Waves A N. Hardy Powerpoint. What we will learn. Causes of Waves How Waves move Wave characteristics Orbital motion Wind Waves Waves approaching shores.

Waves

A N. Hardy Powerpoint

What we will learn.

• Causes of Waves• How Waves move• Wave characteristics• Orbital motion• Wind Waves• Waves approaching shores• Tsunamis

Where do waves come from?

• All waves created by a disturbing force– Cause 1: interface between fluids of different

density• All called surface waves• Air-air interface = atmospheric wave

– We’re not studying these in this powerpoint.

• Water-water interface = internal wave– In pycnocline– Sometimes very very tall

• Air-water interface = ocean waves– What we are studying

Where do waves come from?

– Cause 2: disturbances• Mass movement into ocean create splash waves• Earthquakes

– Cause seismic waves– Also called tsunamis

– Cause 3: gravity• Tides

– Very very very long wavelengths

– Cause 4: human activity• Ships create wake

How Waves move

• Longitudal waves– Compression and decompression of matter• Sound waves

• Transverse waves• Particles move back and forth• Waves created if you shake a rope• Only transmit energy In solids

• Orbital waves

How Waves Move

• Orbital Waves– Matter moves in a circular path.– A mix of longitudal waves and transverse waves

Wave Characteristics

• Parts of a Wave– Crest

• Highest part of the wave

– Trough• Lowest part of the wave

– Wavelength• Distance between crests or troughs

– Still water level• Average water level between crests and troughs

– Wave height• Distance from crest to trough• Amplitude is distance from still water level to crest or trough

– Amplitude = ½ wavelength

Wave Characteristics

• O.K. diagram

Wave Characteristics

• Simple equation for speed in a wave– C = L/T• C = celerity, which is the politically and scientifically

correct term for speed• L = wavelength• T = period

Wave Characteristics

• REAL Waves are constantly gettings smaller due to restoring forces– In smaller waves:• Cohesion creates a property called capillarity • Friction also takes energy away from wave

– These forces are also present in larger waves, but are not significant in comparison to wave breaking• When a wave breaks, a lot of energy is suddenly

released

Wave Characteristics

• Wave Steepness describes how tall a wave relative to its length– Wave steepness = H/L– When wave steepness reaches 1:7, the wave

breaks– The 1:7 rule also allows one to determine the max

height of a wave• A wave of wavelength 7 m can be 1m max height.

Orbital Motion

• Important: Wave Base– As you travel down the water column below a

wave, orbital motion continues, getting smaller and smaller

– The depth where orbital motion is negligible is called the wave base

– Wave bases are ½ of wavelength from still water level

Orbital Motion

Orbital Motion - effects

• Affects of Wave Base• Shallow Water Waves/Long Waves– Celerity affected by water depth– Water depth is less than 1/20 λ

• Transitional Waves– Celerity is ?kind of? Affected by water depth– Water depth is 1/2 λ < depth < 1/20 λ

• Deep Water Waves– Celerity not affected at all by water depth at all– Water depth is greater than than 1/2 λ

Orbital Motion

• Deep water waves– Their waves do not touch wave base– C = (gL/2π)^.5 ; C = 1.25 * (L)^.5– C = 2.56 T– Deep water waves are often called wind waves

because wind waves in the open ocean are deep water waves

– Orbitals are circles

Orbital Motion

• Transitional Waves– Their wave base touches the ocean floor, but only

partly

– Orbitals are slightly elliptical

Orbital Motion

• Shallow Water Wave / Long Wave– C = (gd)^.5 ; C = 3.13(d)^.5– Wave base is heavily affected by ocean floor– Orbital motion is very elliptical– Include tsunamis and tides

Orbital Motion

Orbital Motion

• Stoke’s drift– V = 2 1.25(πHL) sinh2 kh – Describes how fast objects floating in waves move.• I thought waves moved water in circular motion?• they do, however objects floating in water move.

Wind Waves

• Wind waves are generated by wind• Occur in the open ocean• Stuff happens• Best to start with a picture, one of the most

descriptive diagrams ever in science:

Wind Waves

Wind Waves

• All waves start as capillary waves• Then they GROW• Capillary waves have wavelengths smaller than 1.74 in• Troughs are V-shaped

– As waves develop they become gravity waves• Crests are V-shaped• Wave steepness is generally between 1:15 and 1:35

– As waves get bigger, the wind “catches” more of the wave• Maximum speed for a wind wave is the speed of the wind of

that created it.

