| |||
|
Properties of Waves. The Anatomy of a Wave Waves transmit energy, not water mass, across the ocean''s surface Fundamental Principles Ideally, waves represent a propagation of energy, not matter. ѓЬ (but ocean waves are not always ideal). Three kinds: ѓЬ Longitudinal (e.g., sound wave) ѓЬ Transverse (e.g., seismic Ў¡±SЎЁ wave) - only in solids ѓЬ Surface, or orbital wave ѓЬ Occur at the interfaces of two different densities. ѓЬ These are the common wind-generated waves. Some Definitions ѓЬ Wave Period: Time it Takes a Wave Crest to Travel one Wavelength (units of time) ѓЬ Wave Frequency: Number of Crests Passing A Fixed Location per Unit Time (units of 1/time) ѓЬ Frequency = 1/Period ѓЬ Wave Speed: Distance a Wave Crest Travels per Unit Time (units of distance/time) ѓЬ Wave Speed = Wave Length / Wave Period for deep water waves only ѓЬ Wave Amplitude: Wave Height/2 ѓЬ Wave Steepness: Wave Height/Wavelength Introductory wave period standing wave wave diffraction wave refraction wave reflection constructive interference destructive interference Mixed interference wave frequency wave length wave height trough internal wave swell sea surf zone tsunami shallow water wave orbital wave wave train plunging breaker spilling breaker capillary wave storm surge transverse wave crest resonating force wave steepness Tidal Bore Sir Isaac Newton seiche ebb current tidal range tidal period mixed tide semidiurnal tide diurnal tide neap tide quarter moon spring tide full moon new moon vernal equinox autumnal equinox winter solstice summer solstice apogee perigee aphelion perihelion gravitational force Marine science WAVE ANATOMY orbital wave progressive wave Most waves affect only the ocean''s surface layer. Movement ceases at a depth equal to about half the wave''s wavelenght. CLASSIFYING WAVES disturbing force restoring force wavelenght (capillary waves, wind wave, seiche, seismic sea wave-tsunami, tide Arranged from short to long wavelenghths, ocean waves are generated by very small disturbances (capillary waves), wind, rocking of waters in enclosed spaces (seiches), seismic and volcanic activity or other sudded displacement, and gravitation attractions (tides) DEEP-WATER WAVES AND SHALLOW_WATER WAVES Physical Oceanography DEEP-WATER WAVES AND SHALLOW_WATER WAVES Classification Progressive waves - classification depends on their wavelength relative to the depth of water deep-water waves - transitional water waves (L/2-L/20) - shallow awter awves The speed of ocean waves usually depends on their wavelength, with long waves moving fastest. The behavior of a wave depends largely on the relationship between the wave''s size and the depth of water through which it is moving. Wind wavws (dep-water waves) Seismic Sea Waves (Shallow0Water waves) Shallow-Water waves  travel in water that is shallower than 1/20 of the wave’s wavelength; Depth <L/2  Wave speed is function of water depth only  Waves are not ideal and propagate both Energy and Mass (Stokes Drift) Intermediate waves  neither purely “deep” or “shallow”; L/20 < Bottom Depth < L/2  wave speed depends on both wavelength and water depth Breaking Waves  As waves approach the shore, wavelength decreases, while height and steepness increase.  As waves approach the shore, drag of the bottom slows the water motion near bottom  Consequently, there is net forward transport at the surface as the waves steepens  Once height reaches 1/7 wave length, the wave becomes unstable and breaks. Wave Refraction & Reflection  Shallow-Water Wave change direction due to changes in speed; which are in turn due to changes in depth  The net result is a rotation of wave fronts to become parallel with bottom depth contours.  Wave may also be reflected  Consequence of Wave Refraction: wave energy is focused on headlands and away from bays – nature likes a straight coast!  