Coastal processes


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  • FIGURE 13.15 ( p. 354) Water moves up a beach face at an angle with the wave direction but returns directly down the beach slope. Sand picked up by the incoming wave is washed up the beach and is returned seaward a small distance further down the beach in the direction of the waves. Successive waves move sand progressively along the beach—a process known as “longshore drift” or “littoral drift.”
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  • FIGURE 13.18 ( p. 357) Seasonal variation in beach profile. (a) During summer, when waves are relatively gentle, the beach profile is steep, and one or more summer berms form immediately above the high-tide line. (b) Winter or storm waves, especially when combined with storm surges, reach farther up the beach and erode sand from the berm, moving the sand seaward to form one or more offshore bars. The result is a narrower flat-beach foreshore that is cut back to dunes or a winter berm left from periodic extreme winter storms. (c) When storm waves cease, the gentler waves return sand from the offshore bar to the beach, rebuilding its summer profile. If storms are strong in a particular winter, the beach may recede inland, and dunes may be somewhat lowered.
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  • FIGURE 13.21 ( p. 361) Seawalls are designed to protect beachfront property from erosion. (a) The Galveston seawall was constructed after the destructive hurricane of 1900. Since the wall was completed in 1962, the beach has eroded along much of its length, leaving the shoreline more exposed to attack by the highest waves of future hurricanes. (b) The barrier island has continued to retreat since the Galveston seawall was constructed. The beach just beyond the end of the wall has now retreated substantially farther shoreward than the wall’s location as a result of the natural process of barrier island retreat and the consequent reduction in the amount of sand in the littoral drift system that occurred once the beach had been eroded away in front of the wall. (c) Seawalls are undermined from the sides and eventually destroyed as sea level rises. CREDIT: (a-b) © D. Segar and E. Stamman Segar.
  • Coastal processes

    1. 1. Coastal Systems
    2. 2. Coastal Settings <ul><li>Open coasts can be divided into two general categories based on the dominant processes acting on the coast over long periods of time (1000’s of years or more) </li></ul><ul><li>Erosional </li></ul><ul><ul><li>Shorelines dominated by processes that form erosional features along the shore </li></ul></ul><ul><ul><li>Typically high-relief rocky coasts in tectonically active areas (e.g., the Pacific coast of North America) Typically high-relief rocky coasts </li></ul></ul><ul><li>Depositional </li></ul><ul><ul><li>Shorelines dominated by processes that form deposits along the shore </li></ul></ul><ul><ul><li>Include a wide spectrum of environments </li></ul></ul><ul><ul><ul><li>Deltas, barrier islands, reef coasts, glaciated coasts, etc. </li></ul></ul></ul><ul><ul><li>More typical of passive, or trailing-edge, continental margins (e.g., the Atlantic coast of North America) </li></ul></ul>
    3. 3. Features of erosional coasts 11.4-Thurman and Trujillo, 2004
    4. 4. Features of erosional coasts Cabo Rojo, PR Sea arch Wave-cut cliff
    5. 5. Features of depositional coasts 11.7-Thurman and Trujillo, 2004
    6. 6. Features of depositional coasts 21, 25-RPI Barrier islands along the South Carolina coast
    7. 7. A wave moving onto the shore 20.5-Tarbuck & Lutgens, 2005
    8. 8. Wave erosion
    9. 9. Wave erosion <ul><li>Breaking waves exert a great force </li></ul><ul><li>Wave erosion is caused by </li></ul><ul><ul><ul><li>Wave impact and pressure </li></ul></ul></ul><ul><ul><ul><li>Abrasion by rock fragments </li></ul></ul></ul>
    10. 10. Wave erosion 20.6, 20.7-Tarbuck & Lutgens, 2005
