Geoscience Reference
In-Depth Information
(a)
(b)
(c)
(d)
Fine
Flat
beach
Coarse
Steep
beach
Wave
crests
Longshore
drift
Fig. 8.11
Cell equilibrium types: (a) swash-aligned equilibrium where wave crests arrive at the shoreline with equal distribution of
energy alongshore; (b) drift-aligned equilibrium where sediment flux equals the potential transport ability of waves; (c) graded
equilibrium where grain size at any point is too great for wave transport; (d) log spiral or zeta bay.
broad terms). Such cells are commonly defined
in terms of the two-dimensional horizontal plane
arrangement of sediments, known as the coastal
planform (Fig. 8.11). The simplified approach
of May & Tanner (1973) in which longshore
gradients in wave energy at the breakpoint
are quantified, provides one approach to the
delimitation of cells for any given wave condi-
tion. A number of equilibrium cell forms have
been identified (Fig. 8.11) (Carter 1988).
In swash-aligned equilibrium, wave energy
is distributed evenly along a shoreline such that
there is no longshore wave energy gradient to
produce sediment transport. In a graded equi-
librium, the beach sediments become sorted in
response to longshore gradients in wave energy.
The phenomenon is best seen on gravel or cobble
beaches and is manifest by alongshore gradients
in clast size such that the clasts at any given point
are too large to be transported by wave energy
at that point. On coasts with dominantly oblique
wave approach, drift-aligned equilibrium may be
attained at any given point if inputs of sediment
from up-drift sources are matched by losses to
downstream sinks. Particular types of equilib-
rium forms (zeta and log-spiral bays) comprise
both swash-aligned and drift-aligned sections
and occur where a headland obstructs longshore
drift incompletely (Woodroffe 2002).
8.4
PROCESSES AND IMPACTS OF NATURAL DISTURBANCE
AND ENVIRONMENTAL CHANGE
8.4.1 Temporal change in temperate beach and
dune morphology
Beaches and dunes are characterized by a wide
range of types, and grain size provides a first-
order discrimination among boulder, gravel and
sand beaches. Although these types form a con-
tinuum there are distinctive modes of behaviour
and response to environmental forcing for each.
Boulder beaches (Oak 1984) are inactive under
most coastal conditions because of the large
clast sizes involved. Extreme (high magnitude,
low frequency) events (e.g. storms and tsunami)
thus create the only circumstances under which
the beach morphology changes. The boulder
beach gradient at any time reflects a combina-
tion of the intensity of the last dynamically
effective storm, as well as the cumulative work
of a series of such storms. An interesting feed-
back relationship thus exists in that the mor-
phodynamic effectiveness of successive storms
of the same magnitude is reduced by the beach
having moved closer to equilibrium with that
magnitude of storm.
Gravel beaches have been shown, largely by
virtue of their porosity, which inhibits backwash,
