Topic 16 Waves GEOL 2503 Introduction to Oceanography

What is a wave? Energy moving through some medium 2

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How a Wave Begins How do we put energy into the ocean surface to form waves? Called the generating force 4

Generating Force Wind, for most waves Pebble in a pond Earthquake Meteorite impact 5

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 6

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 8

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 9

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 time 10

Fig. 10-2, p. 266 Direction of wave motion AB Wavelength 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 11

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 12

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 Motion

Note the ½ wavelength depth and orbital diameter 14

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.” 15

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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 17

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 18

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 19

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 20

Wave Dispersion 21

Sea—waves in the area of generation 22

Swell— waves after dispersion 23

In a sea, the waves are sharp-crested, choppy, and irregular, with a mixture of wave heights, lengths, and periods 24

Swell—very regular and long-crested waves 25

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 26

Progress of a wave train 27

Group Speed Wave train moves at speed of one-half that of the individual waves Group speed = ½ wave speed = speed of energy transport C V = —— 2 28

Wave Height Wave height depends on 1.Wind speed 2.Wind duration 3.Fetch (area over which the blows) 29

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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 32

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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 34

12 a b Constructive interference (addition) Destructive interference (subtraction) Constructive interference (addition) 35

Episodic Waves Also called “rogue waves” Combination of intersecting waves, changing depths, and currents May be responsible for sudden disappearance of ships 36

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 37

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 38

When S approaches 1/7 (wave crest angle of about 120° ) the wave becomes too steep and breaks 39

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 40

Video Watch the learner.org video on Waves, Beaches and Coasts to see more about shallow water waves mlhttp:// ml 41

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 42

Wave Refraction 43

Wave Refraction Figure

Wave refraction around a rocky point 45

Wave refraction around a rocky island 46

Wave refraction into a small bay 47

Wave Refraction 48

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 49

Deep-water waves transitioning to surf 50

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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. 53

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 54

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Rip Currents Water piled up against the shore by breaking waves returns to sea in narrow currents called rip currents 56

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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 62

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Tsunami Characteristics Extremely long wavelengths ( km) Long periods (10-20 minutes) Low wave heights (1-2 meters) Shallow water waves 64

Tsunami caused by fault 65

2004 Indonesian Tsunami 66

Indonesian tsunami, December 26,

Banda Aceh Shore, Indonesia: before 68

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 72

Internal Waves 73

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 74

--At nodes there is no displacement --At antinodes there are alternately crestsand troughs Standing Waves 75

Water sloshes up and down around a central node 76

Summary: Types of waves 77