1. CHAPTER 10 The Coast: Beaches and Shoreline Processes

2. Overview Coastal regions constantly change.
The beach is a dominant coastal feature.
Wave activity continually modifies the beach and coastal areas through erosion and deposition of sand.
Sea level changes affect the coast.
Humans have attempted various coastal stabilization measures.
Rip Current-

3. 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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5. Other Beach Profile Features
Wave-cut bench – flat, wave-eroded surface
Recreational beach – area above the shoreline
Berm – dry, gently sloping region

6. 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

7. 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 biological material on tropical beaches
Example, Coral reef material
Material is always in transit along the shoreline.

8. 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

9. Wintertime Beach Heavy wave activity Backwash dominates
Sediment moved away from shore
Narrower beach
Flattened beach face
Longshore bars are present
Stormy weather

10. Summertime Beach Light wave activity Wide, sandy berm Steep beach face
Swash dominates
Longshore bars not present
Generally milder storms

11. Summer Beach
Winter Beach

12. 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

13. 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”
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

14. 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

15. 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.
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.

16. Erosional Shorelines Wave erosion increases with
More shore exposed to open ocean
Smaller tidal range
Weaker bedrock

17. 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.
Oregon Dunes

18. Depositional Shorelines
Tombolo
Barrier island
Tombolo (Goat Rock Beach, CA)
Heavily-Developed Barrier Island (Tom’s River, NJ)

19. 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

20. 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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22. Barrier Island
High and low salt marshes – biologically productive wetlands
Generate peat deposits of decaying organic matter
Lagoon – water between barrier island and mainland

23. Barrier Islands Migrate landward over time due to rising sea levels
Older peat deposits found on ocean beach

24. Deltas
Triangular deposits of sediment where rivers empty into oceans or seas
Distributaries carry sediment to ocean

25. 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.

26. 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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28. Submerging Shorelines
Shoreline below current sea level
Features include
Drowned beaches
Submerged dune topography
Drowned river valleys (estuaries)
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29. Changing Sea Level Two major processes can change 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
Global (eustatic) changes in sea level
Sea level changes worldwide due to
Change in amount of available sea water
Change in ocean basin capacity

30. 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

31. 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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32. 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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33. 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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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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37. 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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40. Seawalls Destructive to beach 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

41. Alternatives to Hard Stabilization
Three major alternatives
Construction restrictions
Simplest alternative
Limit building near shorelines
Paradoxically, National Flood Insurance Program encouraged construction
2. Beach replenishment
Sand added to beach/longshore current
Expensive; costs between $5 and $10 per cubic yard
Sand must be dredged from elsewhere.

42. Alternatives to hard stabilization
3. Relocation
Move structures rather than protect them in areas of erosion
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43. End of CHAPTER 10 The Coast: Beaches and Shoreline Processes
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