Costal Erosion: - PowerPoint PPT Presentation

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Costal Erosion:

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  1. Costal Erosion: “Approximately 25 percent of homes and other structures within 500 feet of the U.S. coastline and the shorelines of the Great Lakes will fall victim to the effects of erosion within the next 60 years, according to a study released by the Federal Emergency Management Agency (FEMA).” http://www.fema.gov/nwz00/erosion.shtm

  2. Coastal erosion and sediment transport

  3. Coastal Evolution • Balance between: • Wave action (energy, geology) • Sediment supply • Sea level

  4. Wave base: point at which wave action becomes negligible on the sea floor It is the effective lower limit of wave action and erosion /2 = wave base Erosion and movement by WAVES 

  5. Waves • So, surf is a powerful erosive agent • Its energy is expressed by turbulence… • …violent movements of parcels of water which are able to move and suspend sediment

  6. Oscillatory and Translational Motion

  7. Sediment movement - redistribution by WAVES • Longshore transport: waves hitting coast obliquely, depends on prevailing winds • waves have both a perpendicular and parallel component relative to the coast • the parallel component represents longshore drift; transport of sediment along the beach

  8. Baymouth Bar - Russian River, CA

  9. Wave Refraction • Waves change path when they reach shallow water • Wave energy is concentrated on headlands and spread out in bays

  10. Wave Refraction off Cape Cod

  11. Tombolo - Santa Cruz, CA

  12. Rips • When waves break parallel to a beach, rips occur

  13. Rips, Lake Superior

  14. Rip currents: water that flows straight out to sea from the surf zone. Travel at the surface and die out at depth. Carry fine grained particles out of surf zone to deeper water.

  15. Summer Winter

  16. Sediment movement - redistribution by WAVES • Longshore transport or Beach drift: oblique waves which move sediment along the beach • Storm transport: very rapid erosion, much larger than normal, which tends to narrow beaches and move sediment offshore

  17. Sea Level Change 1. Glaciers • Glaciers Lower Sea Level Slowly • They Melt quickly – sea level rises 2. Mid Ocean Ridges • Mid Ocean Ridge raises sea level if large and active • Lowers sea level if activity slows - becomes smaller

  18. Sea level rise • linked to global warming, either natural or human-induced • over next century, we anticipate sea level to rise by 30-100 cm • along the east coast where the coastal plain rises gently, this rise translates to a shoreline setback of 100-400 feet.

  19. Shoreline shift • Impact of sea level rise is greatest in low coastal areas http://pubs.usgs.gov/circ/c1075/contents.html

  20. Effects of the Pleistocene • Sea level has risen at least 100 meters in the last 18,000 years • Most coastlines globally are submergent • Primary coastlines are very common

  21. (From Barnola, 2006)

  22. Nonmarine Processes - Uplift Convergent Margins

  23. Wave-Cut Platform, California

  24. Sediment/Sand Budget

  25. Artificial structures and beach erosion • Dams • Groins • Breakwaters • Seawalls • Inlets, dredging, and jetties

  26. Human intervention

  27. Santa Cruz Harbor, California Direction of current

  28. Our efforts to control nature can be a expensive struggleSanta Barbara Harbor in 1931 Source:Fairchild air photos 0-139 & E-5780, UCLA Department of Geography Aerial Photo Archives

  29. Santa Barbara Harbor in 1977 Source:John S. Shelton

  30. From Dean (1999) Groins • Designed to trap sediment… • but they instead progressively starve beach of sediment in the direction of longshore-littoral current

  31. Groins off Cape May, New Jersey Source:John S. Shelton

  32. Further effects of groins Highly eroded beach Rockaway beach, New York Eroded beach…compare buildings on either side of groin

  33. Breakwaters • Designed to allow sediment to settle • Smaller waves behind, less sediment is transported From Dean (1999)

  34. Seawalls • Various types: revetments, rip-rap, concrete walls, piles of rubble, etc. • Nearly 100% of the time they damage or destroy a beach which is eroding to begin with Loss of beach Flooding of beach Destruction of seawall From Dean (1999)

  35. Beach-Protection Structures - Riprap Source:Jack Dermid /Photo Researchers, Inc.

  36. Beach-Protection Structures –Seawall Along the Gulf Coast of Louisiana Source:Martin Miller

  37. The consequences of NO BEACH • Hugely costly measures are required to restore beach, e.g., pumping sand, and this is only a temporary solution • Also, houses are now closer to the water and more vulnerable • On either end of the seawall, the beach erodes and is displaced toward the land naturally…so the walled part of the beach is exposed to the ocean and vulnerable to storms

  38. From Dean (1999) Sand bypassing to replenish beach on down-current side of jetty Replenishing sand the old-fashioned way

  39. Inlets • Inlets are created naturally by large storms…they are breaks in a barrier beach • Over time, the inlet will heal naturally • The inlet can heal rapidly, or can linger for years or decades From Dean (1999)

  40. Raccoon Island, Louisiana

  41. Closing of an inlet • The healing time of an inlet depends on: • -amount of sand transported by littoral drift • -amount of water flowing through with tide • Thus, if the tidal flow is low and littoral currents carry abundant sand, the inlet can heal quickly • Flood tide deltas represent a large amount of sand carried inside the inlet’s mouth • This is a natural sink for sand, and the sand remains relatively undisturbed

  42. Inlets are kept open artificially sometimes, e.g., for a harbour, fishing fleet, etc. If so, inlet needs to be dredged periodically The dredged sand is often dumped in deep water offshore…the sand is therefore lost If sand were dumped on the beach down-current, beach might be stabilized…but not an easy task due to the high-energy surf zone Inlets and dredging

  43. Inlets and jetties • Another way to keep inlets open artificially is by building jetties • The jetties serve to “jet” out sand into deeper water…again, a loss • The longer the jetty, the worse the erosion down-current

  44. Jetty in Miami Beach, Florida Source:Townsend P. Dickinson

  45. Artificial modification of inlets • One problem is political…stakeholders may have diametrically opposite viewpoints… • …e.g., tourism operators who need a beach vs. fishing fleets who want easy access to the ocean through the inlet • One solution is sand-bypassing, but this is expensive and inefficient compared to Mother Nature