    11. 11. Wave refraction <ul><li>Wave refraction </li></ul><ul><ul><ul><li>Bending of a wave </li></ul></ul></ul><ul><ul><ul><li>Causes waves to arrive nearly parallel to the shore </li></ul></ul></ul><ul><ul><ul><li>Consequences of wave refraction </li></ul></ul></ul><ul><ul><ul><ul><li>Wave energy is concentrated against the sides and ends of headlands </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Wave energy is spread out in bays and wave attack is weakened </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Over time, wave erosion straightens an irregular shoreline </li></ul></ul></ul></ul>
    12. 12. <ul><li>Wave energy focused on headland </li></ul><ul><li>Wave energy dispersed over bay </li></ul>Wave refraction 9.19b-Thurman and Trujillo, 2004
    13. 13. Wave Reflection and Refraction
    14. 14. Wave Motion/Refraction
    15. 15. 10.20b-Garrison, 2005 Wave refraction, Maili Point, Oahu
    16. 16. <ul><li>Movement parallel to the ( ↔ ) shoreline </li></ul><ul><ul><ul><li>Caused by wave refraction </li></ul></ul></ul><ul><ul><ul><li>Along most shorelines, waves strike the shore at an angle </li></ul></ul></ul><ul><ul><ul><ul><li>Waves that reach the shoreline at an angle cause the sediment to move along a beach in a zigzag pattern called beach drift </li></ul></ul></ul></ul>Sediment movement on the shore
    17. 17. Sediment movement on the shore <ul><li>Movement parallel to the ( ↔ ) shoreline </li></ul><ul><ul><ul><li>Oblique waves also produce longshore currents </li></ul></ul></ul><ul><ul><ul><ul><li>Currents in the surf zone </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Flow parallel to the coast </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Easily moves fine suspended sand and rolls larger sand and gravel along the bottom </li></ul></ul></ul></ul>
    18. 18. Movement of sand by longshore current 20.10-Tarbuck & Lutgens, 2005
    19. 19. Sediment movement along a beach 11.19-Segar, 2007
    20. 20. Beach Drift and Longshore Currents
    21. 21. <ul><li>Movement perpendicular ( ↕ ) to the shoreline </li></ul><ul><ul><ul><li>Caused by breaking waves </li></ul></ul></ul><ul><ul><ul><ul><li>Swash ( ↑ ) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Backwash ( ↓ ) </li></ul></ul></ul></ul>Sediment movement on the shore
    22. 22. Swash -> ← Backwash Domes beach, Rincón, PR
    23. 23. T11.2-Thurman and Trujillo, 2004
    24. 24. 11.2a-Thurman and Trujillo, 2004
    25. 25. 11.2a-Thurman and Trujillo, 2004
    26. 26. Seasonal variation in beach profile 11.19-Segar, 2007
    27. 27. Barrier Systems Sandy Point, Cape Romain, South Carolina 38-RPI
    28. 28. Barrier Systems <ul><li>Thin strips of land built a few to several tens of meters above sea level </li></ul><ul><li>Form due to the combined action of wind, waves, and longshore currents </li></ul><ul><li>Act as a barrier – protect mainland coast from open-ocean wave energy, storm surge, etc. </li></ul><ul><li>Protected environments form behind barriers – bays, lagoons, marshes, tidal creeks </li></ul>
    29. 29. Barrier Systems <ul><li>Most common along passive margins </li></ul><ul><li>Economically important </li></ul><ul><li>Vulnerable to storms, rising sea level, erosion </li></ul><ul><li>One of the major forces impacting barriers today, as well as in the past, is rising sea level </li></ul>
    30. 30. 11.9c-Thurman and Trujillo, 2004 Heavily developed barrier islands Tom’s River, New Jersey Barrier islands north of Charleston, SC 21-RPI
    31. 31. Chesapeake Bay – a drowned river valley Pamlico Sound – a bar-built estuary 12.5-Garrison, 2005
    32. 32. Coastal Erosion and Beach Loss <ul><li>Coastal erosion is a natural process that redistributes sediments along coasts </li></ul><ul><li>Coastal erosion becomes a problem when coastal development impedes or ignores the natural movement of sediment along the shore </li></ul><ul><li>This problem is exacerbated by continued sea level rise </li></ul>
    33. 33. Coastal erosion is a natural process North Island, SC 32-RPI
