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Key Ideas Wind waves form when energy is transferred from
wind to water.
Waves transmit energy, not water mass, across theocean’s surface.
The behavior of a wave depends on the relationbetween the wave’s size and the depth of waterthrough which it is moving.
Waves can change direction by refraction anddiffraction, can interfere with one another, and reflectfrom solid objects.
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Waves: Undulatory Motionof a Water Surface
Parts of a wave - wave crest, wavetrough, wave height (H), waveamplitude, wave length (L),and waveperiod (T).
Wave period - basis for classifyingwaves.
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Geometry of a Progressive Wave
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Most Waves: Wind-Generated
Size and type of wind-generated waves:affected by wind velocity, wind duration,fetch, and original state of the seasurface.
As wind velocity increases, wavelength,period and height increase, but only ifwind duration and fetch are sufficient.
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Progressive Waves
As waves pass, wave form and waveenergy move rapidly forward, but not thewater.
Water molecules move in an orbitalmotion as the wave passes.
Diameter of orbit increases withincreasing wave size and decreaseswith depth below the water surface.
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A Little Math
Wave speed = wave length/wave period
C= L/T
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Relationship of Wave Length toDepth of Wave Motion
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Motion of Water as Wave Passes
Water in the crest of the wave moves in the samedirection as the wave, but water in the trough moves inthe opposite direction.
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Waves are classified on the basis of:Disturbing force
Free waves vs. forced waves
Restoring force
Wavelength
Classifying Waves
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Classifying Waves
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Types of Waves
Waves transmit energy across the ocean’s surface.
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Wind strength - wind must be moving faster than the wavecrests for energy transfer to continue
Wind duration - winds that blow for a short time will notgenerate large waves
Fetch - the uninterrupted distance over which the windblows without changing direction
Factors Affecting Wind WaveDevelopment
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Factors Affecting Wind Waves:Fetch
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Fully developed sea - sea state where thewaves generated by the wind are as large asthey can be under current conditions of windvelocity and fetch
Significant wave height - average of thehighest 1/3 of the waves present; goodindicator of potential for wave damage toships and for erosion of shorelines.
Properties of Ocean Waves
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The Beaufort Scale for Wind Speed
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Wave Dispersion
The longer the wave length, the faster thewave speed.
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Swell Formation and Dispersion
Wave separation, ordispersion, is a function ofwavelength. Waves with thelongest wavelength move thefastest and leave the area ofwave formation sooner. Thesmooth undulation of oceanwater caused by wavedispersion is called swell.
A wave train.
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When waves from different directions meet, theyinterfere with one another.
Wave interference can be:
Destructive interference – two waves that cancel each other out,resulting in reduced or no wave
Constructive interference – additive interference that results inwaves larger than the original waves
Rogue waves - freak waves that occur due to interference andresult in a wave crest higher than the theoretical maximum
Interference And Rogue Waves
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Wave Interference
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Wave Interference
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Waves Entering Shallow Water
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Shallow-Water Waves
When waves enter shallow water:
Wave speed decreases
Wave length decreases
Wave period does not change
Wave height increases
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Wave base - depth to which a surfacewave can move water
Depth greater than wave base - orbitsare circular with no interaction betweenwave and bottom
Depth shallower than wave base - orbitsare elliptical and become increasinglyflattened towards the bottom
Wave Motions
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Deep- and Shallow-Water Motion
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Wind Waves ApproachingShore
What happens when wind waves break againstthe shore?
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Waves Entering Shallow Water
Wave speed decreases as depth decreases.
Wavelength decreases as depth decreases.
Wave height increases as depth decreases.
Troughs become flattened and the wave profilebecomes extremely asymmetrical.
Period remains unchanged. Period is afundamental property of a wave.
Refraction is the bending of a wave crest intoan area where it travels more slowly.
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Wave Steepness
Maximum height for a given wavelength is basedon H/L. If H/L > 1/7, wave becomes too steep.
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Plunging waves break violently against the shore,leaving an air-filled tube, or channel, between thecrest and foot of the wave. Plunging waves areformed when waves approach a shore over asteeply sloped bottom.
