Chapter 15: The Dynamic Coast OCEANOGRAPHY 2014
Coastal classification • There are two systems for classifying coasts: • The Geological Approach: Active Coasts and Passive Coasts • The Coastal Dynamics Approach: Primary and Secondary Coasts • In the first system, coasts are defined based on their position respective to plate boundaries. Those coasts near plate boundaries that are moving, with the presence of volcanic activity and earthquakes are considered “active” – and those located away from these regions are considered “passive”. • In the second system, coasts are defined by the forces that shape them. Coasts formed by non-marine geological forces are considered “primary” – and those formed by marine action are considered “secondary”.
Coastal Dynamics Approach: Shepard Coastal Classification System Secondary Coasts Wave-erosion Coasts • Wave-cut beaches Marine-deposition Coasts • Barrier beaches, islands, and spits Bio-Shaped Marine Coasts • Coral Reefs • Primary Coasts • Sinking Coasts • Drowned River Valley • Land-deposition Coasts • Deltas • Wind-deposition Coasts • Volcanic Coasts • Lava Flows • Tectonic Coasts • Fault-related Coasts
Beach Dynamics • Beaches are dynamic. • What does this mean? • Dynamic can be “active”, “forceful”, and “changing”. Water motion is constantly shaping and changing beaches. Without human intervention to keep some coastlines intact, the appearance of a given beach can vary greatly year-to-year. • Shape and Appearance of a Beach:
Parts of a beach: • The beach is the coastal area where loose sediment has accumulated as land meets the sea. • Off shore • Near shore • Foreshore • Back Shore • The position of these divisions of the beach varies with the tidal and wave forces of the area.
Coastal dunes • Sand dunes are formed by winds blowing sand toward the land from the dry part of the beach. • Well developed dunes:typically have a primary dune at the edge of the beach and secondary dunes located farther inland. • Vegetation - traps sand and promotes dune growth and stability. • Key Dune Formation Factors: • Sand is abundant • Strong and persistent on-shore winds • Large tidal range • Wide and gently sloped Beach
Longshore Drift • Transport of sediment by a longshore current (current that results from waves approaching at an angle due to wind) • Moves sediment down the coast in the direction of the wind
Large-Scale Sand Features • Spits • Bay-mouth Barriers • Tombolo • Barrier Islands • Deltas
Spits and Tombolos • Spit: area of accumulated sand that is attached to land at one end and points in the direction of longshore drift • Often have a hook shape • Can continue to grow across the opening to a bay to form a bay-mouth barrier • Tombolo: spits that extend between two islands, or from and island to the mainland • Decreases the effects of longshore drift because it slows the water down and therefore the current can’t carry as much sand with it.
Barrier Islands • Barrier islands are islands composed of sediment that parallel the coast and form where sand supply is abundant and a broad sea floor slopes gently seaward. • Separated from the mainland by shallow bodies of water and are connected to the ocean through tidal inlets. • Typical Barrier Island has 5 features: • Ocean Beach • Sand Dunes • Barrier Flat • Salt Marsh • Lagoon • Barrier islands are created via: • sand ridges isolated by rising sea level • Sand spits breached during a storm • vertical growth and emergence of longshore sand bars
Two ways a barrier island can form: • Sea level rises to breach the coastline, and fills in the depression behind a ridge – which then becomes an island • A longshore bar develops and grows via sand deposition to form an island Barrier island formation:
deltas • There must be the following conditions for a delta to form: • Large sediment load in the river • Broad continental shelf region in the ocean nearby • Small tidal range • Mild waves and currents • Why are there few deltas in the US? • West Coast: Continental Shelf is too narrow • East Coast: Estuaries are common and usually trap most river sediment
Three Basic Delta Types: • River Dominated • Location: Area with a strong river, and mild ocean waves • River is primary shaping force • Characteristic Triangle Shape • Example: Mississippi River Delta Tide Dominated • Location: Area with strong tidal changes • Tides redistribute sediment, and create tidal sand ridges perpendicular to shore • Characteristic ridges will form a tidal channel over time • Example: Essex River Delta in Massachusetts Wave Dominated • Location: Area with strong waves • Waves redistribute river sediment along beaches • Action forms dunes and spits • Example: Senegal River Delta in west Africa
Chapter 15 Classwork Questions: • PART 1 • For the following scenarios, determine whether the coastline would be considered primary or secondary: • A volcanic island with coastlines of smooth, cooled lava flow • A tropic shoreline bordered by an immense coral reef • A coast that has been eroded away by intense wave action • A shoreline that occurs where a river meets the ocean • An ancient river valley that has been flooded over time by a rise in sea level PART 2 6. How is longshore drift important to coastline shaping? 7. How do tides affect the divisions of a beach? 8. What are the differences between river-dominated, tide-dominated, and wave-dominated deltas? 9. Explain some types of large-scale sand features found along coasts. 10. Why would people want to modify coastlines?
Highlighting Coastal Habitatsand human impacts • PART 3 • 11. Explain the four types of estuaries. • 12. How are estuaries important in the cycling of nutrients? • 13. How do mangroves affect the coast? • 14. What are the characteristics of salt marshes and lagoons? • 15. Where are salt marshes found? • 16. Describe how are marine organisms are able to build coasts. • 17. What are the differences between fringing reefs, barrier reeds, and atolls? • 18. Explain the differences between the following man-made coastal structures: groins, jetties, breakwaters, and seawalls. • 19. What problematic changes occur along the coastline due to human impacts? • 20. Describe beach re-nourishment.
Knowledge check: • Use the book and your notes to complete the questions on the following pages: 15-11, 15-20, and 15-26 • Write the question out completely, and include just the answer below.
Oceanography April 28, 2014 • Beaches • What are the factors that shape a beach’s appearance? • What does it mean to say a beach is “dynamic”? • List the parts of a beach in order moving from inland to the open ocean.
Chapter 15 Practice Extended Response Questions • Explain the difference between primary and secondary coasts and give an example of each. • How is a beach dynamic? What factors affect the dynamics of a beach? • Describe two ways a barrier island can form. • Characterize the roll that coral reefs can play in coastal shaping. • In what ways do humans impact coastlines. Provide at least two specific examples.