1. CHAPTER 10 The Coast: Beaches and Shoreline Processes
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Overview
Coastal regions constantly change.
The beach is a dominant coastal feature.
Wave activity continually modifies the beach and coastal areas.
Waves affect deposition and erosion of sand and subsequent coastal features.
Sea level changes affect the coast.
Humans have attempted various coastal stabilization measures.
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Coastal Regions
General Features
Shore – the zone that lies between the low tide line and the highest area on land affected by storm waves
Coast – extends inland as far as ocean related features are found
Coastline – boundary between shore and coast
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Beach Features
Beach – entire active area of a coast affected by waves. Consists of
Shore – divided into
Backshore – above high tide line; covered with water only during storms
Nearshore – from low tide water line to where waves break at low tide
Offshore – area beyond low tide breaking waves
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6. Other Beach Profile Features
Wave-cut bench – flat, wave-eroded surface
Recreational beach – area above the shoreline
Berm – dry, gently sloping region
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7. Other Beach Profile Features
Beach face – wet, sloping surface between berm and shoreline
Longshore bar – underwater sand bar parallel to the coast
Longshore trough – separates longshore bar from the beach
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8. Composition of Beaches
Formed from locally available material
May be coarse or fine grained sediment
Boulders from local cliffs
Sand from rivers
Mud from rivers
Significant biologic material on tropical beaches
Example, Coral reef material
Material is always in transit along the shoreline.
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9. Sand Movement Along Beach
Two Major Types
Perpendicular to shoreline (toward and away)
Swash – water rushes up the beach
Backwash – water drains back to the ocean
Parallel to shoreline (up-coast or down-coast)
Longshore current – transports sand along the beach
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Swash and Backwash
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Longshore Transport
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Wintertime Beach
Heavy wave activity
Backwash dominates
Sediment moved away from shore
Narrower beach
Flattened beach face
Longshore bars are present
Stormy weather
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Summertime Beach
Light wave activity
Wide, sandy berm
Steep beach face
Swash dominates
Longshore bars not present
Generally milder storms
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Longshore Current
Parallel motion of water along shoreline
Caused by wave refraction
Causes zigzag motion of water in surf zone
Longshore currents travel at speeds up to 4 km (2.5 miles) per hour
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Longshore Transport
Also called longshore drift, beach drift, or littoral drift
Only occurs in the shallow water surf zone
Transports beach sediment in a zigzag fashion in the direction of the longshore current
Beaches sometimes called “rivers of sand”
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Longshore Transport
Millions of tons of sediment moved yearly
Direction of transport changes due to wave approach
In general, net sediment movement is southward along the Atlantic and Pacific coasts of the United States
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18. Two Major Types of Shores
Erosional Shores
Well-developed cliffs
Exist where tectonic uplift of coast occurs
U.S. Pacific coast is one example
Depositional Shores
Gradually subsiding shore
Barrier islands and sand deposits are common
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Erosional Shores
Protruding bits of land called headlands absorb much wave energy.
Wave cut cliffs and sea caves are other features carved out by wave activity.
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Erosional Shores
Sea arches form where sea caves in headlands erode all the way through.
Sea stacks form when the tops of sea arches erode away completely.
Bedrock uplift generates a marine terrace.
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Erosional Shorelines
Wave erosion increases with
More shore exposed to open ocean
Smaller tidal range
Weaker bedrock
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22. Depositional Shorelines
A bay barrier, or bay mouth bar, seals off a lagoon from the ocean.
A Tombolo is an sand bar that connects an island to the mainland.
Barrier islands are long offshore sand deposits that parallel the coast.
A spit connects at one end to the mainland and hooks into a bay at the other.
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23. Depositional Shorelines
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24. Depositional Shorelines
Tombolo
Barrier island
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Barrier Islands
Common along East and Gulf coasts of the United States
Do not exist along erosional shorelines
Protect mainland from high wave activity
Can migrate landward over time
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26. Barrier Island Anatomy
Ocean beach
Dunes
Barrier flat
High salt marsh
Low salt marsh
Lagoon
Barrier Island Anatomy
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Barrier Island
Ocean Beach – closest part of the island to the ocean
Dune – stabilized by grasses; protect lagoon from strong storms
Barrier flat – grassy area that forms behind dunes
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Barrier Island
High and low salt marshes – biologically productive wetlands
Generate peat deposits of decaying organic matter
Lagoon – water between barrier island and mainland
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Barrier Islands
Migrate landward over time due to rising sea levels
Older peat deposits found on ocean beach
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Deltas
Triangular deposits of sediment where rivers empty into oceans or seas
Distributaries carry sediment to ocean
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Beach Compartments
Three major components:
Rivers that supply beach sediment
Beach itself
Offshore submarine canyons
Beach starvation – human activities block supply of sand to beach compartments.
