Geoscience Reference
In-Depth Information
The erosional response model of sea-level
rise is one that envisages maintenance of the
nearshore profile by shoreface accretion at the
expense of beachface and berm/dune erosion as
the beach profile moves upward and landward.
This simple model envisages a layer of sediment
being preserved on the shoreface as sea-level
changes. A numerical form of this response was
optimistically termed the Bruun 'Rule' (after the
engineer who invented it) by Schwartz (1967),
although subsequent studies have shown that,
in addition to the lack of field support, it is a
flawed concept because of the very restricted
conditions in which it is likely to operate and
the limited range of processes it considers (List
et al. 1997; Pilkey & Cooper 2004).
The rollover response takes place on low
barriers where waves overtop the berm, thereby
eroding and transporting beachface sediment
to the back-barrier area. In a sediment-limited
environment this causes the barrier to migrate
landward. The migration rates of such systems
are dependent largely on the slope of the under-
lying surface over which they migrate.
In situ
drowning is a phenomenon of barriers that
respond more slowly to dynamic forcing than
the rate of sea-level rise. Thus the system does
not reach equilibrium with a change in sea-level
before it is stranded below the level of effec-
tive coastal processes. Examples cited include
gravel barriers, which have a response time that
is slower than sand barriers and are thus left
stranded on the shelf as sea-level rises (Carter
& Orford 1993). Diagenesis (beachrock and
aeolianite formation) may fulfil a similar func-
tion and several drowned shorelines on the South
African and Australian coasts are preserved as
beachrock/aeolianite ridges (Cooper 1991).
Studies of stratigraphical successions that have
resulted from sea-level rise have identified dif-
ferent modes of preservation depending on the
rate of sea-level rise and the supply of sedi-
ment (Belknap et al. 2002; Thom 1984). At the
millennial scale, the balance between the rate
of sea-level rise and sediment supply are the
dominant controls on coastal evolution. At these
time-scales, variations in wave dynamics are
considered to be masked by the other processes
(interestingly, the reverse is true of shorter term
observations).
The response of coastal dunes to changing
sea-level has received comparatively little atten-
tion other than their consideration as part of
the beach profile. Dunes are, however, subject
to a range of distinctive processes that may vary
as sea-level changes. Carter (1991) presented a
set of models of potential dune response to sea-
level change that incorporated consideration
of sediment supply and vegetation cover, two
factors that are critical in dune sedimentology
(Fig. 8.19).
Changes of beach planform in response to
sea-level changes have been much less studied,
and are more difficult to relate to sea-level changes
as opposed to temporal changes in shoreline
dynamics. In the case of gravel barriers, Carter
et al. (1987) identified a number of idealized
models of planform response mediated largely
by sediment supply and antecedent topography.
The lack of equivalent studies in sandy environ-
ments may reflect the more complex dynamics
of those systems. Similarly, the direct relation-
ship tentatively identified by Orford et al. (1995)
between gravel barrier retreat and sea-level
rise is probably masked in sand systems by the
seaward return mechanisms and the range of
processes operating in the more dynamically
responsive sand systems.
Relative sea-level is falling on some temperate
coasts as a result of isostatic uplift, particularly
in paraglacial areas, and also as a result of tecton-
ism. Coastal sedimentary responses to falling
sea-level are less well studied than rising sea-
level. Typical landforms of regressive coasts are
beach-ridge plains. These lines of shore-parallel
beach-sand ridges are stranded as sea-level falls,
isolating earlier formed ridges from the sand
source. Several such plains have been attributed
to falling relative sea-level (Dominguez et al.
1987; Orford et al. 2003). A series of prograding
beach ridges in north-east Ireland were deposited
during a fall in sea-level between 6000 and 2000
yr BP (Orford et al. 2003). Firth et al. (1995)
described a series of beaches and spits deposited
in an embayment in eastern Scotland during fall-
ing sea-level as shelf sand was carried onshore
