Coastal Dunes
•Basic Terminology of Dune Morphology
•Formation
•Restoration and artificial stabilization
•Example Dune Coasts
Wind Deposition Coasts - Dunes
Dune Line
Toe
a. Sand dunes are common features along sandy
coastlines around the world
Crest
b. The only climatic zone lacking extensive coastal
dunes is the frozen Arctic and Antarctic
Blowout
c. Sediment supply is the most crucial factor controlling
growth of dunes; lack sufficient loose sediment
NW Denmark
Dune means a ridge or hill of loose, windblown, or artificially placed earth,
the principal component of which is sand
Toe means the lowest point on a slope of a dune or bluff
Primary dune = most seaward major dune
•seaward limit is the landward limit of its fronting beach
•landward limit of a primary dune is 25 feet landward of its landward toe.
Secondary dune = dune immediately landward of the primary dune
•seaward limit of a secondary dune is the landward limit of its fronting
primary dune
•landward limit of a secondary dune is 25 feet landward of its landward toe
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Stable sand dunes
•buffer against wave and wind damage during storms
Dune Formation and Source of Sand
•protecting the land behind from salt water intrusion
•Backshore most common source of dune sands
•allows the development of more complex plant communities in areas
protected from salt water inundation, sea spray and strong winds.
•Foreshore if it dries between tidal cycles
•reservoir of sand, to replenish and maintain the beach at times of
erosion.
•A flat or low-relief area inland of the coastline is needed to accommodate
the dunes
•predominant onshore or alongshore winds for at least part of the year
-10 mi/hr, or 16 km/hr will put fine sand in motion
-If the wind blows too hard, any existing dunes will actually be
destroyed, as can happen in hurricanes
Plants
act as a windbreak and trap sand particles
ability to grow up through the sand
continually produce new stems and roots as the dune grows
American beach grass = United States
northeast and on the west coast.
Vegetation can be destroyed by:
•storms, cyclones, droughts or fire
•clearing, grazing, vehicles or excessive foot traffic
strong winds may cause 'blowouts' or gaps in the dune ridge
unless repaired, these increase in size
Sea oats = Gulf coast and southeast
Reduced reservoir = erosion of the beach and coastal recession
•tolerant of salt spray and occasional
inundation by salt water
•Growth is stimulated by sand burial
•Regrowth occurs even after rapid
deposition of sand up to 1 m thick
•Plant growth is by seed and by rhizome
extension
Dune Restoration
Blowout
Sand Fences and Vegetation
• Sand fences create areas of lower wind speed both in front of and
behind the fence, which encourages sand deposition
• The amount of sand trapped depends on the fence height, the size
of spaces between fence slats, and the wind speed.
• Wooden slat fences are most commonly used
• Initial dune crest is located by installing a sand fence parallel to the
high tide line
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Dune Restoration
(1) Foredunes. Mounds or ridges directly
by the beach. Serve as storm buffer.
Examples
(2) Parabolic dunes. Arcuate sand ridges
with the concave portion facing the
beach.
(3) Barchan dunes. Crescent-shaped
dunes with the extremities (horns)
extending downwind.
(4) Transverse dune ridges. Ridges
oriented perpendicular or oblique to the
dominant winds.
(5) Longitudinal (seif) dunes. Dune
ridges elongated parallel to the wind
direction and symmetrical in profile.
(7) Attached dunes. Formations of sand
that have accumulated around obstacles
such as rocks
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Lake Michigan = largest dune field associated with a lake
Namibia, largest barrier dune system
•developed over a period of many millions of years
•sand carried into the Atlantic Ocean by the Orange river
•Benguela current subsequently moved it northwards
•surf pushed the sand back onto land
•coastal dunes developed, wind continuously restacks the sands
of the huge shifting dunes
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The impact of horse grazing on the dunes at Assateague Island
Ocean City Inlet
Horse Population
21 in 1965
180 in 2001
Vegetation promotes dune formation
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