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Unformatted text preview: Waves and Water Dynamics Most waves winddriven Moving energy along ocean/air interface Origin of waves Wind main disturbing force Boundary between and within fluids with different densities Air/ocean interface (ocean waves) Air/air interface (atmospheric waves) Water/water interface (internal waves) Internal waves Associated with pycnocline Larger than surface waves Caused by tides, turbidity currents, winds, ships Possible hazard for submarines Other types of waves Splash wave Seismic sea wave or tsunami Tides Sea floor movement Coastal landslides, calving icebergs Wake Gravitational attraction among Moon, Sun, and Earth Ships Types of ocean waves Waves Measurements of a wave Wave height the distance between a trough and a crest Wavelength the horizontal distance between successive crests (or troughs) Wave period the time interval for one full wave to pass a fixed position Waves As the wave travels, the water passes energy along by moving in a circle Waveform moves forward At a depth of about onehalf the wavelength, the movement of water particles becomes negligible (the wave base) Progressive waves Longitudinal (Primary wave) Transverse (Secondary wave) Orbital (Surface wave) Orbital or interface waves Waves on ocean surface Crest, trough, wave height (H) Wavelength (L) Orbital waves Wave characteristics Wave steepness = H/L If wave steepness > 1/7, wave breaks Wave period (T) = time for one wavelength to pass fixed point Wave frequency = inverse of period or 1/T Waves the interface between ocean and atmosphere Derive their energy and motion from wind Parts of a wave Represent energy traveling along Crest Trough Circular orbital motion
Water particles move in circle Movement up and down and Back and forth Orbital motion Fig. 8.5 Diameter of orbital motion decreases with depth of water Wave base = L Hardly any motion below wave base due to wave activity Water depth is greater than wave base (>1/2L) Wave speed proportional to wavelength Deep-water waves Characteristics of both deep and shallowwater waves Speed depends on both water depth and wavelength Transitional waves Water depth is < 1/20L Wave speed is proportional to depth of water Shallow-water wave Changes that occur when a wave moves onto shore Waves Measurements of a wave Wave height the distance between a trough and a crest Wavelength the horizontal distance between successive crests (or troughs) Wave period the time interval for one full wave to pass a fixed position The basic parts and movement of a non-breaking wave Waves Wave height, length, and period depend on Wind speed Length of time the wind blows Fetch the distance that the wind travels Wave development from a hurricane Wave energy Fully developed sea Swell Maximum wave height, wavelength for particular fetch, speed, and duration of winds at equilibrium conditions Uniform, symmetrical waves that travel outward from storm area Long crests Transport energy long distances Admiral Halsey's pacific fleet twice encountered typhoons during WW II Swell Longer wavelength waves travel faster and outdistance other waves Wave train = group of waves with similar characteristics Sorting of waves by their wavelengths is wave dispersion Wave train speed is speed of individual wave Swell wave train Wave interference patterns Different swells coming together Constructive interference Destructive interference Mixed interference Inphase wave trains with about the same wavelengths Outofphase wave trains with about the same wavelengths Two swells with different wavelengths and different wave heights Wave interference patterns Surf Beat Mixed interference produces the type of wave best suited for surfing. Maximum wave height ever recorded from a Rogue Wave Reliable measurement Wave height 34 m or 112 ft Waves approach shore Deepwater swell waves shoal Transitional waves Shallowwater waves Wave speed decreases Wavelength decreases Wave height increases Wave steepness increases Waves break Shoaling waves Fig. 8.16 Breakers in surf zone
Top of wave topples over base because of decrease in wave speed due to friction with seafloor Wave form not sustained Different types of breakers associated with different slope of seafloor Spilling breaker Water slides down front slope of wave Gently sloping seafloor Wave energy expended over longer distance Plunging breaker Curling crest Moderately steep seafloor Wave energy expended over shorter distance Best for board surfers Wave refraction As waves approach shore, they bend so wave crests are nearly parallel to shore Wave speed proportional to depth of water (shallowwater wave) Different segments of wave crest travel at different speeds Wave refraction along an irregular coastline Beaches and shoreline processes Wave refraction Bending of a wave Wave arrives parallel to shore Results Wave energy is concentrated against the sides and ends of headland Wave erosion straightens an irregular shoreline Wave energy distribution at shoreline Energy focused on headland Headland eroded Energy dissipated in bay Bay filled up with sediment Fig. 8.18 b Wave refraction Beaches and shoreline processes Longshore transport Beach drift sediment moves in a zigzag pattern along the beach face Longshore current Current in surf zone Flows parallel to the shore Moves substantially more sediment than beach drift Beach drift and longshore currents End of Waves Surging breaker Breakers on shore Steepest seafloor Energy spread over shortest distance Best for body surfing Standing waves Two waves with same wavelength moving in opposite directions Water particles move vertically and horizontally Water sloshes back and forth Tsunami or seismic sea wave Sudden changes in seafloor caused by Long wavelengths (> 200 km or 125 m) Shallowwater wave Speed proportional to water depth so very fast in open ocean Sea level can rise up to 40 m (131 ft) when tsunami reaches shore Earthquakes, submarine landslides, volcanic eruptions Tsunami or seismic sea wave Fig. 8.21a Tsunami or seismic sea wave
Most occur in Pacific Ocean (more earthquakes and volcanic eruptions) Damaging to coastal areas Loss of human lives Example, Krakatau eruption (1883) in Indonesia created tsunami that killed more than 36,000 people Example, Aura, Japan (1703) tsunami killed 100,000 people Tsunami watches and warnings Pacific Tsunami Warming Center Seismic waves forecast possible tsunami Tsunami watch Tsunami warning Evacuate people from coastal areas and send ships from harbors Increasing damage to property as more infrastructure constructed near shore Waves as a source of producing electricity Lots of energy associated with waves Mostly with large storm waves Environmental issues How to protect power plants How to produce power consistently Offshore power plants? Building power plants close to shore Interfering with life and sediment movement Wave power plant at Islay, Scotland Fig. 8.25b Global coastal wave energy resources Fig. 8.26 End of CHAPTER 8 Waves and Water Dynamics Fig. 8A Most ocean waves windgenerated Capillary waves (ripples) formed first Wave development Increasing energy results in gravity waves Increasing energy results in trochoidal waveforms Crests pointed, troughs rounded, greater wave heights Symmetrical waves with longer wavelengths Rounded crests, very small wavelengths Sea = area where waves generated by storm Waves and wave energy bounced back from barrier Reflected wave can interfere with next incoming wave Wave reflection The Dynamic Ocean DeepOcean Circulation Waves Beaches and Shoreline Processes Shoreline Features Stabilizing the Shore Coastal Classification Tides Surface Circulation The Dynamic Ocean DeepOcean Circulation Waves Beaches and Shoreline Processes Shoreline Features Stabilizing the Shore Coastal Classification Tides Surface Circulation Beaches and shoreline processes Beaches are composed of whatever material is available Wave erosion Caused by Some beaches have a significant biological component Material does not stay in one place Wave impact and pressure Breaks down rock material and supplies sand to beaches Wave development Wave energy Factors that control wave energy Wind speed Wind duration Fetch ...
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- Summer '07