Geology Reference
In-Depth Information
areas where the land is subsiding, will become widespread
and emerging coastlines will become a rarity. Broadly
speaking, low-lying coastal areas will be extensively sub-
merged and their high- and low-tide lines will advance
landwards, covering the present intertidal zone. On steep,
rocky coasts, high- and low-tide levels will simply rise,
and the coastline stay in the same position. It seems likely
that the sea will continue to rise, with little prospect of sta-
bilization. If it does so, then coastal erosion will accelerate
and become more prevalent as compensating sedimen-
tation tails off. The rising seas will reach, reshape, and
eventually submerge 'raised beaches' created during the
Pleistocene interglacials. Forms similar to those found
around the present world's coasts would not develop until
sea level stabilized, which would presumably occur either
when the measures adopted to counterbalance increasing
greenhouse gases worked, or else when all the world's
glaciers, ice sheets, and snowfields had melted, occa-
sioning a global sea-level rise of more than 60 m (Bird
2000, 276).
Specific effects of rising sea levels on different types of
coast are summarized in Figure 13.21. Cliffs and rocky
shores were largely produced by the tolerably stable sea
levels that have dominated over the last 6,000 years.
Rising sea levels will submerge shore platforms and rocky
shores, allowing larger waves to reach cliffs and bluffs,
so accelerating their erosion on all but the most resis-
tant rocks (Figure 13.21a). Some eastern British cliffs are
retreating about 100 cm a year, and this rate will increase
by 35 cm a year for every 1 mm rise of sea level (Clayton
1989). Cliff notches will enlarge upwards as the rising
sea eats into successively higher levels. Rising sea levels
are also likely to increase the occurrence of coastal land-
slides and produce new and extensive slumps, especially
where rocks dip towards the sea. The slump material will
add to sediment supply for beaches, perhaps in part com-
pensating for the rising sea level. The rise of sea level by
1 to 2 mm per year over the last few decades has caused
beach erosion in many places around the world. Accel-
erating sea-level rise will greatly exacerbate this problem.
The seaward advance of prograding beaches will stop and
erosion set in (Figure 13.21b). Where the beach is nar-
row, with high ground behind it, the beach may rapidly
disappear unless nearby cliff erosion provides enough
replenishment of sediment. Beaches fronting salt marshes
and mangals will probably be eroded and over-washed.
Beaches ahead of sea walls will be eroded or be removed by
the scour resulting from the reflection of incident waves.
Beaches will persist wherever the supply of sand or shin-
gle is sustained, or where additional material is provided
by cliff erosion or increased sediment load from rivers.
Most present beaches will probably be lost as sea levels
rise, but on coastal plains with coastal dunes new beaches
may form by the shoreward drifting of sediment up to the
new coastline, along the contour on which submergence
stops.
Salt marshes, mangals, and intertidal areas will all
be submerged beneath rising sea levels (Figure 13.21c).
Small cliffs on the seaward margins of salt marshes
and mangrove terraces will erode faster than at present.
Continued submergence will see the seaward and land-
ward margins move inland. In low-lying areas, this may
produce new salt marshes or mangals, but steep-rising
hinterlands will cause a narrowing and perhaps even-
tual disappearance of the salt marsh and mangal zone.
The loss of salt marshes and mangals will not occur in
areas where sediment continues to be supplied at a rate
sufficient for a depositional terrace to persist. And mod-
elling suggests the salt marshes of mesotidal estuaries,
such as the Tagus estuary in Portugal, do not appear vul-
nerable to sea-level rise in all but the worst-case scenario
with several industrialized nations not meeting the terms
of the Kyoto Protocol (Simas
et al
. 2001). Inner salt
marsh or mangal edges may expand inland, the net result
being a widening of the aggrading salt-marsh or man-
grove terrace. Intertidal areas - sandflats, mudflats, and
rocky shores - will change as the sea level rises. The outer
fringe of the present intertidal zone will become perma-
nently submerged. As backing salt marshes and mangals
are eroded and coastal lowland edges cut back, they will
be replaced by mudflats or sandflats, and underlying rock
areas will be exposed to form new rocky shores.
Estuaries will generally widen and deepen as sea level
goes up, and may move inland. Coastal lagoons will
become larger and deeper, and their shores and fring-
ing swamp areas suffer erosion (Figure 13.21d). The
enclosing barriers may be eroded and breached to form
new lagoon inlets that, with continued erosion and
