmospheric Kelvin-Helmholtz waves are caused when a
certain type of cloud moving horizontally one way interacts
with a stream of air moving horizontally at a different speed.
Eddies develop, making beautiful, unusual, curling waves of
cloud.
http://www.siskiyous.edu/shasta/map/mp/bswav.jpg
Internal waves
Associated with
pycnocline
Larger than surface
waves – up to 100 m
Caused by tides,
turbidity currents,
winds, ships
Possible hazard for
submarines
Fig. 8.1a
Internal waves (wavelength about 2 km) which seem to
move from the
Atlantic ocean to the Mediterranean Sea, at the east of
Gibraltar and Ceuta
http://envisat.esa.int/instruments/images/gibraltar_int_wave.gif
Other types of waves
Splash wave
Coastal landslides, calving icebergs
Seismic sea wave or tsunami
Sea floor movement
Tides
Gravitational attraction among Moon, Sun, and
Earth
Wake
Ships
Wave motion
Waves
transmit energy by
oscillating particles
Cyclic motion of particles in ocean
Particles may move
○ Up and down
○ Back and forth
○ Around and around
Particles
in ocean waves move in
orbital paths
Progressive waves
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Waves that travel without breaking
Types
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Longitudinal – push/pull waves in direction of energy
transmission
•
sound
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Transverse – back and forth motion
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Only in solids
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Orbital
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Combination of longitudinal and transverse
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around and around motion at interface of two fluids
Orbital or interface waves
Waves on ocean surface at water/air interface
Crest, trough, wave height (H)
Wavelength (L)
Orbital waves
Wave characteristics
Wave steepness = ratio of wave height to wave
length H/L
○ If wave steepness > 1/7, wave breaks
Wave period (T) = time for one wavelength to pass
fixed point
Wave frequency = # of wave crests passing fixed
location per unit of time, inverse of period or 1/T
Circular orbital
motion
Water particles
move in circle
Movement up and
down and
Back and forth
Orbital motion
Diameter of orbital motion decreases with
depth of water
Wave base = ½ L
Hardly any motion below wave base due to wave
activity
Types of Waves dependent on
interaction with bottom
Deep-water waves
No interference with ocean bottom
Water depth is greater than wave base ( >
1/2L)
Wave speed (celerity) proportional to
wavelength
Longer the wave, the faster it travels
Shallow-water wave
Water depth is < 1/20L
Wave “feels” bottom, because water is shallower
than wave base
Orbits are compressed elliptical
Celerity proportional to depth of water
The deeper the water, the faster the wave travels
Transitional waves
Characteristics of both deep and shallow-water
waves
Celerity depends on both water depth and
wavelength
Wave development
Most ocean waves wind-generated
Capillary waves (ripples) formed first
Rounded crests, very small wavelengths
Provide “grip” for the wind
Increasing energy results in gravity waves
Symmetrical waves with longer wavelengths
Wave energy
Factors that control wave energy
Wind speed
Wind duration
Fetch – distance of uninterrupted winds
Maximum wave height caused by wind that is
known:
○ Reliable measurement
Measured on US Navy tanker caught in typhoon
○ Wave height 34 m or 112 ft
Fig. 8.10
Wave energy
Fully developed sea
Maximum wave height,
wavelength for particular
fetch, speed, and duration of
winds at equilibrium
conditions
Swell
Uniform, symmetrical waves
that travel outward from
storm area
Long, rounded crests
Transport energy long
distances
http://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter1
Swell
Longer wavelength
waves travel faster
and outdistance other
waves
Wave train = group of
waves with similar
characteristics
Sorting of waves by
their wavelengths is
wave dispersion
Wave train speed is ½
speed of individual
wave
Wave interference patterns
Different swells coming
together
Constructive
interference
In-phase wave trains with
about the same wavelengths
Add to wave height
Rogue waves – unusually
large waves
○ Rare but can happen and be
unusually large
http://www.ethnomusic.ucla.edu/courses/ESM172a
Wave interference patterns
Destructive
interference
Out-of-phase wave
trains with about the
same wavelengths
At least partially cancel
out waves
Mixed interference
Two swells with
different wavelengths
and different wave
heights
Wave height is extremely variable
~50% of all waves are less than 2 m (6-7 ft)
10-15% are greater than 6 m
Up to 15 m in Atlantic and Indian oceans
Up to 34 m in Pacific - long fetch (speed at
102 km/hr)
Largest rogue wave can sink largest vessels
Largest = 34 m (120 ft) high (above theoretical
max)
1:1200 over 3x average height;
1:300000 over 4x height
Waves hitting current may double height suddenly and
break
Most common near strong currents, long fetches,
storms
Rogue waves that rise as high as 10-story
