Chapter 17 Ocean and Coastal Processes Tides are caused by Gravitational Attraction of the Sun and Moon Waves Waves are driven primarily by wind Waves may also be generated by earthquakes submarine landslides and meteorites Wave height is the vertical distance between crest and trough Wave length is the horizontal distance between 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 wavelength Wave Motion Wave motion of the water extends to about onehalf the wavelength 20 to 200m wave base When a wave enters shallow water 1 2 wavelength it is slowed by drag Slowing causes bending of wave parallel to coast refraction Breakers are caused by the crest overtaking the trough Wave Motion Wave Base Refraction 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 is more parallel to shore It focuses wave energy on headlands Refraction Refraction Tsunamis A tsunami is a seismic sea wave caused by an 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 and Sediment Sorting Question Why does sand accumulate at the beach Wave Motion and Sediment Sorting Waves sort sediment particles by size Fine particles silt and clay are kept suspended if water is moving Sand accumulates at the beach and fine particles can only settle out in depths below wave motion 1 2 wavelength Beach Environments A beach is a strip of sand extending from low water line to cliff or zone of permanent vegetation In temperate climates the dominant beach mineral is quartz SiO2 In tropical climates the beach sand mineral is calcite CaCO3 Beach sands may also be volcanic glass Beaches Waves arriving at an angle to the beach will cause a current or long shore drift which moves sand parallel to coast A bay mouth small bay bar is a sand bar that closes a A spit is a small peninsula of sand A tombolo is a sand bar connecting to a rock or off shore outcrop Seastacks Tombolos Beach Environments Rising 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 New England Scandinavia or tectonic activity California or volcanic 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 Coast Emergent Coast Climate Change and Ocean Waves Ocean Wave Heights increasing since 1980 Both in Atlantic and Pacific Average storm wave heights are measured as the largest one third of the waves US northwest coast experienced four storms in 1998 1999 winter that exceeded 100 year norms One storm had average deep water wave height in excess 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 the surface it would be about 2000 m There has been liquid water on the Earth as far back as we can see 4 3 By
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