Topic 16Waves

GEOL 2503Introduction to Oceanography

What is a wave?

• Energy moving through some medium

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How a Wave Begins

• How do we put energy into the ocean surface to form waves?

• Called the generating force

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Generating Force

• Wind, for most waves• Pebble in a pond• Earthquake• Meteorite impact

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Restoring Force

• Force that causes water to return to its undisturbed level

• Surface tension for very small waves (called ripples or capillary waves)

• Gravity for large waves

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Ripples (capillary waves) on top of waves 7

Most Waves of Interest

• Surface waves– travel on the water surface

• Gravity waves– gravity is the major restoring force

• Wind waves– wind is the major generating force

• Progressive waves– Moving forward through the water

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Waves

• Energy moving through a medium• Water is the medium• Sound is also energy moving through air,

water, walls, etc. • Waves move through the medium, but the

medium is not transported

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Wave Terminology• Crest = highest water level• Trough = lowest water level• Height = H = distance from crest to trough• Amplitude = A = distance from still water

level to crest or trough• Period = T = time (in seconds) for two

successive crests to pass fixed point• Frequency = F = number of wave crests to

pass a fixed point in a given amount of time10

Fig. 10-2, p. 266

Direction of wave motion

A BWavelength

Height

Still water level Crest Trough

Frequency: Number of wave crests passing point A or point B each second

Orbital path of individual water molecule at water surface

Period: Time required for wave crest at point A to reach point B

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Wave Motion

• Energy moves through the water, but the water is not transported

• Individual water particles move in a circular path called a wave orbit

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1. As wave trough passes, water particles begin to rise and move backward

2. At the midpoint of crest approach, water particles stop moving backward, continue to rise, and begin moving forward

3. Under crest, particles have stopped rising and are moving forward with speed of the crest

4. As crest passes, particles fall and stop moving forward

5. As trough advances, particles move backward

6. At bottom of trough, maximum backward speed

The bird, cork, boat, or water particles have no net forward motion, they move in circular orbits

Orbital Wave Motion1

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Note the ½ wavelength depth and orbital diameter

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Wave Speed L (wavelength)• C = —— T (time)

C = Celerity, speed at which a wave moves across the sea surface

Once a wave is created, its speed may change, but its period remains the same

The speed of a wave is the distance traveled by its crest per unit time. It is known as celerity, thus the “C.”

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Deep-Water Waves

• Waves in water deeper than one-half the wave’s length

• DWWs move at a speed controlled by their wavelength

• Leads to wave dispersion

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Storm Centers

• Most waves are progressive wind waves• Generated by wind• Restored by gravity• Progress in a particular direction• Formed in local storm centers or in trade wind belts

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Sea

• In the storm center, winds are variable and turbulent

• Sea surface is a mixture of waves of all heights, lengths, and periods

• Called a “sea” (waves in the area of generation)

• Waves move outward in all directions

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Wave Dispersion

• Waves from storm center move at speeds controlled by wavelength

• The greater the wavelength, the greater the speed

• Faster, longer waves gradually move through and ahead of shorter, slower waves

• This is called sorting, or dispersion

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Wave Dispersion

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Sea—waves in the area of generation

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Swell—waves after dispersion

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In a sea, the waves are sharp-crested, choppy, and irregular,with a mixture of wave heights, lengths, and periods

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Swell—very regular and long-crested waves

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Wave Train

• As wave train progresses, leading waves lost

• Energy used to advance wave form into undisturbed water

• Therefore, speed of each individual wave (C) in the group is greater than the speed of the leading edge of the wave train

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Progress of a wave train

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Group Speed• Wave train moves at speed of one-half that

of the individual waves• Group speed = ½ wave speed = speed of

energy transport

CV = ——

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Wave Height

• Wave height depends on

1. Wind speed2. Wind duration3. Fetch (area over which the blows)

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Wave Terminology

• Fully developed sea—when the local wind speed, duration, and fetch have transferred maximum energy, thus creating the largest possible waves for that set of conditions

