CHAPTER 14 - WAVES_ BEACHES AND COASTS

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CHAPTER 14 - WAVES_ BEACHES AND COASTS Powered By Docstoc
					CHAPTER 18 - COASTS AND OCEANS
Overview

     Wind energy is transferred to water surface and forms waves. Wave height
     (distance between crest and trough) is determined by the amount of energy
     transferred. Wave length is the distance between crests. As a wave passes,
     water molecules describe circular orbits, equal to wave height at the surface,
     but decreasing with depth until there is essentially no motion at a depth equal
     to half the wavelength. Waves break because friction with the sea floor causes
     the top of the wave to move faster and it eventually collapses producing surf.

     As waves approach shore, they change direction as the leading edges are
     slowed by contact with the bottom and the waves become more parallel to
     shore. This wave refraction produces longshore currents that flow parallel to
     the shore. Rip currents are localized narrow currents that flow perpendicular to
     shore in areas with low wave height. They may either be in fixed positions
     over channels or shift because of wave interference.

     Tides involve the regular rising and falling of the sea surface and are caused
     by the gravitational attraction between the Earth, the sun and the moon. Two
     bulges are produced on the surface of the oceans that move around the Earth
     and cause water levels to rise (flood tide) or fall (ebb tide), usually on a
     semidiurnal (twice a day) cycle. Tidal range is the difference in water level
     between high and low tide and can range from less than 1 m to over 16 m.
     Areas of the shoreline exposed between high and low tides form an intertidal
     zone.

     Beaches are strips of sand or gravel along which waves break and extend
     inland from the low water line to a cliff or vegetated area. Beaches can be
     subdivided into an offshore marine terrace, a beach face, and an upper berm.
     The form and dimensions of beaches change seasonally, with winter beaches
     being narrow, and summer beaches wider. Tremendous amounts of sediment
     are transported along the beach face by swash and backwash and immediately
     off shore by longshore drift. Spits, baymouth bars and tombolos are the
     natural products of longshore drift. Jetties and groins constructed by humans
     can also interrupt sand transport along a beach. The bulk of beach sand is
     transported to the coast by rivers and streams. Damming this source cuts off
     sand supply and promotes beach erosion.

     Erosional coastlines exhibit features such as sea cliffs, arches, stacks, wave-
     cut platforms and undergo progressive coastal straightening. Depositional
     coastlines are characterized by barrier islands, deltas, and tidal deltas.
     Drowned coastlines are submergent and identified by estuaries or fiords.
     Uplifted coastlines are emergent and marked by uplifted marine terraces.
     Coastlines can also be shaped by organisms such as reefs and mangroves.

     The ocean floor consists of the gently sloping continental shelf, the steeply
     sloping continental slope and the deep ocean floor. Passive continental
     margins are those in areas of little tectonic activity, such as the eastern coast
     of the US, while active continental margins are associated with volcanic and
     earthquake activity, such as the margins of the Pacific Ocean. Continental
     shelves lie on the edge of continents and are mostly covered with land-derived
     sediment, although in areas of little sediment input, reefs may form.
     Continental slopes link continental shelves to the deep ocean and are
     characterized by steep slopes and turbidite deposition. Submarine canyons
     cut into the edge of continental shelves and the upper part of continental
     slopes and allow the transport of large amounts of sediment by turbidity and
      bottom currents to be deposited on submarine fans on the continental rise at
      the base of the slope. Turbidity currents are sediment-laden current that
      move rapidly downslope under the influence of gravity. Contour currents are
      slow moving and flow along (parallel to) the slope. Abyssal plains are flat
      areas at the base of the continental rise, although some areas of the deep
      ocean floor are characterized by rugged topography. Sediments accumulating
      on the sea floor are either terrigenous (derived from land) or pelagic (settle
      from suspension. Pelagic sediments include clays and the skeletons of
      microscopic organisms.

Learning Objectives

      1. Wind energy is transferred to water surfaces to form waves. Wave height
      (distance from crest to trough) reflects wind speed, duration and distance.
      Wavelength is the distance between crests. Waves passing a point move water
      particles in a circular orbit. At the surface, diameter of the orbit equals wave
      height. At depth, effects of wave passage are lost below depths equal to half
      the wavelength.

      2. Wave refraction is the change in direction along a wave crest as it comes in
      contact with the bottom while approaching shore. Wave refraction helps
      develop longshore currents that are flow parallel to the shoreline and transport
      considerable amounts of sediment in the surf zone. Rip currents flow
      perpendicular to shore and carry fine sediment offshore.

      3. Tides are caused by changes in the elevation of the sea surface, usually on
      a semidiurnal cycle. Tides are caused by a force resulting from the
      gravitational attraction between the Earth, sun and moon.

      4. Beaches are strips of sand or gravel that extend from marine terraces
      offshore to cliffs or permanent vegetation zones onshore. The beach face is the
      steepest part and is exposed to wave action. The berm extends landward from
      the beach face.

      5. Summer beaches have wide berms, while winter beaches have narrow
      berms and sandbars offshore. Most sediment on beaches was brought to the
      coast by rivers and streams. Damming free flowing rivers reduces sediment
      supply to beaches and promotes beach erosion.

      6. Longshore drift is the movement of sediment parallel to shoreline by a
      combination of swash and backwash along the beach face, and longshore
      currents. Spits are fingerlike ridges of sediment deposited into open water.
      Baymouth bars extend from headland to headland cutting off bays from the
      ocean. Tombolos connect offshore islands to the mainland. All three
      depositional features are the result of longshore drift processes. Jetties and
      groins are structures made by humans to interrupt sand being transported
      along the shoreline by longshore drift.

