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Explore the role of plate tectonics and coastal processes in shaping Folly Island's morphology and learn about the challenges of managing erosion. Discover strategies to minimize coastal hazards.
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Learning Objectives • Discuss the role of plate tectonics influencing coastal zone morphology and process. • Explain coastal processes, such as waves, beach forms and processes, and rising sea level. • Summarize the effects of sea level rise on coastal processes. • Explain why coastal erosion rates vary along different U.S. coastlines.
Learning Objectives, cont. • Synthesize the coastal erosion hazards. • Summarize the potential link between coastal processes and other natural hazards. • Evaluate how use of the coastal zone affects coastal processes. • Summarize what we can do to minimize coastal hazards. • List options available for coastal management.
Folly Island • Barrier island south of Charleston, South Carolina • Barrier to ocean waves that would strike the mainland • About 10 km (~6 mi) long, less than 1 km (~0.6 mi) wide • Most of island has an elevation of 1.5-3 m (~5-10 ft) above sea level • Typical Atlantic barrier island • Eroding at a high rate • East-facing shoreline has been retreating for at least 160 years • Pair of rocky barriers (jetties) aiding in the erosion • Built to prevent the entrance of Charleston Harbor from being blocked by sand • Facing tough decisions on how to adapt to and manage future coastal erosion
11.1 Introduction to Coastal Hazards • Continental and oceanic processes converge to produce landscapes that are capable of rapid change • Coasts are greatly influenced by plate tectonics • East Coast United States and Canada, Canadian Arctic is passive • Because they are not close to convergent boundary • Have wide continental shelves with barrier islands and sandy beaches • West Coast United States and Canada is active • It is close to convergent and transform boundaries • Have sea cliffs and rocky shorelines
11.1 Introduction to Coastal Hazards, cont. • Coast also influenced by climate and organisms • Alaska, Canada, and Great Lakes affected by seasonal ice or glaciers • Temperate region coastlines affected by marsh vegetation • Tropical and subtropical coasts affected by mangroves and coral reefs • Most serious coastal hazards include • Strong coastal currents • Coastal erosion • Storm surge (Chapter 10) • Tsunamis (Chapter 4)
11.2 Coastal Processes: Waves • Waves are caused by offshore winds producing friction over water • Size of waves depend on • Speed of wind • Duration of wind • Fetch – distance wind blows over water surface • Waves become sorted into groups as they move away from their origin • Rogue waves are exceptions to these groups
Waves, cont. • Rogue waves • Appear to form by constructive interference • Multiple, similarly sized waves intersect to create a much larger wave • Wave may be as high as the sum of the intersecting waves • Can be extremely dangerous to the unsuspecting beachgoer • Can appear out of nowhere • Lives are lost – sweep individual into ocean • Can also appear out of nowhere in the open ocean • Can threaten ships • Waves can reach 30 m (~100 ft) – ships are designed to withstand 15 m (~50 ft) waves
Waves, cont. • Wave form • Wave height • Distance from crest to trough • Wavelength • Distance from crest to crest • Wave period • Time between crests
Waves, cont. • To understand wave energy transmission, useful to study the wave motion • Motion is circular in open ocean • Circles decrease in diameter with increasing depth • Waves in shallow water become ellipses as waves “feel bottom” • When depth is ½ wavelength • May eventually become a narrow ellipse with a back-and-forth movement
Waves, cont. • Wave sets generated by storms are called swells • Swells enter shallow water, become unstable and break • Mathematical equations can be used to predict wave height, period and velocity • Waves move in open ocean with little energy loss • Energy is spent on reaching coastline • Wave energy is proportional to square of wave height • On coast, wavelength and velocity decrease, wave height increases, wave period is constant
Waves, cont. • Variations along a coastline • Irregularities in topography cause variations in wave height as it approaches shore • A single wave is called a wave front • Irregular coastlines have headlands • The shape of the coast is similar underwater to that of the coastline • Water gets progressively shallower close to shore • As the wave approaches the shore, it slows at the headland first • This causes the wave front to bend around the headland (refraction)
Waves, cont. • Effects of wave refraction • Wave normals, perpendicular to wave fronts pointing toward shoreline • Wave refraction causes normal to converge and diverge • Convergence • Wave heights and energy increases • Waves are bigger here • Divergence • Wave heights and energy decreases
Waves, cont. • Breaking waves • Plunging breakers • Waves that pick up quickly • Typical on steep beaches • More erosive • Spilling breakers • Waves that spill gently • Typical on wide, flat beaches • More likely to deposit sand
Beach Form and Processes • Beach consists of loose material which has accumulated by wave action on shoreline • Type of beach material depends on source of sand • White beaches from shell and coral • Black beaches from volcanic rock • Brown beaches from quartz and feldspar
Beach Form and Processes, cont. • The beach onshore • Landward extent of a beach on seashore or lakeshore • Line of sand dunes • Line of permanent vegetation or • Sea cliff or bluff forms from erosion of rock or sediment • Beaches are divided into • Berm • Beach portion that slopes landward • Formed by deposition of sediment by waves • Beach face • Beach portion that slopes toward water • In the swash zone where waves swash and backwash
Beach Form and Processes, cont. • The beach offshore • Swash zone • Zone where waves swash and backwash on the beach • Surf zone • Where turbulent waves move after waves break • Breaker zone • Where the waves become unstable, peak, and break • Longshore bar forms beneath breakers • Longshore trough forms landward from bar
Beach Form and Processes, cont. • Sand transport • Littoral transport • Sand movement parallel to shore • Beach drift • Sand moving in zigzag pattern in swash zone • Longshore drift • Transport of sand by longshore currents • Longshore currents • Current that flows parallel to shoreline as a result of up and back movement of water in swash zone • Updrift and downdrift • Indicate the direction in which sediment is moving or accumulating along the shore