• Maximum wave height—largest recorded• Average wave height—average of all• Significant wave height—average height

of the highest one-third of waves over long time periods

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Wave Interaction

• Waves are likely to meet other waves from other storm centers

• Waves may intersect at any angle• Form interference patterns

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1 2

a

b

Constructive interference

(addition)

Destructive interference (subtraction)

Constructive interference

(addition)

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Episodic Waves

• Also called “rogue waves”• Combination of intersecting waves,

changing depths, and currents• May be responsible for sudden

disappearance of ships

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Usually many different storms, producing waves of different characteristics. They interfere with each other to produce small swell.

When they become synchronized they can produce huge waves that can overwhelm small boats. Waves can have amplitudes of between 5 and 15 m.

Rogue Waves

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Wave Steepness

• There is a maximum possible height for any given wavelength

• Ratio of wave’s height to length• S = H/L• Called wave steepness

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When S approaches 1/7 (wave crest angle of about 120° ) the wave becomes too steep and breaks

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Shallow-Water Waves

• As deep-water waves approach shore, wave orbits interact with sea bottom

• Orbits gradually become ellipses (flattened circles)

• SWW speed is controlled by water depth• Leads to refraction and wave breaking

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Video

• Watch the learner.org video on Waves, Beaches and Coasts to see more about shallow water waves

• http://www.learner.org/resources/series78.html

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Wave Refraction

• Refraction means bending• Waves usually approach shore at an angle• Part in shallower water moves slower• So part of the wave is in deeper water and

moves faster• The wave bends or refracts• Waves end up more parallel to shoreline

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Wave Refraction

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Wave Refraction

Figure 20-544

Wave refraction around a rocky point

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Wave refraction around a rocky island

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Wave refraction into a small bay

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Wave Refraction

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Wave Breaking

• The surf zone is the shallow area along the coast where waves slow, steepen, break, and expend their energy

• Waves in the surf zone do transport water forward

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Deep-water waves transitioning to surf

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The type of breaking wave depends on the steepness of the seafloor

• Spilling breaker: Top of wave crest ‘spills over’ wave. Energy released gradually across entire surf zone.

• Plunging breaker: Crest ‘curls over’ front of wave. Energy dissipates quickly. Common at shorelines with steep slopes

• Surging breaker: Never breaks as it never attains critical wave steepness. Common along upwardly sloping beach faces or seawalls. Energy released seaward.

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Longshore Current

• Water is transported shoreward in breaking waves

• Waves often approach the beach at an angle• So water is moved along the beach in the

direction of travel of the waves

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Rip Currents

• Water piled up against the shore by breaking waves returns to sea in narrow currents called rip currents

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rip currents

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Tsunamis

• Seismic sea waves• Japanese for “harbor wave”• Not tidal waves• Generating force is seismic event

– Earthquake– Submarine volcano– Submarine landslide

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Tsunami Characteristics

• Extremely long wavelengths (100-200 km)• Long periods (10-20 minutes)• Low wave heights (1-2 meters)• Shallow water waves

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Tsunami caused by fault

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2004 Indonesian Tsunami

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Indonesian tsunami, December 26, 2004

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Banda Aceh Shore, Indonesia: before68

Banda Aceh Shore, Indonesia: after 69

Gleebruk Village (1): before 70

Gleebruk Village (1): after 71

Internal Waves

• We’ve been discussing surface waves• Some waves form on boundary between

water of slightly different densities• Slower than surface waves

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Internal Waves

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Standing Waves

• Waves that do not progress• Often formed when waves are reflected

back on themselves• Crests and troughs move up and down in

place

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--At nodes there is no displacement--At antinodes there are alternately crests and troughs

Standing Waves

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Water sloshes up and down around a central node 76

Summary: Types of waves

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