Geography Reference
In-Depth Information
reflected back from the shore and trapped inside
the breaker zone. At the other extreme,
'dissipative' beaches exhibit low-angle, wide, finer-
grained surf zones that strongly attenuate
incoming wave energy (Wright and Short 1984;
Masselink and Short 1993). Examination of long
time-series has shown that these end-member
states are quite rare and unstable and that most
moderate- to high-energy coasts alternate
between the four intermediate domains (Wright
et al
. 1985; and see Sonu and James (1973) for
transition matrices and Markov chain modelling
of these processes). Useful though this
morphodynamic approach has proved, it is best
suited to relatively short-term coastal behaviour
and has rarely been extended along long stretches
of coastline. In this regard, the concept of the
coastal or littoral cell, open to energy throughflow
but more closed with respect to sediment transfers,
becomes of particular value (Carter and Woodroffe
1994).
Cell boundaries may be fixed at topographic
limits (such as headlands, estuaries or deltas) or,
more contentiously, be mobile with free
boundaries delimited by shore-parallel wave flux
or littoral drift rates (Carter 1988) on open coasts.
As wave directions change on such shorelines, so
cell structure changes with migrations and
mergings (Figure 8.3;
ibid
.)
.
Over time, cells may
reach a static equilibrium, either the coast being
in all places parallel to the approaching wave
refraction (or 'swash-aligned'), or where longshore
currents are nullified ('drift-aligned') or where
beach sediments are graded in such a way that the
threshold for particle entrainment is never reached
(ibid.)
. However, most cells exhibit patterns of
sediment exchange over time, between beaches
and beach ridge and dune systems, between
estuaries and coasts, and between coastal and
offshore environments. These exchanges may be
formalised through the calculation of sediment
budgets, although this is easier in theory than in
practice; few published budgets appear to balance.
As one moves into larger temporal scales, then
the role of extreme events becomes important:
thus at Moruya Beach, southeast Australia, the
'normal' range of beach response was forced into
Figure 8.3
Variations in coastal cell structures and
boundary positions with varying wave approach,
Magilligan Point, Northern Ireland.
Source:
After Carter 1988.
a quite different state, from which recovery took
six years (Thom and Hall 1991). However, this
work still suggests some form of equilibrium.
Recent research, however, has begun to investigate
the idea that shoreline behaviour may be non-
linear (e.g. Phillips 1992), focused around a set of
ideas under the heading of large-scale coastal
behaviour (LSCB; de Vriend
et al.
1993), applied
to timescales of decades and distances of tens of
kilometres where boundary conditions are set by
geological time (and refer back to Box 8.1). Where
high-quality data sets are available, such as the
Dutch JARKUS bathymetric database, such
approaches really begin to provide a rigorous,
detailed picture of the space-time complexities in
the nearshore zone (Wijnberg andTerwindt 1995)
and, by implication, erosion and deposition
patterns at the shore.
Cells, and sediment movements within and
between cells, provide a useful framework within
