Lecture 12 Ocean Waves
Focus Question: Why do waves break?
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1. Ocean Waves-today we will look at one of the most obvious aspects of the ocean-the waves that we see. We will consider classes of waves, look into what deep and shallow waves are, how waves form, and why they break. There are 2 simple but useful formulas to take note of:
Wave speed = wave length / wave period (same idea as speed = distance/time)
Wave Steepness = Height / wave length
2. What is a wave?
A wave is a disturbance caused by the movement of energy through a medium
As the energy passes through, the medium moves in
specific ways
Waves move across the surface but water particles do not move along with the
wave
Graphic: Garrison, Fig. 10.1, pg. 266 (pg 230).
3. Parts of a Wave
Crest -highest part of the wave
Trough - lowest part of the wave
Wavelength - distance between adjacent crests
Wave height -vertical distance from the trough to the crest
Period - time needed for the wave to move a distance of one wavelength
Graphic: Garrison, Fig. 10.2, pg. 266 (pg 230).
4. Classifying Ocean Waves
Ocean waves are classified by:
- the disturbing force that creates them
- the restoring force that tries to flatten them
- the influence of the disturbing force once they are formed
- their wavelength
Disturbing forces
Restoring forces
wind
gravity
low atmospheric pressure due to storms
surface tension
disturbance of the sea floor
5. Ocean Waves Classified by Restoring Force
Gravity ("gravity waves"):
- wavelength > 2 cm
- wind waves, seiches, tsunamis, tides
- can travel long distances
- can carry a lot of energy
Surface tension ("capillary waves):
- wavelengths < 2 cm
- very small wind waves (ripples)
- vertical motions of the sea surface
- do not travel long distances
- carry very little energy
6. Ocean Waves Classified by Disturbing Force
Wave
Type
Disturbing
Force
Typical Wavelength
Wind
wave
wind
60-150 m
Seiche
changes in atmos.
pressure
basin-scale
Seismic sea
landslide,
volcanic
200 km
wave (tsunami) eruption, earthquake
Tide
gravity, rotation of Earth 1 /
2 Earth's circumference
7. Forces and Ocean Waves
Graphic: Garrison, Fig. 10.5, pg. 269 (pg 232)
8. Orbital Motion
Water particles move in "orbits" as a wave passes
Wave motion is the result of the combined motion of all of the orbiting water
particles
Water particles do not move along with most ocean waves - most ocean waves
transfer
energy not mass
Graphic: Garrison, Fig. 10.3, pg. 267 (pg 230).
9. Deep vs Shallow Water Waves
Deep water waves:
- water deeper than 1/2 wavelength
- circular orbits that do not extend to the bottom
Shallow water waves:
- water shallower than 1/20th wavelength
- elliptical orbits that extend to the bottom
Transitional waves:
- water depths between 1/2 and 1/20th wavelength
Graphic: Garrison, Fig. 10.6, pg. 269 (pg 233).
10. How Do Wind Waves Grow?
Wind waves form when wind transfers energy to the water
Many wind waves grow from capillary waves
Capillary waves disrupt the ocean's surface, allowing more efficient transfer
of energy
from the waves to the water
As the wind continues to blow, waves grow and become more peaked
Garrison, Fig. 10.8, pg. 271 (pg 234).
11. Factors Affecting Wind Wave Development
- How stong the wind blows
- How long the wind blows
- The distance over which the wind blows (fetch)
12. Fully Developed Seas
Are attained when additional forcing by the wind does
not increase wave height
Occurs when energy added by the wind is equal to the energy removed by wave
breaking
Fully developed seas can have large or small waves
13. Wave Dispersion
Long waves leave the region of formation first
Short waves leave the region of formation last
Dispersion sorts waves over time
Graphic: Garrison, Fig. 10.11, pg. 273 (pg 236)
14. Wave Interference - Building a Rogue Wave or a Superwave
When two waves meet, they interfere
Destructive interference - waves cancel each other out, forming smaller waves
Constructive interference - waves add together, forming bigger waves (can
produce very
dangerous "rogue" waves)
Graphic: Garrison, Fig. 10.15, pg. 275 (pg 239).
15. Why Do Waves Break at the Shore?
As waves approach shore:
- deep water waves become shallow water waves and begin losing energy to the
sea bottom
(the wave "feels" the bottom)
- interaction with the bottom slows the wave and reduces the wavelength
- waves break when they become sufficiently steep
- As wavelength shortens, height grows, and they reach a 1: 7 ratio of height
to wavelength. They then topple over.
Graphic: Garrison, Fig. 10.17, pg. 277 (pg 241)
16. Plunging and Spilling Breakers
Plunging breakers
- Hollow tube formed between the falling crest and the foot of the wave
- Form over steeply sloping bottoms
17. Terminology Introduced in this Lecture
Wave
capillary wave
crest deep
water wave
trough shallow
water wave
wavelength fully developed sea
wave height fetch dispersion
dispersion steepness
period
swell
disturbing force constructive interference
restoring force destructive interference
forced wave plunging breaker
free wave spilling breaker
gravity wave
Next Lecture: Tides
Focus Question: Why does Earth have tides?