Oceanic Overwash

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Oceanic Overwash
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Beach Diagram
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Oceanic Overwash

• The washover fan is a fan-shaped accumulation of
  sand and shell that is deposited in a thin layer
  during intense storm conditions when part or all
  of the beach-dune system is overtopped or
  breached by incoming waves and storm surges
• Fans may come together to form a washover apron
• Sediment accumulations may be a few tens of
  centimeters or up to 1 meter thick
• Sediment will cover vegetation on the back side
  of the island, but plants can grow up through the
  sediment, reworking the layering of the sediments

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Overwash
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Oceanic Overwash
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Oceanic Overwash and Channels
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Overwash and Storms

• Strong wind and waves causes storm surges to wash
  over low dunes or create breaks in the dune
  system
• New inlets can be formed in low lying areas on
  the island

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Overwash
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Overwash effects in Buxton, NC
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Graph of changes after Dennis to Buxton, NC
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Overwash after Hurricane Dennis
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Washover After Dennis
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Overwash and Transgression

• Sediment is transferred to the back side of the
  island by overwash
• Sediment may bury marshland on the lagoon side,
  building up the back side of the island
• Overwash process erodes sediment from the front
  side and carries it to the back
• The combination of erosion on the front and
  buildup on the back of the island are factors
  that lead to island transgression

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Man-made Interventions

• Groins-A shore protection structure built
  (usually perpendicular to the shoreline) to trap
  lateral drift or retard erosion of the shore
• Jetties-structures intended to keep sand from
  flowing into a channel intended to protect an
  inlet
• Sea walls-rock or concrete structures built
  parallel to the shoreline intended to protect
  buildings by repelling the sea
• Beach replenishment-Building up the beach system
  by depositing sand

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Beach Replenishment

• Beach replenishment is used to add dredged sand
  or sediment from an inland source to the beach
  system.
• Procedure is expensive and not a permanent
  solution
• Some prefer this temporary fix to the more
  permanent structures that may cause more damage
  in the long run

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Photos of Renourishment at Kure Beach
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Groins
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Groins
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Jetty
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Sea Wall
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Effects of Sea Walls
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Sea Wall at Shell Island
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Sea Wall
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Myrtle Beach
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Disappearance of Islands
Long-term rates of change between 1855 and 1989
show both accretion and erosion for barrier
island shorelines facing Louisianas coast.
Between the late 1800s and the 1980s, deposits
left from washovers caused some of these
shorelines to migrate landward at rates up to
24.0 meters per year. Erosion, in contrast, has
caused loss on other shorelines at rates from 1
to 5 meters per year.
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Disappearance of Islands
The timeline above shows anticipated years of
disappearance for several of Louisianas barrier
islands through 2110. Although these barrier
islands are in danger, other more stable barriers
exist. For example, Grand Isle has a life
expectancy about 900 years longer than that of
Timbalier Island
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On-line References

• http//books.nap.edu/books/030904806/html/70.html
  pagetop
• http//coastal.er.usgs.gov/hurricanes/dennis/buxto
  n/
• http//members.nbci.com/igorz/MIWO.html
• http//www.beachbrowser.com/Archives/Environment/A
  /Barrier-Islands-Always-changing.ht
• http//www.csc.noaa.gov/products/nchaz/htm/lidtopo
  .htmhttp//www.lacoast.gov/Programs/CWPPRA/Waterma
  rks/Fall99/barrier.fa99.htm

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References

• http//www.ncsu.edu/coast/shell/migration/migratio
  n.html
• http//www.ncsu.edu/coast/shell/terms.html
• http//www.salem.mass.edu/lhanson/gls214/gls214_t
  ec_clas.htm
• http//www.seafriends.org.nz/oceano/beacheng.htms
  ea20walls
• http//www.seashell.com/kbphotos.htm
• http//www.vcrlter.virginia.edu/bph/NSFTalk/NSFTa
  lk.html

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References

• Davis, Richard, 1994, The Evolving Coast W.H.
  Freeman and Company, New York, pp 166-183.
• Fox, William, 1983. At the Seas Edge,
  Prentice-Hall, Inc., pp. 151-175.
• Pilkey, Orrin, 1990. The Beaches are moving
  (videorecording).
• Schwartz, Maurice, 1973, Benchmark Papers in
  Geology Barrier Islands Dowden, Hutchinson and
  Ross, Inc., Stroudsburg, Pa.