Dunes - Getting Started
Dunes provide a buffer to the coastal system - they protect the
regions behind them from wind and waves, and provide a substrate,
or ground, for plants to form communities upon. Three things are
required for dune formation to occur: a large supply of sand, wind
speeds capable of moving it, and an ideal location for its accumulation.
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Sand Supply
 Sand
comes from many locations and environments, and may be derived from
either rock or biotic
sources. It can have a source as close as rockfalls from seacliffs,
or be transported hundreds to thousands of miles by streams and
rivers. A portion is even carried as dust and sand in the air. Once
the sand is has been washed into the ocean longshore drift moves
and collects it in locations - prompting dune formation.
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Sand Type
 The
majority of the sand you'll see in temperate regions is composed
of minerals eroded from bedrock. It is generally a combination of
many mineral types and might look something like the sample at right.
The primary component of rock-derived sand is silica, what both
quartz and glass are made of, and is very resistant to erosion.
It remains after other minerals have been broken down mechanically
and altered chemically by their environments. Another mineral can
also be found in sand - calcium carbonate, or CaCO3, in higher percentages
the closer to the equator the sample comes from. Calcium carbonate
is a biotic by-product - it is grown as a shell or skeleton by marine
organisms suck as molluscs and corals. Many other minerals are also
present in beach sand; among these black sand and green sand are
explored in the links below.
Dig deeper - silica sand
Dig deeper - calcareous
sand
Dig deeper - green sand
Dig deeper - black sand |
Wind
Once sand begins to pile up, ripples and dunes can form. Wind continues
to move sand up to the top of the pile until the pile is so steep
that it collapses under its own weight. The collapsing sand comes
to rest when it reaches just the right steepness to keep the dune
stable. This angle, usually about 30-34°, is called the angle
of repose. Every pile of loose particles has a unique angle
of repose, depending upon the properties of the material it's made
of, such as the grain size and roundness. Ripples grow into dunes
with increase of wind and sand input.
Fort Funston, Golden Gate NRA. |
The repeating cycle of sand inching up the windward side to the
dune crest, then slipping down the dune's slip face allows
the dune to inch forward, migrating in the direction the wind blows.
As you might guess, all of this climbing then slipping leaves its
mark on the internal structure of the dune. The image on the right
shows fossil sand dune structure preserved in the Merced Formation
at Fort Funston, Golden Gate National Recreation Area. The sloping
lines or laminations you see are the preserved slip faces of a migrating
sand dune. This structure is called cross-bedding,
and can be the result of either wind or water currents. The larger
the cross-bedded structure, however, the more likely it is to be
formed by wind, rather than water.
A breeze of 10 mi/hr, or 16 km/hr will put fine sand in motion,
but will not form dunes. Dune formation relies upon the weight and
size of the sand grains in relation to the prevailing, or normal,
wind. If the wind blows too hard, any existing dunes will actually
be destroyed, as can happen in hurricanes. Once the wind drops back
down to normal the dunes start to form again.
Dig deeper - hurricane effects on dune sand |
Sand Trap
 Waves
move the sand provided by longshore drift - the migration of sand
along the coast by tide and wave action. Waves push the sand up
on the beach, and then pull it back out, however the current will
cause the overall movement to be in a certain direction. This can
be seen in the image at left, the waves are breaking towards the
viewer, pushing sand in that direction. When the landscape changes
and the longshore drift is disrupted, beaches and dune fields can
be cut off from their supply of sand, and shrink. For this reason
engineers are very careful of where channels and sea-walls, rip-rap,
and groins are placed.
Dig
deeper - limited sand resources; the loss of beach sand and dunes
Dune Stability
 Because
they are entirely composed of loose, or unconsolidated, sand, dunes
tend to be very fragile, mobile, and susceptible to deterioration
and erosion. They act as a buffer zone, protecting the land behind
them from the force of ocean wind and waves. Plants that colonize
the dunes act to stabilize the dunes and entrain blowing sediment.
These "pioneer" species also act to give other species
a foothold, enabling a progression of species that act to further
stabilize and enlarge the dune field. Animals, humans and severe
weather can all impact negatively on this stabilization. The image
to the above right is an experimental plot showing the impact of
grazing by horses on dune stability at Assateague National Seashore.
For more on this study, click the link below.
Dig deeper - the impact of horse grazing on the dunes at Assateague
National Seashore
Parks with Coastal
Dunes | Parks with Terrestrial
Dunes |
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