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shorter period. These beaches tend to permit
propagation of incident wave energy closer to
the shore and thus waves break closer to the
shoreline with more residual energy than on dis-
sipative beaches. Excess energy is reflected from
the beach face, which tends to be steep. Waves
are often not fully refracted at the shoreline and
are thus more likely to generate secondary wave
motions and currents. The presence of beach
cusps, and high longshore sediment transport
rates are physical manifestations of such condi-
tions. Intermediate beaches between these two
extremes display both dissipative and reflective
elements. On such beaches, a low-angle dissip-
ative facet is backed by a steep, reflective facet,
and wave energy is accommodated by a com-
bination of dissipation and reflection.
Empirical data have been used to relate beach
state to various morphodynamic indices includ-
ing Dean's parameter (combining grain size,
wave height and period) and surf scaling para-
meter (combining nearshore slope, wave height
and period) (Wright & Short 1984; Masselink
& Short 1993). These beach state models provide
a set of 'expectation criteria' for beach morpho-
logy in a given dynamic setting. A number of
authors (e.g. Hegge et al. 1996) have questioned
the universal applicability of beach state models
and it is likely that factors such as sediment
supply, underlying topographic control and vari-
ations in wave climate may well create a wider
variety of beach states than is accommodated
within existing models.
Within the beach state approach, temporal
changes in factors such as wave height are
matched by corresponding changes in beach
morphology that take place in a predictable
sequence. These changes are largely described
by variations in profile shape, driven by cross-
shore sediment transport. With decreasing wave
energy, changes take the form of bar migration
onshore and eventual welding onto the beach-
face. Increasing wave energy leads to a lowering
of beach gradient and bar formation on the
shoreface. Beaches with strongly seasonal wave
climate variation may traverse the full range
of beach states (Shaw 1985), whereas others
remain within the same state perennially. There
are many beaches that do not appear to fit the
beach state models and the role of sediment
transport thresholds remains a constraint on
such models, whereby beach morphology might
reflect only higher energy events.
8.4.2 Planform variability
The planform morphology of beaches and
barriers in relation to sediment supply and
accommodation space has been considered in
the previous section. Some aspects of the gross
morphology of beaches and barriers relate to
spatial and temporal variations in dynamics. The
most commonly recognized control is the relative
importance of wave and tidal processes. This was
identified in a regional study of the eastern USA
(Hayes 1979; Davis & Hayes 1984). In terms
of barriers and beaches these studies identified
wave-dominated and mixed wave- and tide-
dominated systems. Wave-dominated barrier
islands tended to have relatively few tidal inlets
(a result of small tidal prisms), small ebb deltas
(a result of wave reworking) and well-developed
flood deltas. Mixed-energy coasts in contrast
tended to have more frequent inlets (larger tidal
prisms requiring more inlets for tidal exchange),
larger ebb deltas (stronger tidal currents) and
more extensive marshes (a consequence of flood-
tidal current deposition of fine sediments). Such
a division has not been recognized on gravel
barriers, perhaps because of their latitudinally
restricted distribution.
Beaches and barriers are subject to many
additional changes in their overall planform.
Typically these changes are more gradual than
the types of change envisaged in beach state
models. Changes related to sea-level rise are
discussed below. Here we are concerned with
changes that may be effected independently
of sea-level change. Beach planform adjusts,
as do profiles, to minimize variations in wave
energy. The planform adjustment is markedly
constrained by the topographic setting of the
beach, and the sediment supply. Abundant sedi-
ment supply leads to progradation (Fig. 8.13).
Such progradation involves successive welding
of bars, which provide sediment supply for
