Coasts - Key Ideas l Coasts are temporary structures, often subject to rapid change. l The location of a coast depends on tectonic activity and the level of water in the ocean. l Coasts are classified as primary or secondary. l Human interference in coastal processes rarely increases the long term stability of a coast.
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Classifying Coasts
Coasts can be classified by a method that takes into account tectonic position and sea level.
Primary coasts are young coasts dominated by terrestrial influences.
Secondary coasts are older coasts that have been changed by marine influence.
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Types of Primary Coasts
• Drowned rivers
• Coasts built out by land processes (e.g., rivers, glaciers)
• Volcanic coasts
• Coasts shaped by tectonic processes
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Primary Coasts
Many of the islands of Martha’s Vineyard and Nantucket are 4 examples of a primary coast.
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Processes Shaping Coasts
• Waves and currents
• Stream erosion
• Wind abrasion
• Freeze/thaw cycles
• Slumping
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Secondary Coasts
Waves are one of the forces that shape secondary coasts. 6
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Alternative Classification of Coasts l Erosional Coasts
l Features are: headlands, cliffs, stacks, arches, wave-cut platform l Depositional Coasts
l Features are: barrier islands, deltas, glacial deposits
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Other Types of Coasts l Drowned Coasts
l Estuaries, fiords l Emergent or Uplifted Coasts
l Uplifted marine terraces l Coasts affected by organisms
l Reefs, mangroves
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Beaches
A beach is a zone of loose particles that covers a shore.
Note that larger particles are associated with more sloped beach.
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Features of Beaches
• berm • berm crest • backshore • foreshore • beach scarp • longshore trough • longshore bars
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Beaches
The features of a beach.
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Wave Refraction and Longshore Current
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Longshore Drift l Features formed by longshore drift
l Spit, baymouth bar, tombolo l Human interference with longshore drift
l Jetties, groins, breakwaters l Sources of sand
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Large Scale Features of Secondary Coasts
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Origin of Barrier Islands
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Coastal Cells
Sections of coast in which sand input and sand output are balanced are referred to as coastal cells. 25
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Coasts Formed by Biological Activity
Coral Reefs are the most dramatic of the coasts formed by biological activity.
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Coral Reef Classifications
Fringing Reefs cling to the margin of land.
Barrier Reefs are separated from land by a lagoon.
Atolls are ring-shaped islands of coral reef enclosing lagoons.
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Storm Surges
A storm surge is an abrupt bulge of water driven on shore by a tropical cyclone or a frontal storm.
Storm surges are short-lived.
Storm surges consist of only a crest, so they cannot be assigned a period or wavelength, and cannot be called a wave.
Storm surges are sometimes called storm tides.
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Human Interference on Coastal Processes
• Groins
• Seawalls
• Importing sand
These methods are largely ineffective in solving beach erosion.
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Coasts
A map showing shore erosion by region. One example of shore erosion is the lighthouse on Cape Hatteras, which was moved during 1998 and 1999 to protect it from destruction. It was threatened by rising sea levels and a changing shoreline.34
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Characteristics of U.S. Coasts
The Pacific Coast - An actively rising margin where indications of recent tectonic activity can be observed.
The Atlantic Coast - A passive margin on the trailing position of the North American plate.
The Gulf Coast - Smaller wave size and a smaller tidal range characterize the Gulf Coast.
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Summary
A few of the many types of measures taken to slow and prevent beach erosion. Some of these methods are actually counterproductive.
In many cases these methods help serve as a reminder that shorelines and beaches are constantly changing, and are not under human control.
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Estuaries
An estuary is a body of water in which fresh river water mixes with ocean water. Estuaries can be classified by their origin.
• Drowned river mouths
• Fjords
• Bar-built
• Tectonic
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Estuaries
The four types of estuaries. 39
Salt-Wedge Estuary (river-dominated)
Examples: Mouths of Mississippi, Hudson and Columbia Rivers
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Well-Mixed Estuary (tide-dominated)
Examples: Shallow estuaries such as Chesapeake and Delaware Bays
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Partially Mixed Estuary
Examples: Puget Sound and San Francisco Bay
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Fjord-Type Estuary
Examples: Glaciated coastal mountains
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Estuarine Water Budget
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Positive Water Balance Estuary
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Negative Water Balance Estuary
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