    34. 34. Figure 20.16 Wave erosion caused by strong storms, Long Island, New York Coastal development makes erosion a problem
    35. 35. Sea-level rise makes the problem worse Studies show a 150x erosion multiplier for sea level rise on sandy shorelines. Hence, for a mean 0.24 m rise by 2050, beaches will recede 36 m (118 ft). ( Leatherman et al., 2000)
    36. 36. Stabilizing the shore <ul><li>Shoreline erosion is influenced by several local factors including </li></ul><ul><ul><ul><li>Proximity to sediment-laden rivers </li></ul></ul></ul><ul><ul><ul><li>Degree of tectonic activity </li></ul></ul></ul><ul><ul><ul><li>Topography and composition of the land </li></ul></ul></ul><ul><ul><ul><li>Prevailing wind and weather patterns </li></ul></ul></ul><ul><ul><ul><li>Configuration of the coastline and nearshore areas </li></ul></ul></ul>
    37. 37. Stabilizing the shore <ul><li>Three basic responses to erosion problems </li></ul><ul><ul><li>Building structures (Hard stabilization) </li></ul></ul><ul><ul><li>Beach nourishment </li></ul></ul><ul><ul><li>Relocation </li></ul></ul>
    38. 38. Stabilizing the shore <ul><li>Hard stabilization perpendicular to the shore </li></ul><ul><ul><ul><li>Jetties </li></ul></ul></ul><ul><ul><ul><ul><li>Usually built in pairs to develop and maintain harbors </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Extend into the ocean at the entrances to rivers and harbors </li></ul></ul></ul></ul>
    39. 39. Jetties are built to prevent deposition Figure 20.17
    40. 40. Stabilizing the shore <ul><li>Hard stabilization perpendicular to the shore </li></ul><ul><ul><ul><li>Groins </li></ul></ul></ul><ul><ul><ul><ul><li>Built to maintain or widen beaches </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Constructed at a right angle to the beach to trap sand </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Because of increased erosion on the downdrift side of the groin, additional groins may be built resulting in a groin field </li></ul></ul></ul></ul>
    41. 41. 11.20-Thurman and Trujillo, 2004 Interference of sand movement <ul><li>Hard stabilization like the groin shown here interferes with the movement of sand along the beach, causing deposition of sand upstream of the groin and erosion immediately downstream, modifying the shape of the beach. </li></ul>
    42. 42. Figure 20.18 Groins along the New Jersey Shore at Cape May Longshore transport
    43. 43. Jetties and Groins <ul><li>Jetties are always in pairs </li></ul><ul><li>Groins can be singular or many (groin field) </li></ul><ul><li>Both trap sand upstream and cause erosion downstream </li></ul>11.22-Thurman and Trujillo, 2004
    44. 44. Stabilizing the shore <ul><li>Hard stabilization parallel to the shore </li></ul><ul><ul><ul><li>Breakwater </li></ul></ul></ul><ul><ul><ul><ul><li>Barrier built offshore and parallel to the coast </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Protects boats from the force of large breaking waves </li></ul></ul></ul></ul>
    45. 45. Groins and Breakwaters 11.7-Jones and Jones, 2003
    46. 46. Figure 20.19
    47. 47. Coastal Stabilization Structures
    48. 48. Stabilizing the shore <ul><li>Hard stabilization parallel to the shore </li></ul><ul><ul><ul><li>Seawall </li></ul></ul></ul><ul><ul><ul><ul><li>Barrier parallel to shore and close to the beach to protect property </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Stops waves from reaching the beach areas behind the wall </li></ul></ul></ul></ul><ul><ul><ul><li>Often the building of structures is not an effective means of protection </li></ul></ul></ul>
    49. 49. Seawalls and beaches <ul><li>Seawalls are built to reduce erosion on beaches </li></ul><ul><li>Seawalls can destroy recreational beaches </li></ul><ul><li>Seawalls are costly and eventually fail </li></ul>11.25-Thurman and Trujillo, 2004
    50. 50. Seawalls are designed to protect beachfront property from erosion 13.21-Segar, 2007
    51. 51. Figure 20.20 Seawall in Seabright, New Jersey Where’s the beach?
