LECTURE NOTES: OCEANOGRAPHY (MARSC 100), SNYDER, L.
CH. 12 Coasts
Coastlines (Coastal Zone)
Long, narrow geographic features
Where ocean & land meet (sea level)
Margins of continents & islands
Beaches, bluffs, dunes, estuaries (swamps, salt marshes), coral atolls, sea stacks
Shoreline: limited to beach or littoral zone (rocky intertidal, sandy beach)
Coastline Location & Shape Always changing (long & short-term)
Location controlled by:
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Plate Tectonics
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Volume of water in Ocean
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Shape controlled by:
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Tectonic uplift & subsidence (sinking)
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Sea level fluctuations
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Erosion
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Deposition
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Volcanism
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Human activity
World Sea Level Change (Eustatic)
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Lower during ice ages (glaciation)
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Higher in warm periods (Glaciers melt, warmer water expands)
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Seafloor spreading & continental erosion: Displaces H2O = higher on continents
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Global warming: seawater expands (takes up more space), ice melts o Raises sea level
Current sea level: stable for ~2,500 years

In past 200 million years, varied from current level:

6m (20ft) above (warm period)
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125m (410ft) below (glaciation) – 18,000 yrs. ago
Bering Land Bridge:
Exposed during glaciations (sea level decreased)
Last Exposed: ~22,000-7,000 Yeags Ago
Allowed for migration of animals & humans to/from Eurasia & North America
Classifying Coasts
By dominant shaping processes
2 Types:
1. Erosional – removal of material (shrinking)
2. Depositional – sediments are added & accumulate (growing)

Erosional Coasts:

Attacked from LAND & sea o River erosion - river valleys cut into land during last glaciation
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Drowned by seawater as sea level rose o Glacial erosion (fjords) – Moving glaciers cut U-shaped troughs o Plate Tectonics (fault zones) – Continental crust torn from coast
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Gulfs form as seawater intrudes (Ex: Tomales Bay CA – along
San Andreas Fault)
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Attacked from land & SEA
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Wave & surf action – water & sediment abrades shore
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Erosion rate depends on: o Rock hardness (Granite vs. Sandstone) o Wave energy & degree of protection o Low-energy Coasts (Gulf, bays): protected location, few large waves
Features of Erosional Coasts: Wave cut platforms, sea caves, sea stacks
Depositional Coasts (Growing):
Surface composed of sediment, not rock
Sediments added = coastline grows
1. Rivers transport from land (main source)
2. Transported from other coastal area
Beaches (loose particles cover all or part of shore): Silt, sand, cobbles, boulders
1. River Transport of Sediment:
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Carry sediments from land, mountains, etc.
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Deltas: Mouth of many large rivers (Mississippi, Nile, Ganges – India) o Extensive coastal build-up o Broad continental shelf (sediment platform) o Low-energy coasts (protected) o Tectonically stable areas (No subduction /convergence) o Decreased Sediment supply to delta leads to erosion, subsidence, compaction
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Dams & levees along river decrease sediment supply to delta
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EX: Mississippi Delta loses ~1 cm/yr
2. Sediment Transport Along the Coast:
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Longshore transport (drift) carries sediment along coast o By wave & current action o Where seafloor slope is not steep
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Lonshore current moves sediment between:
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Surf zone
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Upper limit of wave action

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Wind waves approach shore at an angle
Rip Currents:
On High-energy coast (high surf)
Incoming waves pile excess water on land side of surf zone
Faster than longshore current carries it downcoast
Excess water breaks through & returns toward sea (look for mushroom shaped sediment plume)
Escape by swimming parallel to shore
Southern CA Coastal Transport Cells
Sand introduced by rivers
Moved South by longshore transport
Trasported offshore into submarine canyons
Beaches are Shrinking due to human activities
Biologic Coasts
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Coral Reefs: Colonial animals Secrete a calcium carbonate exoskeleton o Left behind at death o Found in Warm, shallow water
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Estuaries: Where Rivers meet the Sea o Semi-enclosed o Vegetation can tolerate salt
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Swamps – have trees, tropical & subtropical latitudes
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Salt marshes – no trees (grasses & succulents), temperate
Estuaries Originate as:
1.
2.
3.
Drowned river mouths: When sea level rises, lower portion floods
Fjords: as glaciers melt, sea level rises & glacial valleys flood
Bar-built: built up sediment blocks tidal
4. Tectonic: Faults cause subsidence (sinking)
•Depressions fill with water
Value of Estuaries: flow (bars, islands)
High Primary Productivity
Food web support
Oxygen
Fish & Wildlife habitat
Flood Control
Water quality improvement
Shoreline stabilization & bank protection
Groundwater recharge
Recreation & aesthetics

Estuarine Destruction
~60% lost in lower 48 states
75% lost in Southern California
Many being restored (Batiquitos Lagoon in 1997, Bolsa Chica 2004-2007)
But, many continue to be disturbed by adjacent human land use (trains, roadways – PCH, power plants, sanitation treatment plants, housing, hotels, retail, golf courses, harbors…..)
Human Assault on Coastlines
>53% U.S. population live in coastal areas
Pacific Coast population more than doubled since 1960
Coasts: natural balance between erosion & deposition
Humans build structures that oppose coastal processes
Cause cell imbalances
Seawalls
Blocks wave erosion of cliffs or bluffs
To protect structures
Reflected wave energy increases beach erosion in front of & beside seawall
Blocks natural bluff erosion
Decreased sediment input for beach replenishment
Importing Sand
In response to human induced erosion & decreaseddeposition
We import sand for beach replenishment!
New sand dredged from offshore
Very expensive (millions)
Damages benthic ecosystems
Erodes faster than beach sand (finer)
WE HAVE TO KEEP DOING IT!
81% of California’s Beaches are Critically Eroded
(only 14% are stable)
80% of Lower Mississippi critically eroded
Coastal development paid for by taxpayers:
Subsidize growth through federal flood insurance
Beach restoration projects
Road repairs & other public works