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CHAPTER 10 The Coast: Beaches and Shoreline Processes
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Overview Coastal region constantly changes Primarily due to waves
Erosion Deposition Many people live in coastal regions 80% of people in U.S. live within easy access of coast
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Coastal Processes Wave Refraction and the Longshore Current
Convergence and Divergence of Wave Energy Sediment Cells Sediment Budget Grain Size and Beach Slope
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S Fig. 10-1
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P Fig. 10.3b
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Composition of beaches
Locally available material May be coarse or fine Boulders from local cliffs Sand from rivers Mud from rivers Significant biologic material at tropical beaches Example, Coral reef material
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Longshore current P Wave refraction causes water and sand to move parallel to shore Zigzag motion in surf zone Longshore current Longshore transport
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Longshore transport Millions of tons of sediment moved yearly
Direction of transport changes due to wave approach In general, sediment transported southward along Atlantic and Pacific coasts of U.S.
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S Wave Refraction
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S
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P
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S Convergence Divergence
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S Lower energy, sand settles out
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Beach compartments Rivers supply sediment Beach
Offshore submarine canyons “drain” sediments from beach
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Erosional Coastline S Fig. 10.4
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Depositional shorelines
Primarily deposited by longshore drift Beach Spit Bay barrier Tombolo Barrier island Delta Beach compartment Fig. 10.7
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Depositional shorelines
Fig. 10.8
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P
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P Fig
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S
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P
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S
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S Winter Summer
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10_02a S Summer
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10_02b S Winter
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10_Ar S
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10_26Al P
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10_26Al P
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Changing sea level Local tectonic processes
Example, Pacific Coast of U.S. and active plate margin Isostatic adjustments Ice-loading Global (eustatic) changes in sea level Changes in sea floor spreading rates Lake buildup or destruction Ice volume changes
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Eustatic changes in sea level
P Ice build up (glaciation) Ice melting (deglaciation) Thermal contraction and expansion of seawater About 120 m (400 ft) change in sea level
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P Fig
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Global warming and changing sea level
P Global warming and changing sea level About 0.6oC (1.1oF) warmer over last 130 years Sea level rose cm (4-10 in) over past 100 years If global warming continues, higher sea level Fig
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U.S. coasts Erosion or deposition dominates Type of bedrock
Tidal range and wave exposure Active tectonics Eustatic changes in sea level U.S. coasts S Fig
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Alternatives to hard stabilization
Construction restrictions Limit building near shorelines National Flood Insurance Program encouraged construction Beach replenishment Sand added to beach/longshore current
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Alternatives to hard stabilization
Relocation Move structures rather than protect them in areas of erosion Fig. 10B
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