chapter 1 clickers chapter 10 lecture essentials of ... of oceanography eleventh edition alan p....

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The Coast:

Beaches and

Shoreline Processes

Chapter 1 Clickers

Essentials of OceanographyEleventh Edition

Alan P. Trujillo

Harold V. Thurman

Chapter 10 Lecture


Chapter Overview

• 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.


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



• 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



• 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



• 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



• 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


Cliffed Coastal Region


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.


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


Summertime Beach

• Light wave activity

– Wide, sandy berm

– Steep beach face

– Swash dominates

• Longshore bars not present

• Generally milder storms


Wintertime Beach

• Heavy wave activity

– Backwash dominates

– Sediment moved away from shore

– Narrower beach

– Flattened beach face

• Longshore bars are present

• Stormy weather


Light vs. Heavy Wave Activity


Longshore Current

• Zigzag movement of water along shore

• Longshore currents travel at speeds up to 4 km (2.5 miles) per hour


Longshore Drift

• Also called longshore transport, beach drift, orlittoral drift

• Transports beach sediment in a zigzag fashion in the direction of the longshore current

• Occurs in surf zone


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.


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 headlandsabsorb much wave energy.

• Wave-cut cliffs and sea caves are other features carved out by wave activity.


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.


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


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


Depositional Coast Features


Bay Barrier, Martha’s Vineyard, MA


Tombolo, Goat Rock Beach, CA


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


Barrier Islands

• Separated from mainland by lagoon

• Attractive building sites because of proximity to ocean

• Many structures destroyed by ocean or required relocation


Heavily Developed Barrier Island, NJ


Barrier Islands

• More than 2000 barrier islands identified worldwide

• Almost 300 along Atlantic and Gulf coasts of U.S.


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.


Barrier Island Features


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 –branching channels carry sediment to ocean


Deltas

• Delta shoreline is smoothed when erosion exceeds deposition.

• Nile River Delta currently eroding


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


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


Submerging Shorelines

• Shoreline below current sea level

• Drowned beaches

• Submerged dune topography

• Drowned river valleys


Changing Sea Level

Two major processes can change sea level:

• Local tectonic processes raise or lower Earth’s crust

• Worldwide changes in sea level


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.


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


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


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).


U.S. Coasts

• Three coasts:– Atlantic coast

– Pacific coast

– Gulf coast

• Each has its own unique characteristics.


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.


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).


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


Atlantic Coast

• Barrier islands

• Drowned river valleys

– Common

– Form large bays


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.


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


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.


Hard Stabilization

• Four major types of stabilization structures:

1. Groins and groin fields

2. Jetties

3. Breakwaters

4. Seawalls


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


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.


Jetties

• Similar to groin

• Built perpendicular to shore

• Built in pairs

• Built to protect harbor entrances


Jetties at Santa Cruz Harbor, CA


Breakwaters

• Built parallel to a shoreline

• Designed to protect harbors from waves

• Can cause excessive erosion, requiring dredging to keep area stable


Breakwater at Santa Barbara, CA


Breakwater at Santa Barbara, CA

• Between 1931 and 1949, breakwater disrupted longshore transport of sand


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


Seawall Damage


Alternatives to Hard Stabilization

• Three major alternatives

1. Construction restrictions

2. Beach replenishment

3. Relocation



• Construction restrictions

– Simplest alternative

– Limit building near shorelines

– Paradoxically, National Flood Insurance Program encouraged construction.



• Beach replenishment

– Sand added to beach/longshore current

– Expensive; costs between $5 and $10 per cubic yard

– Sand must be dredged from elsewhere.


Beach Replenishment



• Relocation

– Move structures rather than protect them in areas of erosion

– Can allow humans to live in natural balance with beach processes


Relocation of Cape Hatteras Lighthouse, NC


End of CHAPTER 10The Coast:

Beaches and Shoreline Processes

chapter 1 clickers chapter 10 lecture essentials of ... of oceanography eleventh edition alan p....

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