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32. Beach Compartments (Continued)
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Emerging Shorelines
Shorelines above current sea level
Marine terraces – flat platforms backed by cliffs
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34. Submerging Shorelines
Shoreline below current sea level
Features include
Drowned beaches
Submerged dune topography
Drowned river valleys (estuaries)
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Changing Sea Level
Two major processes can change sea level:
Local tectonic processes
Global (eustatic) changes in sea level
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Changing Sea Level
Local tectonic processes
Example: the Pacific coast of the United States is currently being uplifted.
Isostatic adjustments – rebound of Earth’s crust after removal of heavy loads or sinking with application of heavy loads
Ice-loading from glaciers during ice ages
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Changing Sea Level
Global (eustatic) changes in sea level
Sea level changes worldwide due to
Change in amount of available sea water
Change in ocean basin capacity
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38. Eustatic Changes in Sea Level
Some Mechanisms
Ice ages lock seawater up in ice (glaciation) – sea level goes down
Ice melting after an ice age (deglaciation) – sea level rises
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40. Eustatic Changes in Sea Level
Also caused by thermal expansion and contraction of seawater
Physical property of water: warmer water expands and cooler water contracts.
Sea level rises and falls in response to seawater temperature.
This is roughly 2 meters (6.6 feet) per 1°C (1.8°F) change in temperature.
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41. Pleistocene Epoch and Today
From about 2 million to 10,000 years ago, a series of four ice ages affected Earth.
Sea level was at least 120 meters (400 feet) below today’s sea level.
If all remaining ice on Earth melted today, sea level would rise another 70 meters (230 feet).
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42. Global Warming and Changing Sea Level
Globally averaged temperatures – about 0.6°C (1.1°F) warmer over last 130 years
Sea level rose cm (4-10 in) over past 100 years
As global warming continues, we will see a higher sea level.
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United States Coasts
Atlantic coast
Pacific coast
Gulf coast
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Atlantic Coast
Most coasts open to storm wave attack
Barrier islands common from Massachusetts south
Bedrock
Florida bedrock is resistant limestone.
Northward through New Jersey is comprised of easily erodable recent deposits.
New York through Maine has glacier-affected rocks.
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Atlantic Coast
Strong storms called nor’easters can damage the coast north of Cape Hatteras, NC.
Nor’easters can generate storm waves up to 6 meters (20 feet).
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Atlantic Coast
Drowned river valleys common
Average erosion is 0.8 meter (2.6 feet) per year; sea is migrating landward
Delaware, New York, and Georgia have the most serious erosion problem.
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Atlantic Coast
Barrier islands
Drowned river valleys
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Gulf Coast
Low tidal range
Generally low wave energy
Tectonically subsiding
Mississippi delta dominates
Locally sea level rises due to compaction of delta sediments
Average rate of erosion is 1.8 meters (6 feet) per year
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Pacific Coast
Tectonically rising
Experiencing less erosion than Atlantic or Gulf coasts
Open exposure to high energy waves
Average rate of erosion meter (0.016 feet) per year
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Hard Stabilization
Structures built to decrease coastal erosion and interfere with sand movement
Also called armoring of the shore
Often results in unwanted outcomes
Some structures may increase wave erosion
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Hard Stabilization
Four major types of stabilization structures:
Groins and groin fields
Jetties
Breakwaters
Seawalls
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52. Groins and Groin Fields
Built perpendicular to the beach
Often made of rip rap, or large blocky material
Traps sand upcoast, which can cause erosion downstream of the longshore current
May necessitate a groin field, or a series of groins built along a beach
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Jetties
Built perpendicular to shore
Built in pairs
Built to protect harbor entrances
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Breakwaters
Built parallel to a shoreline
Designed to protect harbors from waves
Can cause excessive erosion, requiring dredging to keep area stable
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Seawalls
Destructive to environment
Designed to armor coastline and protect human developments
One large storm can remove beach
Wave activity eventually undermines seawall structure; need continual repair or will collapse
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56. Alternatives to Hard Stabilization
Three major alternatives
Construction restrictions
Beach replenishment
Relocation
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57. Alternatives to Hard Stabilization
Construction restrictions
Simplest alternative
Limit building near shorelines
Paradoxically, National Flood Insurance Program encouraged construction
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58. Alternatives to Hard Stabilization
Beach replenishment
Sand added to beach/longshore current
Expensive; costs between $5 and $10 per cubic yard
Sand must be dredged from elsewhere.
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59. Alternatives to hard stabilization
Relocation
Move structures rather than protect them in areas of erosion
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60. End of CHAPTER 10 The Coast: Beaches and Shoreline Processes
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