11.3 Sea Level Change • The level of the sea is constantly changing • Relative sea level • Position of the sea at the shore • Influenced by movement of both the land and water • Eustatic sea level • Global sea level • Controlled by processes that affect overall volume of water in the ocean and shape of the basins
11.3 Sea Level Change, cont. • Eustatic sea level (global sea level) • Rises or falls when the amount of water in the world’s oceans increases or decreases • Climate/average air temperature • Temperature increases cause volume of water to expand • Temperature decreases cause contraction of water • Changes in temperature cause ice on land to melt or snowfall to increase • Volume of water in ice sheets, glaciers increases, ocean water linked • Tectonic processes • Changes ocean basin shape over long period of time
11.3 Sea Level Change, cont. • Relative sea level • Glacier melt or earthquakes can cause uplifting of land • Decrease in sea level • Rates of deposition, erosion, or subsidence makes the level rise or fall • Tides caused by gravitational pull of the moon cause daily and seasonal changes • Weather conditions • Changes in wind speed • High winds pile up water and increase water height in open water • Swell increases both water level and wave heights when it reaches the shore • Changes in atmospheric pressure • Can add a meter or more to height of storm surge
11.4 Geographic Regions at Risk from Coastal Hazards • Coastal hazards are present on both seacoasts and lakeshores • Atlantic, Pacific, Gulf, and Arctic coasts in North America • Large lakes such as the Great Lakes, Great Bear Lake, Lake Winnipeg have coastal conditions similar to the ocean • Coastal erosion is a more universal hazard • All 30 U.S. states bordering the ocean • Canadian provinces bordering the ocean • Those also bordering the Great Lakes
11.4 Geographic Regions at Risk from Coastal Hazards, cont. • Strong nearshore currents are also common problems on coastlines that have regular, strong surf conditions • California and Hawai’i • Any coast with breaking waves • Rip currents on U.S. Gulf and East Coast beaches • Astronomical tides in narrow bays and channels • Coastlines that are subsiding are at particular risk • Coastlines that are close to sea level • Gulf and East coasts • Arctic Canada and North Slope of Alaska • Atolls in the Pacific
11.5 Effects of Coastal Processes • Create hazard for both individuals and communities • Individuals • Safety hazards during swimming • Safety and property hazards during storm surges and tsunamis • Communities • Long-term hazards related to erosion and sea level rise • Barrier islands and spits • Shoreline is constantly shifted • Can move an entire island, the inlets between islands, or the separation of a spit from the mainland • Poor places for coastal development
Rip Currents • Powerful currents that move large amounts of water away from shore • Develop when waves pile up water between longshore bar and swash zone • Becomes concentrated in narrow zones • Serious coastal hazard • In United States, kill more than hurricanes and earthquakes annually • Currents are narrow; widen and dissipate once they reach line of breaking waves • Escape requires swimming parallel to shore; don’t panic • Recognized as a relatively quiet area in the surf zone
Coastal Erosion • Becoming recognized as a serious worldwide problem • Continuing global sea rise • Extensive development in the coastal zone • A more continuous, predictable process than other natural hazards • Large amounts of money spent but solutions are temporary
Coastal Erosion, cont. • Beach erosion • Beach Budget – easy way to visualize erosion • Input • Longshore and beach drift bringing sediment from upshore • Local erosion of dunes and cliffs • Output • Longshore and beach drift bringing sediment away from shore • Storm waves • On-shore winds • Storage • Sediment on the beach • Beach grows when input exceeds output • Beach erodes when output exceeds input
Coastal Erosion, cont. • Cliff erosion • Sea cliffs and lakeshore bluffs erode due to wave action, running water, and landslides • Causes the cliffs and bluffs to retreat • Human activities increase erosion rate • Increase surface runoff • Increase groundwater discharge • Addition of weight to cliff • Can be monitored using LIDAR
11.6 Linkages between Coastal Processes and Other Natural Hazards • Earthquakes, volcanic eruptions, tsunamis • Change the shape of shoreline • Storm waves, storm surge, and coastal flooding • Increase coastal erosion • Landslides • Caused by eroding cliffs and bluffs • Climate change • Storm frequency and intensity change with climate conditions
11.6 Linkages with Other Natural Hazards, cont. • 2010 Oil Spill • Emphasizes the fact that coastal processes are linked to environmental problems • Predicting the movement of the oil along the beaches and in salt marshes requires detailed coastal information on • Wave height and frequency • Direction and rate of longshore transport • Strength of tidal flow into and out of barriers and island inlets • How far inland tidal flow inundates salt marshes • How salt marsh vegetation and sediment interact with the oil • The effects of the oil on life on beaches and salt marshes
11.7 Natural Service Functions of Coastal Processes • Pleasing landscapes • Cliffs and bluffs are result of wave action and erosion • Beaches • Maintained or created by erosion and deposition • Provide a number of their own service functions • Renewal of ecosystems • Recreation • Swimming, sailing, fishing, and sunbathing
11.8 Human Interaction with Coastal Processes • Human interference has caused considerable erosion • Most problems in highly populated and developed areas • Atlantic, Gulf, and Pacific coasts • Great Lakes • Some parts of Canada • Efforts to stop coastal erosion involve engineering structures that impede littoral transport • Interrupt movement of sand • Cause beaches to grow in some areas and erode in others • Damage beachfront property
The Atlantic Coast • Characterized by barrier islands • Long narrow islands of sand separated from the mainland by a lagoon or bay • Many have been altered by human use • Removal of coastal dunes • Increased vulnerability to storms • Jetty construction • Interrupts longshore drift • Increases erosion at some locations
