Chapter Chapter 1 10 Clickers Lecture Essentials of Oceanography Eleventh Edition The Coast: Beaches and Shoreline Processes Alan P. Trujillo Harold V. Thurman
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
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
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
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
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
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, or littoral 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 headlands absorb 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
Alternatives to Hard Stabilization Construction restrictions Simplest alternative Limit building near shorelines Paradoxically, National Flood Insurance Program encouraged construction.
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.
Beach Replenishment
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
Relocation of Cape Hatteras Lighthouse, NC
End of CHAPTER 10 The Coast: Beaches and Shoreline Processes