Oceanic Overwash
Beach Diagram
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
Overwash
Oceanic Overwash
Oceanic Overwash and Channels
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
Overwash
Overwash effects in Buxton, NC
Graph of changes after Dennis to
Buxton, NC
Overwash after Hurricane Dennis
Washover After Dennis
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
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
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
Photos of Renourishment at Kure
Beach
Groins
Groins
Jetty
Sea Wall
Effects of Sea Walls
Sea Wall at Shell Island
Sea Wall
Myrtle Beach
Disappearance of Islands
Long-term rates of change between 1855 and 1989 show both accretion and
erosion for barrier island shorelines facing Louisiana’s 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.
Disappearance of Islands
The timeline above shows anticipated years of disappearance for several of Louisiana’s
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
On-line References
• http://books.nap.edu/books/030904806/html/70.ht
ml#pagetop
• http://coastal.er.usgs.gov/hurricanes/dennis/buxton
/
• http://members.nbci.com/igorz/MIWO.html
• http://www.beachbrowser.com/Archives/Environm
ent/A…/Barrier-Islands-Always-changing.ht
• http://www.csc.noaa.gov/products/nchaz/htm/lidto
po.htmhttp://www.lacoast.gov/Programs/CWPPR
A/Watermarks/Fall99/barrier.fa99.htm
References
• http://www.ncsu.edu/coast/shell/migration/migrati
on.html
• http://www.ncsu.edu/coast/shell/terms.html
• http://www.salem.mass.edu/~lhanson/gls214/gls21
4_tec_clas.htm
• http://www.seafriends.org.nz/oceano/beacheng.ht
m#sea%20walls
• http://www.seashell.com/kbphotos.htm
• http://www.vcrlter.virginia.edu/~bph/NSFTalk/NS
FTalk.html
References
• Davis, Richard, 1994, The Evolving Coast: W.H.
Freeman and Company, New York, pp 166-183.
• Fox, William, 1983. At the Sea’s Edge, PrenticeHall, 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.