Waves striking beach at an angle produce Longshore transport of sediment Transport Obstructed by Groins Wave Generation. WIND WAVES Generation of Waves ƒÜ Most surface waves generated by wind; therefore called wind waves Waves are also generated by ƒÜ Earthquakes, landslides ¡X tsunamis ƒÜ Atmospheric pressure changes (storms) ƒÜ Gravity of the Sun and Moon ¡X tides Height of Wind-Generated Waves depends on: ƒÜ Wind Speed ƒÜ Duration of Wind Event ƒÜ Fetch - the distance over which wind can blow without obstruction Waves and Swell ƒÜ Once generated, waves can propagate as swell without wind ƒÜ Dispersion: Large waves travel faster than small ones ƒÜ Because waves are not ideal, energy is dissipated and waves die out ƒÜ Small ones die out, or damp out, faster. Wave Speed and Water Depth Deep-Water waves ƒÜ travel in water that is deeper than 1/2 the wave¡¦s wavelength; Depth > L/2 ƒÜ Speed is function of wavelength only ¡V long wavelengths move faster ƒÜ Waves have nearly ideal shape and thus propagate energy but very little mass swell wind strength wind duration fetch Wind strength and duration determine the wavelength and speed of wind waves. wave steepness Global wave heigth wavelength records Interference Destructive interference constructive interference Crossing waves can ¡§interfere¡¨ to create either a bigger wave (constructive interference) or smaller wave (destructive interference). the freak wave=a rogue wave waves can interfere with one another, resulting in larger or smaller waves. Wind waves Approaching Shore surf zone plunging waves WAVE REFRACTION Most wave change shape and speed as they approach shope. They may plunge or spill at the surf zone and bend to breack nearly parallel to shore. LONG WAVES TIDAL WAVES STORM WAVES storm tides dome of water OTHER WAVES Other Waves  Tsunamis  Storm Surges & Sieches  Internal Waves  Planetary Waves Tsunamis  Tsunamis (sometimes improperly called tidal waves) are large amplitude, long wavelength waves that propagate on the ocean surface Tsunamis can be generated by  Earthquakes  Landslides  Volcanic Eruptions  Meteorite/Asteroid Impact Properties of Tsunamis  Tsunamis have long periods and long waves lengths (as long as 1 hour and 100 km respectively).  For tsunamis, wavelength is always greater than twice the water depth.  Therefore, tsunamis always behave as shallow water waves (speed depends on depth).  In water of average depth (4000 m), a tsunami will travel at 700 km/hr. The December, 26 2004 Sumatran Earthquake and Tsunami  Generated by a magnitude 9.3 earthquake as the Indian Plate thrust under the Sunda Plate  Calculated vertical displacements were as much as 5 meters  The fault ruptured along more than 1200 km  One of the largest earthquakes in past 100 years.  May have generated submarine landslides  Extensive damage and loss of life throughout the Eastern Indian Ocean Can we predict tsunami’s and save lives? Generating a Tsunami Warning  Earthquake occurs  Seismic waves travel through the Earth (at about 8 km/sec) to seismometers  Earthquake detected by seismometers  Determine location - did it occur in the sea?  Determine size - is it big enough to generate a tsunami?  Pacific Tsunami Warning Center issues bulletin  About 75% are false alarms - most earthquakes don’t generate damaging tsunamis  Governments must then take action to warn & evacuate  Detect tsunamis using seabed detectors - NOAA’s DART system: only 6 detectors deployed so far – most in the northeast Pacific; 32 planned for the Pacific by 2007 (at a cost of over $1 million a piece). What areas are vulnerable to tsunamis?  Pacific Rim is most vulnerable  Hawaii is particularly vulnerable  Japan, Alaska, S. America  Pacific Northwest  The plate boundary system in the Pacific NW is behaves very similarly to the Indonesia one, with very large, very infrequent earthquakes. Geologic evidence and Japanese historical records indicate a very large tsunami generated there in 1700.  Indian Ocean  Atlantic and Caribbean  Many of the Caribbean islands are subduction zone volcanoes - in addition to earthquakes, volcanic eruptions and volcanic landslides could generate tsunamis  Eastern Mediterranean  Both volcanically and seismically active  Salt beds beneath Mediterranean are particularly subject to landslides. Other Waves Seiches – resonant oscillations Internal Waves – waves that propagate along density boundaries within the ocean Kelvin and Rossby waves:  Low amplitude, long wavelength waves related to wind changes – notably El Niño |
