# Waves - Introduction to Oceanography - Lecture Slides, Slides for Oceanography. Gujarat University

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These are the lecture slides of Oceanography. Key important points are: Waves, Wind Blowing, Turbidity Currents, Coastal Landslides, Waves Move Energy, Movement of Particles, Seagull Resting, Ocean Surface, Orbital Waves...
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Waves

Waves

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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

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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!

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• 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

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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

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Wave motion is negligible below a depth of one half of the wavelength

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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

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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

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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

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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

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Wavelength is the most useful measure of wave size

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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

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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

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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…

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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’

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Gee, that’s just swell… • This results in swells; uniform, symmetrical

wind waves that have traveled out of their area of origination

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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: additiveDestructive: subtractive (cancellation)

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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

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• Constructive: crests of waves coincide • Destructive: crest and trough of waves coincide

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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

Just how big can waves be?

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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

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