• Cliff-top dunes: climbing dunes that have remained at
the top of a cliff due to erosion, usually disconnected
from the original source of sand. Falling dunes are clifftop dunes that are provided with excess of sand supply
so that the dunes keep moving down the side of the
cliff lee.
• Echo dunes: elongated mounds with a more or less linear ridge, generated near a subvertical topographic barrier and parallel to this barrier.
• Wrap-around dunes: dunes that surround the barrier
and their shape is adapted to the obstacle. Nebkha – or
nabkha, also named hummock or coppice dune – is a
particular type formed in and around clumps of grass,
herbs, bushes, etc. (Figure 2).
Other classifications are based on biological, ecological, and morphological traits, where different dune areas
are related with different habitats colonized by characteristic plant species in each case, which often coincide with
particular geomorphological situations. Corresponding to
different stages of dune maturity and/or evolution, white
(carbonate rich sands due to shell content) and yellow
(siliceous sand) dunes are referred to mobile dunes with
scarce or null vegetation cover, where sand surface is
clearly visible, while grey dunes are referred to dunes that
are semistabilized and stabilized (fixed) with vegetation
(mainly grass), where a major organic matter content in
the soil would provide a darker color to the sands
(Sanjaume et al., 2011). On the other hand, the dune classification carried out by Cooper (1967) is based on factors
driving the dune genetics and development, and it can be
sometimes related with the dune conservation status and
vulnerability. It groups dunes according to factors such
as the sediment exchange with the beach: primary dunes
derive directly from the beach sand transported by the
wind and maintain an active sediment exchange with the
beach while secondary dunes are derived from the erosion
of primary dunes.
Dune formation
The origination and development of coastal dunes are
determined by a complex interaction of factors, the main
ones being wind velocity and direction, sand supply availability, influence from marine processes, moisture content, vegetation, and topography of the beach and its
adjacent continental zone (Ranwell, 1972; Klijn, 1990;
Hesp, 2002; Sanjaume et al., 2011). Coastal dunes are
originated when the amount of sand that reaches the beach
from the sea is greater than the amount that the beach
loses, for instance, by erosion during storms, and this sand
surplus is transported inland by the wind toward a dry
zone of the beach, out of the reach of normal waves and
tides (Olson, 1997).
Once the sand is deposited in a wide and dry enough
area of the beach, a selection of grain sizes occurs in relation to the wind intensity and grain traits. Four types of
Aeolian transport of particles take place: very coarse sand
(1–2 mm diameter) and granules (2–4 mm) are rolled or
slid forward (traction or creep) and generally remain in
the beach and absorb part of the wind transport energy.
On the other hand, very fine grains (silt and clay) that have
not been washed away by waves during the transport of
beach sediments tend to be carried upward in turbulent
eddies (suspension) and finally winnowed out of the
beach and dune. As a result, middle size-range grains
consisting mostly of fine to coarse sand, ranging, respectively, from 0.125 to 0.25 mm (fine sand), 0.25–0.5 mm
(medium sand), and 0.5–1 mm diameter (coarse sand)
remain to the formation and development of the dunes.
These sand grains are transported by wind by saltation
or by reptation. During saltation, the sand grains are lifted
up into the air and describe a parabolic trajectory for a
short distance before falling back to the surface. During
reptation, particles on the ground may set in motion at
wind velocities lower than those required to move them
by wind alone, due to collisions with grains that are
already in motion by saltation (Bagnold, 1941; Olson,
1997). This transport by wind takes place almost entirely
within 0.5 m of the ground surface, with nearly 90 % of
this movement within 2.5 cm of the surface (Bagnold,
1941).
Dune formation begins with the deposition of sand
transported by the onshore winds, to the lee of the roughness elements by reverse flows. It is therefore necessary to
set some obstacles in the beach surface, natural or artificial, such as vegetation, pebbles and stones, shells, marine
debris, or some irregularities existing out of the reach of
waves and tides (Ranwell, 1972; Olson, 1997). More usually, tidal litter initiates sand accumulation at the top of the
backshore. This is firstly colonized by annual plant species and subsequently by perennial grasses capable of
growing up through sand accretion, forming shadow
dunes, mounds, and nebkhas. Although other inert obstacles are capable of producing sand deposition, these incipient dunes are not able to grow once the obstacle is buried.
The development of embryo dunes and their transformation from ephemeral to permanent features is therefore
favored by vegetation establishment and succession
(Ranwell, 1972; Hesp, 2002). Scattered embryo dunes
may coalesce to form a continuous band parallel with the
strandline and would grow upward and in width to form
a foredune. In situations of high availability of sand supply, the resulting morphology is consistent with a series
of parallel foredunes of increased age from seaward to
landward, separated by interdune depressions or slacks
(Figure 3). Otherwise, a foredune regression and a dune
topography dominated by washovers would occur.
Rates of beach-dune sediment exchange (particularly
the balance between the beach and frontal dune sediment
budgets) strongly influence the frontal dune development
and morphology. In this regard, Hesp (1988) differed
5 foredune types (Fa – Fe), corresponding to gradual situations associated to the loss of vegetation cover and
foredune disintegration. Under conditions of positive balance of beach sediment supply, the formation of the
coastal dune continues until a reduction in sand supply
198
DUNES
the top of a cliff due to erosion, usually disconnected
from the original source of sand. Falling dunes are clifftop dunes that are provided with excess of sand supply
so that the dunes keep moving down the side of the
cliff lee.