• Area where waves are created is called the “sea area”

Wind Waves

• Factors affecting Wave Energy1. Wind Speed2. Duration Wind Blows3. Wind Fetch• Distance wind blows in a given direction

Wind Waves

• Wave height is directly related to the energy in the wave– More energy = more steep– When steepness = 1/7, the wave releases energy

and break• Open ocean breakers are called whitecaps

• Beaufort Wind scale describes the state of a sea, in comparison to wind speed– Created by Admiral Sir Francis Beaufort if the British

Navy

Wind Waves

Wind Waves

• Winds in the southern hemisphere between the 40s-60s latitudes are intense– Known as the “furious forties,” the “furious fifties,” and

the “screaming sixties”• 60 foot rule– Said that no wave could grow to be over 60 feet in height– According to the US navy in the early 1900s– Broken by the USS Ramapo in 1933, which accurately

recorded a wave to be 34 tall (112 feet) during 67 mph winds

Wind Waves

• How did they measure it?– geometry

Wind Waves

• Now we measure wave height with satellites such as POSEIDON– Look at this image showing average wave height:

Wind Waves

– Fully developed sea• When waves cannot grow any bigger under vertain

conditions, an equilibrium condition called a “fully developed sea” is reached.

• Basically, energy lost = energy gained (equilibrium)– Most energy lost from whitecaps

• Look at this graph showing the maximum qualities for a wave under a given wind speed, duration, and fetch created by data from the TOPEX/Poseidon satellite

Wind Waves

• Diagram linking wind speed, fetch, and duration to wind energy– Coming soon, when I find this diagram online, its

in my book So I know it has to be online somewhere.

Wind Waves

• Swell– As waves leave their origin, they eventually travel

faster than the wind around them• Separate and become long, even, crested waves called

swell

– Transport energy long distances

Wind Waves

• Real waves travel in wave trains– Wave dispersion = sorting of waves by their

wavelength– Distance to become a uniform swell from chop is

the decay distance• in a wave train, the leading wave dies out, and

a new wave is created in the back of the train– Effectively, speed of a wave train = ½ of the an

individual wave

Wind Waves

Wind Waves

• Interference patterns– When swells run together, they class or interfere

with one another.– Constructive interference occurs when waves

with the same wavelength overlap in phase– Destructive interference occurs when waves with

the same wavelength overlap out of phase– Mixed interference occurs when waves of various

heights and lengths overlap

Wind Waves

Waves Approaching Shore

• The zone of breaking waves is called the surf zone– Many special things happen in this zone that don’t

happen in the open ocean

Waves Approaching Shore

• Wave shoaling– Leads to waves breaking on shore without gaining any energy– Caused by any shallowly submerged obstacle– How does it work?

• Only occurs in shallow water waves and transitional waves• Orbital motion is interfered with by ocean bottom, leading to a decrease in

wave speed.• As one wave slows in a train, the following waveform, which is still moving at

original speed, moves closer to the slower wave, causes a decrease in wavelength

• Some energy is lost to friction, but most stays and has to go somewhere, so the wave height increases

• Decrease in wavelength and increase in wave height = increase in wave steepness.

• When the wave reaches the critical 1:7 point, it breaks

Waves Approaching Shore

• Swells are smoother waves, and break nearer to shore.

• Locally generated waves are usually choppy, and break further away from shore

• Are waves usually transitional or shallow water waves when they break?– transitional

Waves Approaching Shore

• Different types of breaking waves– Spilling breakers• Mass of are and water run down the front slope of the

wave as it breaks• On gently sloping beaches

– Plunging breaker• Long curling crests• On moderately steep beaches

– Surging breaker• Build up and break right at the shoreline

Waves Approaching Shore

• Refraction– Caused by uneven wave slowing– Not going to waste more time on this because we

know it

Waves Approaching Shore

• Wave reflection– When a wave hits a barrier, it is reflected similarly

how mirrors reflect– Sometimes, standing waves are formed when two

waves reflect off of each other

Tsunamis

• Despite popular belief, tsunamis are not always caused by earthquakes– Tsunami is a general term for waves with a very large

wavelength– Not always tall

• Causes:– Slippage on underwater faults– Underwater avalanches– Collapse of large oceanic volcanoes– Underwater volcanic eruption– Giant splash waves

Tsunamis

• All tsunamis are shallow water waves because of math– Super long wavelengths

• Coastal effects– When slippage occurs on an ocean fault,

• The wave traveling from the lower side will be led by the trough

• The wave traveling from the higher side will be led by the crest

– Can be destructive

Tsunamis

• Notable Tsunamis– Eruption of Krakatau

• Super loud eruption• Tsunami occurs 35m high.• Historians estimate deaths at 36,000• Occurred in 1883, Indonesia on the island Krakatau

– The scotch cap, Alaska/Hila, Hawaii Tsunami• Generated in Alaska by a 7.3 magnitude earthquake, hit Hawai

– Bathymetry of the Hilo bay focused the wave at Hawaii

• 1946• Waters reached 31m above sea level

Tsunamis

– Papua New Guinea• July, 1998 in Papua New Guinea• 15m high tsunami• Earthquake caused a turbidity current that generated

the huge wave

– Indian Ocean• More well known• Hurt lots of people• 2004• Earthquake magnitude 9.3

Tsunamis

• My personal favorites:– Lituya Bay, Alaska• 1958, a 7.9 magnitude earthquake triggered a huge

rockslide that generated a wave that right 530m, or 1740 ft high.• Measured by tree line

– The asteroid that killed the dinosaurs• Still looking for source