Chapter
Clickers
Chapter110
Lecture
Essentials of Oceanography
Eleventh Edition
The Coast:
Beaches and
Shoreline Processes
Alan P. Trujillo
Harold V. Thurman
© 2014 Pearson Education, Inc.
Chapter Overview
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Coastal regions have distinct coastal features.
The beach is a dominant coastal feature.
Waves affect deposition and erosion of sand.
Sea level changes affect the coast.
Different coasts have different characteristics.
Humans have attempted various coastal
stabilization measures.
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Defining 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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Defining Coastal Regions
• Backshore – part of shore above high tide
shoreline
• Foreshore – part of shore exposed at low tide
and submerged at high tide
• Shoreline – water’s edge that migrates with
the tide
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Defining Coastal Regions
• Nearshore – extends seaward from low tide
shoreline to low tide breaker line
• Offshore – zone beyond low tide breakers
• Beach – wave-worked sediment deposit of
the shore area
– Area of beach above shoreline often called
the recreational beach
• Wave-cut bench – flat, wave-eroded surface
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Defining Coastal Regions
• Berm – dry, gently sloping, elevated beach
margin at the foot of coastal cliffs or sand dunes
• Beach face – wet, sloping surface extending
from berm to shoreline
– Also called low tide terrace
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Defining Coastal Regions
• Longshore bars – sand bars parallel to coast
– May not always be present
– Can cause approaching waves to break
• Longshore trough – separates longshore bar
from beach face
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Cliffed Coastal Region
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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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Sand Movement Along Beach
• 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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Summertime Beach
• Light wave activity
– Wide, sandy berm
– Steep beach face
– Swash dominates
• Longshore bars not
present
• Generally milder
storms
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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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Light vs. Heavy Wave Activity
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Longshore Current
• Zigzag movement of water along shore
• Longshore currents travel at speeds up to 4 km
(2.5 miles) per hour
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Longshore Drift
• Also called longshore transport, beach drift, or
littoral drift
• Transports beach sediment in a zigzag fashion
in the direction of the longshore current
• Occurs in surf zone
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Longshore Drift
• Millions of tons of sediment moved yearly
• Direction of transport changes due to wave
approach
• Net sediment movement is southward along the
Atlantic and Pacific coasts of the United States.
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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.
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Erosional Shorelines
• Uplift of wave-cut
bench generates a
marine terrace.
• Wave erosion
increases with
– More shore exposed
to open ocean
– Smaller tidal range
– Weaker bedrock
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Depositional Shorelines
• Bay barrier, or bay mouth bar
– seals off a lagoon from the ocean
• Tombolo
– sand bar that connects an island to the
mainland
• Barrier islands
– long offshore sand deposits that parallel the
coast
• Spit
– connects at one end to the mainland and hooks
into a bay at the other
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Depositional Coast Features
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Bay Barrier, Martha’s Vineyard, MA
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Tombolo, Goat Rock Beach, CA
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Barrier Islands
• Extremely long
offshore deposits of
sand parallel to coast
• Do not exist along
erosional shorelines
• Protect mainland from
high wave activity
• Appear to have
developed at end of
last ice age 18,000
years ago
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Barrier Islands
• Separated from
mainland by lagoon
• Attractive building
sites because of
proximity to ocean
• Many structures
destroyed by ocean
or required relocation
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Heavily Developed Barrier Island, NJ
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Barrier Islands
• More than 2000
barrier islands
identified worldwide
• Almost 300 along
Atlantic and Gulf
coasts of U.S.
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Barrier Island Features
• 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
• Salt marshes – inland of barrier flat
– Low marsh extends from mean sea level to
high neap tide line.
– High marsh extends to highest spring tide line.
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Barrier Island Features
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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 –
branching channels
carry sediment to
ocean
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Deltas
• Delta shoreline is
smoothed when erosion
exceeds deposition.
• Nile River Delta currently
eroding
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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
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Beach Compartments
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Emerging Shorelines
• Shorelines above current sea level
• Marine terraces – flat platforms backed by
cliffs
• Stranded beach deposits
– Indication that former shoreline has risen
above sea level
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Submerging Shorelines
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Shoreline below current sea level
Drowned beaches
Submerged dune topography
Drowned river valleys
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Changing Sea Level
Two major processes can change sea level:
• Local tectonic processes raise or lower
Earth’s crust
• Worldwide changes in sea level
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Changing Sea Level
• Tectonic Movements
– Include crustal uplift or subsidence and
localized folding, faulting, and tilting
– Example: The Pacific coast of the United
States is currently being uplifted.
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Changing Sea Level
• 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
• Eustatic sea level changes – worldwide
• Can be caused by
– Formation or destruction of inland lakes
– Sea floor spreading rate changes
– Formation or melting of glaciers
– Thermal expansion or contraction of seawater
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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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U.S. Coasts
• Three coasts:
– Atlantic coast
– Pacific coast
– Gulf coast
• Each has its own unique characteristics.
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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 composed
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
• 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.
• Northern Maine may still be rebounding from
last ice age – sea level dropping
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Atlantic Coast
• Barrier islands
• Drowned river valleys
– Common
– Form large bays
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Gulf Coast
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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 0.005 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:
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
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Groins and Groin Fields
• Upcoast trapping of
sand may necessitate
a groin field, or a
series of groins built
along a beach.
• Sand is distributed
differently, but no
additional sand is on
the beach.
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Jetties
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Similar to groin
Built perpendicular to shore
Built in pairs
Built to protect harbor entrances
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Jetties at Santa Cruz Harbor, CA
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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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Breakwater at Santa Barbara, CA
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Breakwater at Santa Barbara, CA
• Between 1931 and 1949, breakwater
disrupted longshore transport of sand
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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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Seawall Damage
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Alternatives to Hard Stabilization
• Three major alternatives
1. Construction restrictions
2. Beach replenishment
3. Relocation
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Alternatives to Hard Stabilization
• Construction restrictions
– Simplest alternative
– Limit building near shorelines
– Paradoxically, National Flood Insurance
Program encouraged construction.
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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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Beach Replenishment
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Alternatives to Hard Stabilization
• Relocation
– Move structures rather than protect them in
areas of erosion
– Can allow humans to live in natural balance
with beach processes
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Relocation of Cape Hatteras Lighthouse, NC
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End of CHAPTER 10
The Coast:
Beaches and Shoreline Processes
© 2014 Pearson Education, Inc.