• Echo dunes: elongated mounds with a more or less linear ridge, generated near a subvertical topographic barrier and parallel to this barrier.
• Wrap-around dunes: dunes that surround the barrier
and their shape is adapted to the obstacle. Nebkha – or
nabkha, also named hummock or coppice dune – is a
particular type formed in and around clumps of grass,
herbs, bushes, etc. (Figure 2).
Other classifications are based on biological, ecological, and morphological traits, where different dune areas
are related with different habitats colonized by characteristic plant species in each case, which often coincide with
particular geomorphological situations. Corresponding to
different stages of dune maturity and/or evolution, white
(carbonate rich sands due to shell content) and yellow
(siliceous sand) dunes are referred to mobile dunes with
scarce or null vegetation cover, where sand surface is
clearly visible, while grey dunes are referred to dunes that
are semistabilized and stabilized (fixed) with vegetation
(mainly grass), where a major organic matter content in
the soil would provide a darker color to the sands
(Sanjaume et al., 2011). On the other hand, the dune classification carried out by Cooper (1967) is based on factors
driving the dune genetics and development, and it can be
sometimes related with the dune conservation status and
vulnerability. It groups dunes according to factors such
as the sediment exchange with the beach: primary dunes
derive directly from the beach sand transported by the
wind and maintain an active sediment exchange with the
beach while secondary dunes are derived from the erosion
of primary dunes.
Dune formation
The origination and development of coastal dunes are
determined by a complex interaction of factors, the main
ones being wind velocity and direction, sand supply availability, influence from marine processes, moisture content, vegetation, and topography of the beach and its
adjacent continental zone (Ranwell, 1972; Klijn, 1990;
Hesp, 2002; Sanjaume et al., 2011). Coastal dunes are
originated when the amount of sand that reaches the beach
from the sea is greater than the amount that the beach
loses, for instance, by erosion during storms, and this sand
surplus is transported inland by the wind toward a dry
zone of the beach, out of the reach of normal waves and
tides (Olson, 1997).
Once the sand is deposited in a wide and dry enough
area of the beach, a selection of grain sizes occurs in relation to the wind intensity and grain traits. Four types of
Aeolian transport of particles take place: very coarse sand
(1–2 mm diameter) and granules (2–4 mm) are rolled or
slid forward (traction or creep) and generally remain in
the beach and absorb part of the wind transport energy.
On the other hand, very fine grains (silt and clay) that have
not been washed away by waves during the transport of
beach sediments tend to be carried upward in turbulent
eddies (suspension) and finally winnowed out of the
beach and dune. As a result, middle size-range grains
consisting mostly of fine to coarse sand, ranging, respectively, from 0.125 to 0.25 mm (fine sand), 0.25–0.5 mm
(medium sand), and 0.5–1 mm diameter (coarse sand)
remain to the formation and development of the dunes.
These sand grains are transported by wind by saltation
or by reptation. During saltation, the sand grains are lifted
up into the air and describe a parabolic trajectory for a
short distance before falling back to the surface. During
reptation, particles on the ground may set in motion at
wind velocities lower than those required to move them
by wind alone, due to collisions with grains that are
already in motion by saltation (Bagnold, 1941; Olson,
1997). This transport by wind takes place almost entirely
within 0.5 m of the ground surface, with nearly 90 % of
this movement within 2.5 cm of the surface (Bagnold,
1941).
Dune formation begins with the deposition of sand
transported by the onshore winds, to the lee of the roughness elements by reverse flows. It is therefore necessary to
set some obstacles in the beach surface, natural or artificial, such as vegetation, pebbles and stones, shells, marine
debris, or some irregularities existing out of the reach of
waves and tides (Ranwell, 1972; Olson, 1997). More usually, tidal litter initiates sand accumulation at the top of the
backshore. This is firstly colonized by annual plant species and subsequently by perennial grasses capable of
growing up through sand accretion, forming shadow
dunes, mounds, and nebkhas. Although other inert obstacles are capable of producing sand deposition, these incipient dunes are not able to grow once the obstacle is buried.
The development of embryo dunes and their transformation from ephemeral to permanent features is therefore
favored by vegetation establishment and succession
(Ranwell, 1972; Hesp, 2002). Scattered embryo dunes
may coalesce to form a continuous band parallel with the
strandline and would grow upward and in width to form
a foredune. In situations of high availability of sand supply, the resulting morphology is consistent with a series
of parallel foredunes of increased age from seaward to
landward, separated by interdune depressions or slacks
(Figure 3). Otherwise, a foredune regression and a dune
topography dominated by washovers would occur.
Rates of beach-dune sediment exchange (particularly
the balance between the beach and frontal dune sediment
budgets) strongly influence the frontal dune development
and morphology. In this regard, Hesp (1988) differed
5 foredune types (Fa – Fe), corresponding to gradual situations associated to the loss of vegetation cover and
foredune disintegration. Under conditions of positive balance of beach sediment supply, the formation of the
coastal dune continues until a reduction in sand supply
198
DUNES