GEOG 100--Lecture 17--Coastal Geomorphology


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GEOG 100--Lecture 17--Coastal Geomorphology

  1. 1. Coastal Geomorphology: The Oceans, CoastalProcesses, and Landforms Chapter 12
  2. 2. Ocean Water:Salinity, Temperature, and Density• Salinity – Dissolved mineral salts (sodium chloride, magnesium, sulfur, calcium, potassium, etc.) – Differences in salinity are related to evaporation rates and fresh water inflow • Avg. max. salinity found in subtropics (high evap., low precip.) • Avg. min. salinity found in tropical regions (fresh water from high precipitation, low evaporation due to high atmospheric saturation)
  3. 3. Ocean Water• Temperature –Decreasing temperatures with increasing latitude •Polar water = cold, equatorial water = warm –Decreasing temperatures with increasing depth •Deeper waters are colder (sunlight can’t reach, there is little or no mixing) –Western sides of oceans are warmer due to ocean circulation patterns (see next slide)
  4. 4. Warm and Cold Ocean Currents
  5. 5. Ocean Water• Density – Related to temperature • low temp. = high density – Molecules are less active, so cold water contracts – Related to salinity • high salinity = high density – More solids in the water (salts) increases overall density – Related to depth • deep water = high density – Pressure of water above compresses deep water, increasing density
  6. 6. The Movements of The Oceans• Tides• Currents• Wave motion
  7. 7. Tides• A “bulge” in the world’s oceans, caused by the gravitational pull of the moon and sun Fg = G m1 m2 d2
  8. 8. Tides• Tidal range—the difference between high and low tide• Affected by the shape of the coastline and seafloor• Spring tides—highest tides, strong and quick – Occur when sun, moon, and Earth line up (the sea “springs” up and back)• Neap tides—lowest tides – Sun and moon at right angles with respect to Earth – Neap = A low incline of bend (when graphed)
  9. 9. Monthly Tidal Cycle
  10. 10. Extreme High Tides • The Bay of Fundy • A 50’ (15m) tidal fluctuation is common (x2) •A tidal bore (several in. to several ft. high) rushes miles up the Petitcodiac River in New Bruswick
  11. 11. Landforms shaped by extreme tides
  12. 12. Extreme tides: Mont Saint Michel, FranceLow tide High tide 12
  13. 13. Currents: Surface
  14. 14. Currents: Thermohaline Circulation
  15. 15. Waves and Wave Dynamics
  16. 16. Waves and Wave Dynamics• Period—The time it takes two successive waves (from crest to crest, or from trough to trough) to pass a given point• Fetch—The distance over which the wind blows, creating waves
  17. 17. Waves and Wave Dynamics• Factors affecting open ocean waves: –Fetch •The greater the distance over which the wind blows, the larger the waves –Wind strength •The stronger the wind, the larger the waves –Wind duration •The longer the wind blows, the more waves will be created
  18. 18. Waves and Wave Dynamics Wave base
  19. 19. Waves of Oscillation (Transition) andWaves of Translation• Water molecules on the open ocean move in a circular motion – The motion passes through the water, but the water doesn’t move forward (to oscillate is to move up and down) – What moves the water forward is wind blowing over the surface and the movement of currents• Water molecules that reach the shore have their circular motion interrupted – Their energy is translated into the shore face (it passes from the water to the land, where it does work to move material—like sand—around)
  20. 20. Waves and Wave Dynamics Waves of translation Waves of oscillation
  21. 21. Wave Break
  22. 22. Wave Refraction andLongshore Current• As waves reach the shore and “feel” the bottom, they slow and break• The direction of wave break follows the underwater topography• This causes the wave to “bend” and become more parallel to the shoreline – This bending is called wave refraction• As it breaks from one side to the other, it creates a current – This current, called the longshore current picks up and moves sediment (sand) down the shore
  23. 23. Wave Refraction,Longshore Current and Beach Drift
  24. 24. Erosional Environments• If there is not enough sediment replacing what’s being lost through wave erosion/longshore current, erosion will occur, creating a rocky coastline – Dammed or channelized streams • sediment can’t get to the beach• Erosive environments create distinctive landforms – rocky headlands and pocket beaches, sea arches and sea stacks, wave-cut cliffs, wave-cut platforms, wave- built terraces, etc.
  25. 25. Wave Energy is Concentrated atHeadlands and Dissipated in Bays
  26. 26. Wave Motion and WaveRefraction
  27. 27. Wave Motion and WaveRefraction
  28. 28. Laguna Beach, CA Headlands
  29. 29. Headland Erosionand the Formation of Sea Arches
  30. 30. Stacks and CliffsVictoria, Australia
  31. 31. The Holderness Coast is one of Europes fastest eroding coastlines. Theaverage annual rate of erosion is around 2 metres per year. The main reasonfor this is because the bedrock is made up of till. This material was depositedby glaciers over 18,000 years ago.
  32. 32. Depositional Environments:The Structure of a Beach
  33. 33. Common Depositional Landforms
  34. 34. Barrier Island Structure
  35. 35. Barrier Island Coast Padre Island, Texas 35
  36. 36. Lagoons, Marshlands, and theFormation of New Coastlines
  37. 37. Coastal Stabilization andHuman Impact“Any serious researcher would be hard-pressed to find a marina, a sea wall, or anyother human structure along the shorelinethat does not pose some long-termdeleterious effects to both the natural andcultural environments it attempts to protect.” --Physical Geography: Earth’s Interconnected Systems Angela Orr, 2007
  38. 38. Coastal Stabilization and HumanImpact• Damming and channelizing streams –Causes a loss of sediment where streams empty into the sea. Without sediment, the beach will erode away.• Groynes (groins)• Seawalls• Breakwaters and jetties 38
  39. 39. Coastal Stabilization Structures
  40. 40. Groynes (groins) 40
  41. 41. • Can you tell which way the longshore current is moving material?• Once you put up one groyne, you need to keep building them all along the shore to keep erosion from destroying property downshore from your first structure. Note the severe erosion at the top of the photo where the groynes stop. 41
  42. 42. Breakwater
  43. 43. Breakwater• These breakwaters are creating tombolos in the wave shadow behind them. 43
  44. 44. The Example of Marina del Rey, CA