    52. 52. Wine Island? 10.26-Merritts et al., 1998 <ul><li>Seawalls can’t compete with sea-level rise </li></ul>
    53. 53. Responses to Coastal Erosion and Beach Loss <ul><li>Hard stabilization </li></ul><ul><ul><li>“ Armoring” the shoreline </li></ul></ul>
    54. 54. Responses to Coastal Erosion and Beach Loss <ul><li>Hard stabilization </li></ul><ul><ul><li>“ Armoring” the shoreline </li></ul></ul>
    55. 55. Responses to Coastal Erosion and Beach Loss
    56. 56. USGS Illegal extraction of beach sand is one of the major causes of coastal erosion in Puerto Rico
    57. 57. Morelock et al. Illegal extraction of beach sand is one of the major causes of coastal erosion in Puerto Rico
    58. 58. Stabilizing the shore <ul><li>Alternatives to Hard Stabilization </li></ul><ul><ul><ul><li>Beach nourishment </li></ul></ul></ul><ul><ul><ul><ul><li>The addition of large quantities of sand to the beach system </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Only an economically viable long-range solution in a few areas </li></ul></ul></ul></ul><ul><ul><ul><li>Abandonment and relocation of buildings away from the beach </li></ul></ul></ul>
    59. 59. Responses to Coastal Erosion and Beach Loss <ul><li>Beach nourishment </li></ul>Wrightsville Beach, NC 10.24-Merritts et al., 1998
    60. 60. Figure 20.21 Beach nourishment, Miami Beach
    61. 61. USGS Locations of three submerged sand deposit areas on the insular shelf of Puerto Rico Aerial photograph of the Escollo de Arenas, a subtidal sand and gravel deposit that extends some 6 kilometers north-northwestward of Punta Arenas on the northwest coast of Vieques Island, east of Puerto Rico.
    62. 62. Responses to Coastal Erosion and Beach Loss <ul><li>Caveats of beach nourishment </li></ul><ul><ul><li>Not permanent solution </li></ul></ul><ul><ul><li>Expensive </li></ul></ul><ul><ul><li>Need appropriate sand source </li></ul></ul>
    63. 63. Responses to Coastal Erosion and Beach Loss <ul><li>Still, with appropriate financial and sand resources, nourishment can be a viable solution in some settings </li></ul>
    64. 64. Responses to Coastal Erosion and Beach Loss <ul><li>Abandonment and relocation </li></ul>
    65. 65. Alternatives to hard stabilization <ul><li>Restrict the building of structures too close to the shore </li></ul><ul><li>Eliminate programs that encourage construction in unsafe locations </li></ul><ul><li>Relocate structures as erosion threatens them </li></ul>Figure 10C Relocation of the Cape Hatteras lighthouse, North Carolina
    66. 66. Coastal Hazards <ul><li>Increasing coastal development and rising sea level puts more people and property at risk </li></ul>Combined wind and storm surge damage Iniki flooding reached >800 ft inland >25 ft above sea level
    67. 67. New Directions? <ul><li>Avoid development of eroding lands </li></ul><ul><li>Discourage additional development in erosion hazard zones </li></ul><ul><li>Enforce “setbacks” </li></ul><ul><li>Acquire high-value coastal lands </li></ul><ul><li>Construction guidelines for hazard areas </li></ul><ul><li>Nourish eroding shorelines </li></ul><ul><li>Natural resources are those that are derived from the Earth and that exist independent of human activity </li></ul><ul><li>- Cutter and Renwick, 2004 </li></ul>
    68. 68. New Directions? <ul><li>Without a sound understanding and appreciation of the dynamics of the coastal zone, and thoughtful environmental management of the shore </li></ul><ul><li>We may lose one of our most precious natural resources </li></ul>