Transcript Beach Compartments

CHAPTER 10
The Coast:
Beaches and Shoreline Processes
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.
http://www.youtube.com/watch?v=Cvc5aUM4b8g
Rip Current-
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
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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Other Beach Profile Features
• Wave-cut bench –
flat, wave-eroded
surface
• Recreational beach –
area above the
shoreline
• Berm – dry, gently
sloping region
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
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.
Sand Movement Along Beach
Two Major Types
1. Perpendicular to shoreline
(toward and away)
– Swash – water rushes up
the beach
– Backwash – water drains
back to the ocean
2. Parallel to shoreline (upcoast or down-coast)
– Longshore current –
transports sand along the
beach
Wintertime Beach
• Heavy wave activity
– Backwash dominates
– Sediment moved away
from shore
– Narrower beach
– Flattened beach face
• Longshore bars are
present
• Stormy weather
Summertime Beach
• Light wave activity
– Wide, sandy berm
– Steep beach face
– Swash dominates
• Longshore bars not
present
• Generally milder storms
Summer Beach
Winter Beach
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
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
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
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.
Erosional Shorelines
• Wave erosion
increases with
– More shore
exposed to open
ocean
– Smaller tidal range
– Weaker bedrock
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
Depositional Shorelines
Tombolo
Tombolo (Goat Rock Beach, CA)
Barrier
island
Heavily-Developed Barrier Island (Tom’s River, NJ)
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
Barrier Island
Anatomy
•
•
•
•
•
•
Ocean beach
Dunes
Barrier flat
High salt marsh
Low salt marsh
Lagoon
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
Barrier Islands
• Migrate landward over
time due to rising sea
levels
• Older peat deposits found
on ocean beach
Deltas
• Triangular deposits of sediment where rivers empty into oceans or seas
– Distributaries carry sediment to ocean
Beach Compartments
• Three major components:
1. Rivers that supply
beach sediment
2. Beach itself
3. Offshore submarine
canyons
• Beach starvation – human
activities block supply of
sand to beach
compartments.
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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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
– 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
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
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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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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Global Warming and Changing Sea Level
• Globally averaged
temperatures – about 0.6°C
(1.1°F) warmer over last
130 years
• Sea level rose 10-15 cm
(4-10 in) over past
100 years
• As global warming continues,
we will see a higher sea level.
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• Atlantic coast
• Pacific coast
• Gulf coast
United States Coasts
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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:
1. Groins and groin fields
2. Jetties
3. Breakwaters
4. Seawalls
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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 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
Alternatives to Hard Stabilization
• Three major alternatives
1. 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.
Alternatives to hard stabilization
3. Relocation
– Move structures rather than protect them in areas
of erosion
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End of CHAPTER 10
The Coast:
Beaches and Shoreline Processes
© 2011 Pearson Education, Inc.