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Landform Geography
Coastal Processes and Landforms
July 24, 2013
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Oceans and Seas
• Oceans – largest bodies of water: Pacific, Atlantic, Indian, Southern, and Arctic
• Seas – next largest water bodies: Black, Mediterranean, Barents, etc.
• Gulf – next largest, usu. opens to larger water body: Mexico, Alaska, Guinea, etc.
• Bay – smaller still: Fundy, Biscay, etc.
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Oceans and Seas
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Water as Solvent
• Salinity – concentration of dissolved solids in seawater – global seawater salinity 34-37 parts per thousand (‰)
• Brine – water with >35‰ salinity
• Brackish water - <35‰ salinity
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Shaping the Coastline
• Key to shaping coastline is movement of water
• Eustatic Change - changes in water level in ocean – due to tectonic uplift or hydro cycle variation
• Land above sea level forms river valleys that extend to sea level – when sea level rises, valley floods, as with Chesapeake Bay or Delaware Bay
• Ria – river valley flooded by rising sea level
• Fjord – glacial valley flooded by rising sea level
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Eustatic Sea Level Changes
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North American Coastline Through Time
Lowest sea levels occurred during glaciations when water was tied up in glaciers (130k & 19k bp) – highest sea levels in interglacial periods (120k bp)
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Tides
• Regular, predictable oscillations of sea level – due to gravitation of moon (56%) & Sun (44%)
• High tide on side of Earth facing moon and on side away from moon – ellipsoid shape
• Long, narrow bays usually have highest tides – up to 16 meters in Bay of Fundy in eastern Canada
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Waves
• Oscillations in water due to force of friction from wind blowing across its surface
• Waves travel horizontally, but most movement of water is vertical, up-and-down
• Near coast seafloor slopes upward – wave base intersects ocean floor – wave pushes water up as it slows – waves pile up from behind - wave height exceeds 7X wavelength, and forms a breaker
• Only horizontal movement of water from breaker to beach, called surf – erosional agent
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Wave Compression
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Tsunami• Caused by undersea earthquakes with vertical
displacement, volcanic eruptions, or landslides• Vertical displacement of water causes fast wave
with long wavelength – no harm at sea, but massive as it hits coastline
• March 2011– Pacific plate subducts under North American plate which snaps upward – massive tsunami hits Indian Ocean
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2011 Japan Earthquake
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Littoral Processes
• Transport & deposition of sediment in shore zone• Longshore current – forms when wave hits beach
at oblique angle – water deflects downwind, parallel to beach
• Longshore drift – process of longshore current eroding & carrying sediment down shore
• Beach drift – zig-zag motion of sediment down beach due to swash & backwash
• Littoral Drift – longshore & beach drift together
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Littoral Drift
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Coastal LandformsErosional Coastlines
• Breaking waves have great power to erode
• Headland – promontory that juts into ocean or sea – made of resistant rock
• Waves slow & pivot around headlands – erosive power of waves concentrated on headland
• Retrogradation – retreat of coastline due to erosion
• Wave-Cut Bluff – basic erosional landform of coastlines – near-vertical cliff at water’s edge
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Coastal Erosional Landforms
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Depositional Coastlines
• Progradation – process of coastline extending outward into water through deposition
• Beach – dynamic transition from sea to land– Offshore – under water, where waves break– Foreshore – rise & fall of tides– Offshore Bar – between offshore & foreshore– Beach Ridge – at high water line– Backshore – flat, only covered in storms
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Beach Cross Section
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Spits and Baymouth Bars
• Longshore current carries sediment down beach
• Current slows upon reaching bay – sand deposited as a Spit extending out into bay – current in bay turns spit toward land in hook shape
• Baymouth Bar – spit extends across bay, isolating it from ocean – bay now called Lagoon
• Tombolo – longshore currents from 2 directions meet – sand extends out to island or sea stack
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Depositional Landforms
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Barrier Islands
• Elongated bars of sand that form parallel to shore
• Likely formed from sand deposited on cont. shelf during last glaciation – waves & wind shaped sand
• Lagoons w/mudflats form behind barrier islands
• Mudflats develop into vegetated salt marsh
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Coral Reefs
• Coral polyps excrete external skeletons of calcium carbonate (limestone) – Coral Reefs
• New reefs form on top of old, dead reefs
• 30º N - 25º S latitude, water warmer than 20º C
• 3 settings:– Fringing reef – on shallows around island– Barrier reef – line of coral parallel to shore– Atoll – semicircular reef around degraded volcanic
island
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Development of Atoll
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Global Distribution of Coral Reefs
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Human Impacts on Coastlines
• 37% of world pop. live <60 mi. from shore (2 billion) – 50% within 120 miles of shore
• In US, 53% of pop. live near coastline
• Coastal Engineering Purposes:– Protecting shore & property from hazards– Stabilizing & nourishing beaches– Maintaining traffic & trade into ports
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Mitigating Coastal Hazards
• Raise buildings on stilts so waves roll underneath
• Sea wall – vertical, concrete wall to absorb energy of waves
• Revetment – slope covered with large rocks (rip-rap) to absorb energy of waves
• These work locally, but they aggravate erosion up and down shore from wall due to wave refraction
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Beach Nourishment
• Bringing sand to beaches that have eroded – over $336M spent in Florida since 1960s
• How?– Trucking in sand from remote location– Limit loss of sand through groins – low walls built at
right angles to beach – intercept longshore drift
• Jetties – stone or concrete structures to keep channel open, keep sand to side
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Sea Wall
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Groins and Jetties