Geoscience Reference
In-Depth Information
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Transgressive
dune barrier
SL
Prograded
barrier
(Strandplain)
Stationary
barrier
Mainland beach
Headland spit
Fig. 8.13
Features of temperate coasts associated
with abundant and scarce sediment. Progradation is
associated with sediment retention in the beach and
shoreface. With strong wind dispersal, a transgressive
dune may form, dispersing sediment landward. Vertical
dune growth may lead to a stationary barrier with
onshore sediment transport. Longshore sediment supply
may enable spit elongation and formation of recurves
as the spit terminus advances. With sediment scarcity,
barriers migrate landward over underlying lagoonal
deposits. These may be exposed intertidally seaward of
the barrier. Mainland-attached beaches may undergo
narrowing and seaward dispersal of sediment.
(After Roy et al. 1994.)
Receded barrier
Lagoonal
sediment
outcrop
Mainland beach
embryo dune and then foredune growth. With
continuing sedimentation the foredune is increas-
ingly isolated from sediment supply, and from
maritime influences, and thus vegetation becomes
increasingly terrestrial in nature and the dune
increasingly stable.
Progradation at the end of a spit may take
place through longshore transport of sediment
and accumulation at the terminus of a sediment
cell and thus spits may extend and eventually
enclose embayments. Successive recurves may
be preserved that mark previous positions of the
spit terminus. Sediment abundance may lead to
closure of tidal inlets if a period exists in which
wave energy is able to seal an inlet against tidal
currents. Such conditions are normally associated
with storms (see below). If an inlet closes, a
residual body of sediment may be left in the tidal
deltas, which, in the exposed ebb delta, is likely
to be redistributed by wave action and retained
in the beach sediment budget. Tidal delta sand
in the sheltered, back-barrier environment may
remain intact or be redistributed by locally gen-
erated lagoonal waves (Cleary et al. 1976).
In areas where the modern coastal sediment
was originally derived from a relict source (such
as the continental shelf, or reworked glacial sedi-
ments), the supply may eventually be exhausted.
Under such conditions coastal behaviour involves
reworking of a finite sediment volume, although
it is inevitable that losses will occur through
leakage either offshore (to the shelf ), alongshore
(to adjacent systems) and/or onshore (to dunes).
In the absence of sediment supply, a beach or
barrier may experience recession and landward
dunes then become new sediment sources as they
are eroded and their sediment is reintroduced
into the beach sediment budget.
As a finite sediment volume adjusts to variations
in wave energy along a beach, reorganization