buildings and can sink large ships are far
more common than previously thought,
imagery from European Space Agency
satellites has shown. A rogue wave is seen in
this rare 1980 photo taken aboard a
supertanker during a storm near Durban,
South Africa. (Reuters)
http://www.allhatnocattle.net
Storm surges
• Large wave moving with a storm (not just
hurricanes)
• Low pressure above water water level rises at
center
• Up to 3-4 m higher than normal
• Preceded by low sea-level in front of storm
• Added to increased wind waves + high tide
most damage
Hurricane Katrina – 2005
Record storm surge in Pass Christian, MS - ~27.8 ft
Waves approach shore
Deep-water swell waves shoal
Transitional waves
Become shallow-water waves (< L/2)
Wave base “touches” sea bottom
Waves approach shore
During transition to shallow-water waves
Wave speed and wavelength decreases
Wave height and steepness increases
Waves break
Period remains constant
Breakers in surf zone
Different types of
breakers associated
with different slope of
sea floor
Spilling
Plunging
Surging
http:// www.mikeladle.com
Spilling breaker
Water slides down
front slope of wave
Gently sloping sea
floor
Wind “onshore”
Wave energy
expended over
longer distance
http://www.winona.edu/geology/oceanography
Plunging breaker
Curling crest
Moderately steep sea
floor
Wind “offshore”
Wave energy expended
over shorter distance
Best for surfers
http://www.seagrant.umn.edu/seiche/2002
Surging breaker
Breakers on shore
Steepest sea floor
Energy spread over
shortest distance
Challenging for
surfers
http://www.bbc.co.uk/wales/surfing/images/ecards/400_232/hawaii/sandy_beach_bridgey.jpg
Wave refraction
As waves approach shore, they bend so wave
crests are nearly parallel to shore
Wave speed proportional to depth of water
(shallow-water wave)
Different segments of wave crest travel at
different speeds
Sets – series from relative calm to
largest waves
• Interference in wave train cancel
some, adds to others
• Destructive interference lull
“between sets”
•
Rip currents are
wave energy
escaping shoreline
Stream of water
returning out to
sea through surf
zone
Flows up to a few
hundred meters
offshore then
dissipates
http://www.ripcurrents.noaa.gov/overview.shtml
http://www.ocean.udel.edu/mas/wcarey
Wave energy distribution at shoreline
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Energy focused on headland
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Headland eroded
Energy dissipated in bay
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Bay filled up with sediment
Fig. 8.17b
Tsunami or seismic sea wave
Sudden changes in sea floor caused by
Earthquakes, submarine landslides, volcanic
eruptions
Long wavelengths ( > 200 km or 125 m)
Shallow-water wave characteristics (<L/2)
Speed proportional to water depth so
very fast in open ocean
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Not steep when generated (low H/L ratio)
Crest of only 1-2 ft over 16 min period
Move very fast -- up to 212 m/sec (470
mile/hr)
As crest arrives on shore, slows but grows
in height quickly
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Sea level can rise up to 40 m (131 ft) when
tsunami reaches shore
Fast, onrushing flood of water rather than a
huge breaker
Series of waves
Warning initial rushing out of water from
shore
Tsunami or seismic sea wave
Most occur in Pacific Ocean (more earthquakes
and volcanic eruptions)
Damaging to coastal areas
Loss of human lives
Krakatau eruption (1883) in Indonesia created
tsunami that killed more than 36,000 people
Aura, Japan (1703) tsunami killed 100,000 people
Indonesia (Dec. 26, 2004) tsunami killed over
229,000 around Indian Ocean
Speed of tsunami
Undersea earthquake at 6:59
AM
Scale of tsunami damage on
Sumatran coast in Aceh province
Landsat image before tsunami:
13-Dec. 2004
** Note sediment
covered area
impacted by
tsunami 1-5 km
inshore
Landsat image after tsunami: 29www.jpl.nasa.gov/news
Dec. 2004
Tsunami watches and warnings
Pacific Tsunami Warning Center
Seismic waves forecast possible
tsunami
Issues tsunami watches and
warnings
Increasing damage to property
as more infrastructure
constructed near shore
Evacuate people from coastal
areas and send ships from
harbors
Water “sucked” out before first
http://www.drgeorgepc.com/tsuStationsTravelChart.jpg
Waves as a source of producing
electricity
Lots of energy associated with waves
Mostly with large storm waves
How to protect power plants
How to produce power consistently
Environmental issues
Building power plants close to shore
Interfering with life and sediment movement
Offshore power plants?
Wave power plant at Islay, Scotland
Fig. 8.25b
Ocean Literacy Principles
1.c – Throughout the ocean there is one
interconnected circulation system
powered by winds, tides, force of the
Earth’s rotation, the Sun, and water
density differences. The shape of ocean
basins and adjacent land masses
influence the path of circulation.
5.h - Tides, waves, and predation cause
vertical zonation patterns along the
shore, influencing the distribution and
diversity of organisms.