      7. Coastal areas are classified as erosional, depositional, drowned, uplifted, or
      shaped by organisms. Erosional coasts are subject to coastal straightening
      (erosion on headlands and deposition in bays), and exhibit sea cliffs, wave-cut
      platforms, stacks and arches.

      8. Depositional coasts have barrier islands, deltas, tidal deltas, and may
      preserve glacial deposits such as moraines. Fiords and estuaries are typical of
        drowned (submergent) coasts. Uplifted coasts exhibit uplifted marine terraces.
        Reefs and mangroves may also shape coasts.

        9. Beyond the shoreline, the ocean floor consists of a continental shelf,
        continental slope and abyssal plain. Passive continental margins are those
        formed as rifted margins, whereas active continental margins are those along
        which tectonic activity occurs.

        10. Continental shelves are usually covered with sediment derived from land
        and moved by currents and waves. Reefs are resistant bodies formed of coral
        and other organisms and develop in areas where sediment supply is low.
        Continental slopes are steeply sloping and link shelves with the deeper ocean.
        Submarine canyons are erosional features that cut into the outer part of
        shelves and the upper slope and transport sediment by turbidity currents. The
        continental rise is a wedge of sediment at the base of the slope and passes
        into the flat abyssal plain.

        11. Ocean crust on the ocean floor is covered with either terrestrial sediment
        derived from land or pelagic sediment consisting of clay and microscopic
        organisms.

Boxes

18.1 ENVIRONMENTAL GEOLOGY – COMMUNITIES AND COASTAL EROSION

        Communities built on shorelines often face severe problems with coastal erosion,
        particularly when shorelines consist of loose and easily eroded sediments. Erosion
        rates are particularly high in areas of Atlantic Canada and along the Pacific coast of
        British Columbia and have resulted in damage to docks, roads, homes and
        lighthouses. Coastal erosion is not restricted to marine areas and shorelines of the
        Great Lakes also experience high erosion rates. Canadian Arctic coasts are often
        protected from wave erosion by sea ice although damage can be considerable during
        years of sparse sea ice development.

18.2 ENVIRONMENTAL GEOLOGY - THE EFFECTS OF RISING SEA LEVEL

        Long term sea level rise is caused by melting glaciers, while short term sea
        level rise is caused by storm surge. Sea level has risen 130 meters during the
        past 15,000 years. Initial sea level rise was 1.3 meters per 100 years, but
        declined to 4 centimeters per 100 years. Since 1930, however, the rate of sea
        level rise has increased six-fold to 24 cm per century. This rapid rise caused
        barrier island migration, increased erosion and allowed storm waves access
        further inland than before. Even small increases in sea level have the potential
        for catastrophic flooding of large portions of the coastal United States because
        these areas are so flat and low. Melting all glacial ice would raise sea level by
        60 meters drowning significant coastal areas. Short-term sea level rise occurs
        from storm surges caused by hurricanes that produce a doming of sea level as
        they pass. This situation is compounded by high winds that drive the seawater
        onto shore. Storm surges can exceed 5 meters and have caused considerable
        damage and loss of life in coastal areas. A storm surge of 8 meters struck
        Galveston, Texas, in 1900, killing 6,000 people. A 5 meter storm surge
        produced by Hurricane Hugo in 1989 caused $10 billion damage to South
        Carolina.


Short Discussion/Essay
      1. How much would sea level rise if 1) the north polar ice cap melted; 2) the
      south polar ice cap melted; 3) all glacial on earth melted?
      2. How do tides affect the operation of coastal processes?
      3. Explain the relationship between wave refraction, longshore currents and
      longshore drift.
      4. Why is beach erosion such a problem along the Atlantic coast?
      5. Explain how jetties and groins work.

Longer Discussion/Essay

      1. If sea level is actually rising, how can both emergent and submergent
      coasts be found around the world?
      2. How might a geologist use the concept of effective wave base to
      differentiate between rocks deposited in "shallow" and "deep" settings?
      3. Why would beach dimensions change with season?
      4. Explain the influence of waves and wind on the character of beach sand (i.e.
      sorting, roundness, size differences).
      5. Why are beach replenishment programs ultimately doomed to failure?

Selected Readings

      Businger, Steven. 1991. "Arctic hurricanes", American Scientist 79 (1): 18-33.

      Davis, R.E. and Dolan, R. 1993. "Nor'easters," American Scientist 81 (5): 428-
      439.

      Horton, T. 1993. "Hanging in the balance: Chesapeake Bay" National
      Geographic 183: 3- 35.

      Pilkey, O. H. 1989. "The engineering of sand. Journal of Geological Education
      37: 308 -311.

      Richardson, P.L. 1993. "Tracking ocean eddies," American Scientist 81 (3):
      261-272.

      Schlee, J.S., Karl, H.A. and Torresoan, M.W.. 1995. "Imaging the Sea Floor"
      U.S. Geological Survey Bulletin 2079.

      Schneider, D. 1997. "The rising seas," Scientific America 276: 112-117.

      Trenhaile, A.S., 1998. Geomorphology: A Canadian Perspective. Oxford
      University Press Canada, 340pp

      Walden, D. 1990. "Raising Galveston." American Heritage of Invention and
      Technology (winter): 8-18.

				
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