Geoscience Reference
In-Depth Information
historical change records, wave modelling,
morphological measurements (e.g. beach width)
and bathymetric changes (sediment budget guide)
to assess the likely future behaviour of the
coast. Future coastal evolution is assessed for
two scenarios: unconstrained (i.e. assuming no
defences or new management practices) and
managed (i.e. assuming present management
practices continue indefinitely). The objective is
to describe the 'behaviour' of a beach system in
its coastal context and thus describe the likely
future 'behaviour' of the same beach.
Sea-level change is a key issue for temperate
coastlines, and is widely associated with problems
of coastal erosion. A global predominance of
rising sea-levels means that this area has received
most attention, however, many temperate coasts
are experiencing sea-level fall. As sea-level simply
mediates the level at which short-term dynamic
processes operate, distinguishing the sea-level-
related signature of coastal change from other
forcing mechanisms discussed above (see sec-
tion 8.4) is not straightforward. Nonetheless a
number of models of coastal response have been
proposed based on field observations, analyses
of historical change and laboratory studies. An
additional source of information lies in strati-
graphical studies (typically spanning several
millennia). A number of idealized modes of
response to sea-level change have been identified.
For a rise in sea-level, simple two-dimensional
shore-normal models (Fig. 8.18) involve either
an erosional response, a rollover response, or
in situ
drowning (overstepping).
8.7
FUTURE ISSUES
Two major issues dominate future concerns
about temperate coastlines - climate change
and sea-level change. In both instances, studies
of coastal response to past changes provide
information on likely future scenarios. As the
human population is now greater than ever, and
is increasingly concentrated in the coastal zone,
options for dealing with future changes in the
shoreline must consider this constraint.
The sedimentary effects of long-term climatic
fluctuation have been analysed in several temper-
ate coastal systems. Two distinct climatic phases
have been noted globally in the late Holocene -
the Holocene Climatic Optimum and the Little
Ice Age. The effects of both have been identified
in coastal dune sequences, where climatic deter-
ioration (cooler and with increased storminess)
is associated with large-scale instability and
development of transgressive sand sheets, and
climatic amelioration is linked to enhanced
vegetation growth and hence stability (Gilbertson
et al. 1996; Wilson et al. 2001). At the decadal
time-scale, variations in climate are character-
ized by ENSO (El Niño-Southern Oscillation)
and the NAO (North Atlantic Oscillation). Both
oscillations have been associated with changes
in coastal behaviour at such time-scales. Goy
et al. (2003) ascribed emplacement of beach
ridges in southern Spain over a 400-year period
to fluctuations in the NAO, with emplacement
during stormy periods separated by periods of
shoreline stability in intervening calm periods.
(a) Erosional
Er
osion
Volume eroded
Sea-level 2
Sea-level 1
Volume deposited
Wave base 2
Wave base 1
(b) Rollover
O
nshore
transport distance
Sea-level 2
Sea-level 1
(c) Overstepping
Sea-level 2
Sea-level 1
Fig. 8.18
Erosional, rollover and overstepping models of
response to sea-level rise. The erosional response (a) involves
seaward dispersal of sediment to raise the nearshore profile in
tandem with sea-level rise. The coastal migration model (b)
envisages retention of a fixed sediment volume that migrates
landward via barrier overwash. With no return mechanism,
the barrier 'rolls' landward to maintain a fixed volume. In the
overstepping mode (c), barriers are unable to respond sufficiently
rapidly to sea-level rise and are drowned. (After Carter 1988.)
