ChapterChapter 1 10 Clickers Lecture Essentials of Oceanography Eleventh Edition
The Coast: Beaches and Shoreline Processes
Alan P. Trujillo Harold V. Thurman
© 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. Terminology
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. Seasonality of beach conditions
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. Summertime Beach
• Light wave activity – Wide, sandy berm – Steep beach face – Swash dominates • Longshore bars not present • Generally milder storms
© 2014 Pearson Education, Inc. Wintertime Beach
• Heavy wave activity – Backwash dominates – Sediment moved away from shore – Narrower beach – Flattened beach face • Longshore bars are present • Stormy weather
© 2014 Pearson Education, Inc. Light vs. Heavy Wave Activity
© 2014 Pearson Education, Inc. Longshore Current
• Zigzag movement of water along shore • Longshore currents travel at speeds up to 4 km (2.5 miles) per hour
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. Beach Drift and Longshore Currents
© 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. Depositional Coast Features
© 2014 Pearson Education, Inc. Bay Barrier, Martha’s Vineyard, MA
© 2014 Pearson Education, Inc. Tombolo, Goat Rock Beach, CA
© 2014 Pearson Education, Inc. 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 © 2014 Pearson Education, Inc. Barrier Islands
• Separated from mainland by lagoon • Attractive building sites because of proximity to ocean • Many structures destroyed by ocean or required relocation
© 2014 Pearson Education, Inc. Heavily Developed Barrier Island, NJ
© 2014 Pearson Education, Inc. Barrier Islands
• More than 2000 barrier islands identified worldwide • Almost 300 along Atlantic and Gulf coasts of U.S.
© 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. Barrier Island Features
© 2014 Pearson Education, Inc. Barrier Islands • Migrate landward over time due to rising sea levels • Older peat deposits found on ocean beach
© 2014 Pearson Education, Inc. Deltas
• Triangular deposits of sediment where rivers empty into oceans or seas • Distributaries – branching channels carry sediment to ocean
© 2014 Pearson Education, Inc. Mississippi
1973 1989 2003
© 2014 Pearson Education, Inc. Deltas
• Delta shoreline is smoothed when erosion exceeds deposition. • Nile River Delta currently eroding
© 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Sand colors say something….
© 2014 Pearson Education, Inc. Sea arches and sea stacks.
© 2014 Pearson Education, Inc. Erosional Features off Owen Point, Victoria
© 2014 Pearson Education, Inc. WA
© 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Primary or Secondary?
© 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. Beach Compartments
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. Submerging Shorelines
• Shoreline below current sea level • Drowned beaches • Submerged dune topography • Drowned river valleys
© 2014 Pearson Education, Inc. Changing Sea Level
Two major processes can change sea level: • Local tectonic processes raise or lower Earth’s crust • Worldwide changes in sea level
© 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. 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).
© 2014 Pearson Education, Inc. U.S. Coasts • Three coasts: – Atlantic coast – Pacific coast – Gulf coast • Each has its own unique characteristics.
© 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. 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).
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. Atlantic Coast
• Barrier islands • Drowned river valleys – Common – Form large bays
© 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. Hard Stabilization
• Four major types of stabilization structures: 1. Groins and groin fields 2. Jetties 3. Breakwaters 4. Seawalls
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. Jetties • Similar to groin • Built perpendicular to shore • Built in pairs • Built to protect harbor entrances
© 2014 Pearson Education, Inc. Jetties at Santa Cruz Harbor, CA
© 2014 Pearson Education, Inc. Breakwaters
• Built parallel to a shoreline • Designed to protect harbors from waves • Can cause excessive erosion, requiring dredging to keep area stable
© 2014 Pearson Education, Inc. Breakwater at Santa Barbara, CA
© 2014 Pearson Education, Inc. Breakwater at Santa Barbara, CA
• Between 1931 and 1949, breakwater disrupted longshore transport of sand
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. Seawall Damage
© 2014 Pearson Education, Inc. Alternatives to Hard Stabilization
• Three major alternatives 1. Construction restrictions 2. Beach replenishment 3. Relocation
© 2014 Pearson Education, Inc. Alternatives to Hard Stabilization
• Construction restrictions – Simplest alternative – Limit building near shorelines – Paradoxically, National Flood Insurance Program encouraged construction.
© 2014 Pearson Education, Inc. 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.
© 2014 Pearson Education, Inc. Beach Replenishment
© 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Coastal Stabilization Structures
© 2014 Pearson Education, Inc. 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
© 2014 Pearson Education, Inc. Cape Hatteras
© 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Salt marshes and mangrove swamps
© 2014 Pearson Education, Inc.