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. 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. Cliffed Coastal Region © 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. 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. 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. Deltas • Delta shoreline is smoothed when erosion exceeds deposition. • Nile River Delta currently eroding © 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