Geology Reference
In-Depth Information
Others form in high-energy coasts with large tidal ranges
and powerful waves. The trailing-edge coasts of conti-
nents (passive margins) and coasts facing marginal seas
appear to favour the growth of large deltas.
Some deltas are triangular in plan, like the Greek let-
ter delta,
of rivers, by building protective structures, and indirectly
by setting in train climatic processes that lead to sea-level
rise. Two important issues focus around beach erosion
and beach nourishment and the effect of rising sea levels
over the next century.
, after which they were named almost 2,500
years ago by Herodotus. But deltas come in a multi-
plicity of forms, their precise shape depending upon the
ability of waves to rework and redistribute the incom-
ing rush of river-borne sediment. Six basic types are
recognized (Box 13.3).
Beach erosion and beach nourishment
To combat
beach erosion
, especially where it threatens
to undermine and ruin roads and buildings, humans have
often built
sea walls
. The idea is that a sea wall will stop
waves attacking the eroding coast, commonly a retreating
cliff, and undermining a slumping bluff or a truncated
dune. Sea walls often start as banks of earth, but once
these are damaged they are usually replaced by stone
or concrete constructions. Other options are boulder
ramparts (also called revetments or riprap) and artificial
structures such as tetrapods, which are made of reinforced
concrete. Solid sea walls, and even boulder barriers and
other artificial structures, are effective and reflect break-
ing waves seawards, leading to a backwash that scours the
beach of material. Such is the demand for countermea-
sures against coastal erosion that the world's coastline is
littered with a battery of artificial structures. Some struc-
tures are successful, but the unsuccessful ones stand in
ruins. Some have helped to maintain beaches, but others,
by promoting eroding backwash, simply worsen beach
erosion.
In an effort to prevent beach loss, the dumping of sand
or gravel on the shore has become a common practice,
mainly in the USA, Western Europe, and Australia. Such
beach nourishment aims to create a beach formation,
depleted by erosion, that 'will protect the coastline and
persist in the face of wave action' (Bird 2000, 160). Many
beach nourishment
programmes were implemented at
seaside resorts, where beaches are needed for recreational
use (Box 13.4). Recently, the value of a beach in absorbing
wave energy has been realized, and nourishment beaches
are sometimes used to defend against further cliff erosion
or damage to coastal roads and buildings. The key to a
successful beach nourishment programme is a thorough
comprehension of coastal geomorphology. Before devel-
oping and implementing a programme, it is necessary
to find out the movement of sand and gravel in rela-
tion to the wave regimes and the effects of any artificial
Coral reefs and atolls
A
coral reef
is a ridge or mound built of the skeletal
remains of generations of coral animals, upon which
grow living coral polyps. Reefs typically grow in shal-
low, clear waters of tropical oceans. The Great Barrier
Reef, in the Coral Sea off the north-east Australian coast
is, at over 2,600 km long, the world's largest living reef,
and indeed the largest living organic features. It com-
prises more than 3,000 individual reefs and hundreds of
small coral islands, ranging in size from about 10 ha to
10,000 ha, formed along the edge of the continental shelf.
An
atoll
is a ring of coral reef and small sandy
islands that encircles a shallow lagoon. Atolls are common
in the tropical Pacific Ocean, where such groups of islands
as the Marshall Islands and Kiribati are chains of atolls.
They form when volcanic islands move away from the
heating anomaly that creates them, as in the Hawaiian
island hot-spot trace (p. 107), and they begin to subside
beneath sea level. Reefs initially form as a fringe in the
shallow waters around a volcanic island, as in Tahiti. With
time, the island erodes and subsides. However, the reef
continues growing upwards to create an offshore barrier
reef separated from the main island by a lagoon, as in the
case of Bora Bora in the Society Islands. The lifting of
a reef above sea level creates a
raised atoll
. These often
have spectacular cave landscapes.
HUMANS AND COASTS
Humans affect erosion and deposition along coasts. They
do so through increasing or decreasing the sediment load
