Ocean Waves: Causes, Characteristics, and Motion
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Presentation Transcript
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 - 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 • 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 • 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 Energy • Wind Speed • Duration Wind Blows • 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 • 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 • 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
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