Spilling waves occur on gradually sloping oceanbottoms. The crest of a spilling wave slides downthe face of the wave as it breaks on shore.
Breaking Waves
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Breaking Waves
In shallow water, wave height increasesand wave length decreases.
Wave steepness - wave height/wavelength (H/L)
When H/L is larger than or equals 1/7 (H/L≥ 1/7), the wave becomes unstable andbreaks.
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Spilling, Plunging and Surging Breakers
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Storm Surge
Rise in sea level due to low atmosphericpressure and build-up of water by stormwinds
Water is deeper at the shore area,allowing waves to progress fartherinland
Especially severe when superimposedupon a spring high tide
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Wave refraction - the slowing and bending of waves inshallow water
Wave diffraction - propagation of a wave around anobstacle
Wave reflection - occurs when waves “bounce back” froman obstacle they encounter. Reflected waves can causeinterference with oncoming waves, creating standingwaves
Wave Refraction, Diffraction,and Reflection
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Wave Refraction
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Wave Refraction
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Wave Diffraction
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Internal Waves
Waves that occur at the boundaries of water layerswith different densities are called internal waves.
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Internal waves
Form within the water column along thepycnocline due to density differences
Internal waves display all the properties ofsurface progressive waves including reflection,refraction, interference, breaking, etc.
Any disturbance to the pycnocline can generateinternal waves, including: flow of water related tothe tides, flow of water masses past each other,storms, or submarine landslides.
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Standing Waves or Seiches
Water surface “seesaws” back and forth
Node - imaginary line on the surfacewhere water does not oscillate
Antinodes - where maximumdisplacement of the surface occurs as itoscillates; usually located at the edge ofthe basin
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Seiches
Water rocking back and forth at a specificresonant frequency in a confined area is aseiche.
Seiches are also called standing waves.
The node is the position in a standing wavewhere water moves sideways, but does not riseor fall.
How does a seiche form?
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Standing Waves or Seiches
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Geometry of basin controls the period ofa standing wave
Standing waves can be generated bystorm surges or earthquakes
Resonance amplifies water disturbanceat the nodes and occurs when theperiod of the basin is similar to theperiod of the force producing thestanding wave.
Standing Waves
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Tsunami are long-wavelength, shallow-water, progressive waves causedby the rapid displacement of ocean water. Tsunami generated by thevertical movement of earth along faults are seismic sea waves. Whatelse can generate tsunami?
landslides
icebergs falling from glaciers
volcanic eruptions
other displacements of the water surface - asteroid impact
Tsunami and Seismic SeaWaves
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TsunamisPreviously called tidal waves, but not related to tides
Consist of long-period waves characterized by very longwavelength (up to 100 km) and high speed (up to 760km/hr) in the deep ocean
Because of their large wavelength, tsunamis are shallow-water to intermediate-water waves as they travel across theocean basin
Become a danger when reaching coastal areas wherewave height can reach 10 m
Tsunamis originate from earthquakes, volcanic explosions,or landslides
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Tsunami and Seismic SeaWaves
A tsunami, in 1946, was generated by a rupture along a submergedfault. The tsunami traveled at speeds of 212 meters per second.
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Tsunami Speed
Because tsunami have extremely long wavelengths, theyalways behave as shallow water waves.
The speed of a tsunami can be calculated using the sameformula used for other shallow-water waves:
C = √gd
g = 9.8 m/s2 (the acceleration due to gravity)
d = depth (a typical Pacific abyssal depth is 4,600 meters)
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Tsunami
Ten of the tsunami that have struck since 1990.
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Generation of a Tsunami
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Sumatra-Andaman IslandEarthquake
Magnitude 9.0 Sunday, Dec. 26, at 7:58 a.m. local time 30 km off the west coast of northern
Sumatra 250 km SSE of Banda Aceh Fourth largest in the world since 1964
Prince William Sound, Alaska, quake More casualties than any other tsunami in
recorded history
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Tsunami Warning Network
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63Parts of an ocean wave
Summary
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