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wave energy dissipation takes place in the surf
zone, this is a zone of high sediment transport
potential.
The processes that affect coastal sand dunes
are quite distinctive as they rely not upon wave
action, but wind. The presence of coastal dunes
requires three criteria to be met. First, there
must be a supply of sand. This is usually the
adjacent beach and beach supply is enhanced
if the sand is dry and transport is not impeded
by coarse fractions. Second, the wind velocity
must be sufficiently strong to entrain grains and
must have a significant onshore component.
The third requirement is an interruption in
the airflow such that velocities drop and sand
is deposited. This is commonly provided by a
physical obstacle on the beach, but may also be
related to topographic features or vegetation,
which cause airflow to diverge. Potential con-
straints on aeolian transport on beaches include
fetch distance, moisture, diagenesis (e.g. salcretes),
surface roughness and armouring (Sherman &
Bauer 1993).
beaches are gentler in gradient. In general, sand
beaches assume lower gradients (usually
10°)
with higher wave energy; the reduced gradient
is associated with dissipation of wave energy.
Paradoxically, this may result in lower residual
energy at the coast than on steeper sand beaches
associated with lower wave energy. In the latter
case, residual energy is reflected from the beach.
The terms dissipative and reflective have thus
been used to describe sand beaches in a number
of morphodynamic classifications. Beaches with
mixed grain sizes tend to show some degree of
spatial segregation of the grains. Typically this
takes the form of a high tide, steep beach facet
composed of coarse material, fronted by a low-
gradient sand 'apron'. Such beaches comprise
both dissipative and reflective elements.
Beaches show a range of features (Fig. 8.8)
that are developed to greater or lesser extents
depending on several factors. The beachface is
the zone in which waves are altered as they
approach the shoreline. The berm is the limit to
which swash action typically extends, and is an
aggradational feature. The back-beach typically
slopes gently landward and is activated by over-
wash processes. It also acts as the source zone
for dune sands. The beach may merge into a
dune system in which ephemeral dunes give way
to a foredune ridge that in turn may be backed
by more stable dunes.
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8.3.4 Coastal morphology
Just as sediment transport is commonly con-
sidered in longshore and cross-shore contexts,
the same is true of beach morphology. Below,
the morphology of beaches is considered first in
a cross-shore (profile) context and second from
an alongshore (planform) perspective. In reality
the two are closely related in the fully three-
dimensional beach morphology.
8.3.4.2 Coastal planforms
The processes of sedimentation outlined above
have high potential to sort and arrange indi-
vidual clasts into distinctive landforms. In most
circumstances, there is a limited sediment sup-
ply, and in all cases, a topographic control on
the distribution of sediments along the coast.
Thus coastal sedimentary systems are discon-
tinuous and form discrete systems. Although
some sedimentary systems are physiographic-
ally distinct from a planform perspective (e.g.
headland-embayment cell), others on linear
clastic coasts are more difficult to compart-
mentalize. The concept of coastal cells enables
identification of semi-closed zones within which
material fluxes may be quantified (even if in
8.3.4.1 Beach profile morphology
The cross-shore geometry of a beach is strongly
influenced by its constituent grains. Boulder
beaches (clast diameter
0.25 m) have low
gradients (typically 6 -14°). This is lower than
the natural angle of repose and results from
the fact that they are flattened by extreme
storms and there is no mechanism to rebuild
them. Gravel beaches (clast diameter
>
>
2 mm)
in contrast tend to be steep (
15°) because the
large pore space promotes infiltration of water
rather than surface backwash. In contrast sand
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