Ocean andCoastal ProcessesChapter 17Tides are caused byGravitational Attraction ofthe Sun and MoonWaves• Waves are driven primarily by wind.• Waves may also be generated by earthquakes,submarine landslides and meteorites.• Wave height is the vertical distance betweencrest and trough.• Wave length is the horizontal distancebetween crest and trough.• Period is the time between crests.Wave Size• Wave height:– Ocean wave heights range from 0.3 to about 5m.– Maximum recorded ocean wave height is 34m (100ft).• Wavelength:– Ocean wave lengths range from 40 to about 400m.– Ocean waves travel at speeds of 25 to about 90 km/h• Wavebase– The depth at which wave motion ceases– Equals about 1/2 wavelengthWave Motion• Wave motion of the water extends to about one-half the wavelength (20 to 200m) (wave base).• When a wave enters shallow water (<1/2wavelength), it is slowed by drag.• Slowing causes bending of wave parallel tocoast (refraction).• Breakers are caused by the crest overtaking thetrough.Wave MotionWave BaseRefraction• Waves travel more slowly in shallow water.• If the water is shallower than the wave base.• This is called refraction• This causes the wave front to bend so it ismore parallel to shore.• It focuses wave energy on headlands.RefractionRefractionTsunamis•A tsunami is a seismic sea wave caused byan earthquake or submarine landslide.• Wave height in deep water of 0.5 to 2 m.• Wave height in shallow water up to 50 m.• Wavelength of 100 km• Wave velocity up to 700 km/h (400mph).Wave Motion andSediment Sorting• Question• Why does sand accumulate at the beach??Wave Motion andSediment Sorting• Waves sort sediment particles by size.• Fine particles (silt and clay) are kept suspendedif water is moving.• Sand accumulates at the beach, and fine particlescan only settle out in depths below wave motion(1/2 wavelength).Beach Environments•A beach is a strip of sand extending fromlow water line to cliff or zone of permanentvegetation.• In temperate climates, the dominant beachmineral is quartz (SiO2).• In tropical climates the beach sand mineral iscalcite (CaCO3).• Beach sands may also be volcanic glass.Beaches• Waves arriving at an angle to the beach willcause a current or long-shore drift whichmoves sand parallel to coast.–A bay-mouth bar is a sand bar that closes asmall bay.–A spit is a small peninsula of sand.–A tombolo is a sand bar connecting to a rockor off-shore outcrop.Seastacks, TombolosBeach EnvironmentsRising and Falling Coasts• Sea level is currently rising at about 2mm/y•An emergent coast is rising faster than the water.– Land may be rising due to glacial rebound (NewEngland, Scandinavia), or tectonic activity (California) orvolcanic activity (Hawaii).– Coastline is rocky•A submergent coast is falling relative to the water.– Southeast Atlantic and Gulf of Mexico Coasts.– Barrier island coast (temperate).– Barrier reef coast (tropical).Submergent CoastEmergent CoastClimate Change and OceanWaves• Ocean Wave Heights increasing since 1980• Both in Atlantic and Pacific• Average storm wave heights are measured as thelargest one-third of the waves.• US northwest coast experienced four storms in1998-1999 winter that exceeded 100 year norms.• One storm had average deep-water wave height inexcess of 14m. (~ 45 ft)The Ocean Basins• Cover about 70% of the Earth’s surface.• Are Geologically Young (<250 My)• Are underlain by basalt and gabbro• are under about 3000 m of water.• If this water was spread evenly over thesurface it would be about 2000 m.• There has been liquid water on the Earth asfar back as we can see (~4.3
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