Shorelines
[50% of US population live within 1 hour drive (50 miles) of a coast]
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Depositional – sandy beaches
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Erosional- seacliffs
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Seasonal variation: erosion (winter); summer (depositional)
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Sand composition reflects local bedrock
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e.g volcanic, granitic, coral
Wind waves
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Wind gives waves energy; expend energy on coasts
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Size of waves: wind velocity, distance (fetch); length of time wind blows.
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L: wavelength- distance between crests
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Wave base: depth of wave = L/2
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Breakers: wave base “feels” bottom
Wave Refraction
• Change in wave velocity in shallow water
• Causes longshore drift (parallel to coast)
• Also causes headland erosion
• Groins: artificial barriers to prevent longshore drift
• Groins cause deposition upcurrent and erosion downcurrent
• Rip currents- seaward currents from longshore current
Types of beaches
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Mainland beach – coastal plain
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Barrier islands: sand bar (lagoons behind)
spit beaches: longshore drift of sand
Corals
• Coral reefs: important marine ecosystem
• Need: warm water (20C; 70F);
• Sunlight and need oxygen (shallow water)
• Occur in low latitudes (+/- 20 deg. Equator)
• Corals are animals with CaCO3 skeletons
• Live in symbiosis with algae (plants)
• Worldwide bleaching: warmer water; more acid ocean; pollution
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Beach erosion
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Sand supply (by rivers) less than removed by longshore drift
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Rising sealevel (thermal expansion; melting ice): 1-3 m next 100 years.
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On coastal plain, 1 inch sea level rise: causes retreat of shoreline 300ft.
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Dams – trap sediment behind- cause erosion of coastline, beach, sand bars, barrier
islands.
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Drought – less sediment transport
Local erosion
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Manmade structures: breakwater, jetties, groins.
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Cause sand depletion upcurrent (narrow beaches)
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Cause sand deposition upcurrent (wide beaches)
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Modern groins now permeable.
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Cliff erosion: causes cliff retreat
Tsunamis
Most caused by earthquakes; some caused by undersea landslides
Pacific ocean more prone, because of active plate boundaries
Ocean tides
Due to gravity of Earth’s Moon on oceans