Understanding
coastal erosion problems in T&T
Trinidad Express
Port Spain
Petroleumworldtt.com
06 24 07
Trinidad and Tobago sits on the edge of the South
American Plate and on a broad boundary zone that
is continuous around the Caribbean Plate to the
north. These
plates are moving in opposite directions. The boundary zone is seismically
active, with active volcanoes in regions of subduction and with the younger
strata much contorted with anticlines and synclines.
In Trinidad there are two major faults running
more or less east and west, and numerous minor
ones. El Pilar is in the north and the Los Bajos
in the south. The shallow strata are petroleum
and gas bearing, with many mud eruptions both on
land and sea in the southern half of Trinidad.
Trinidad and Tobago also sits facing the oncoming
South Equatorial Current from the southeast that
includes also outflow from the South American rivers.
As this major current approaches it splits into
three streams with the southernmost entering the
Columbus Channel where it streams westward at between
one and two knots and enters the Gulf of Paria.
There it forms a permanent circular clockwise
current or gyre, with a stream exiting through
the Bocas into the Caribbean Sea as the Antillean
Current. Trinidad and Tobago is surrounded by sea
and therefore vulnerable to changes in sea level-and
to any other natural phenomena such as ocean currents
and storms, and subsidence or uplift, and are you
really surprised-coastal erosion.
The world is now in an interglacial period. Toward
the end of the last glacial period over eleven
millennia ago much water was locked in ice sheets
in places over a mile thick that covered much of
the northern hemisphere and sea levels were substantially
below present ones. Some authorities suggest that
at the height of glaciation sea levels may have
been over 100 metres below present levels. If you
look at the bathymetry of the nearshore areas,
there would have been dry land between Trinidad
and the mainland and between Tobago and Trinidad,
that land now inundated as sea levels rose. The
initial rate of rise has been assumed to be much
faster than at present.
Erosion of land is a widespread phenomenon and
indeed without it and weathering of rock there
would be no alluvial soils and agriculture. The
more common agents are liquid moving water, or
ice, or wind applying force to the interface with
land. When water, even a raindrop, falls on dry
soil the process of erosion may commence with the
displacement of soil particles.
A river in flood will dislodge soil from its banks.
Waves breaking on ashore will move particles of
sand. Wind may lift sand particles into the air
and deposit them elsewhere. The essential point
to note is that in erosion energy is applied to
the interface and in the process of dissipation
of this energy material is removed from the interface.
Coastal erosion is present wherever moving water
applied to coastal land. What determines the rate
of erosion is the level of energy applied, the
content of the water, the bathymetry of the adjacent
sea, the angle at which wave trains, or currents,
is applied to the land, the geological nature of
the coastal materials, regional geology, boring
organisms and coastal vegetation. A wave train
moving over the years toward a beach contained
by two long headlands, will build a relatively
stable beach, subject mainly to sand movements
to and from the beach.
A similar wave train, or continuous current, applied
along a beach at the foot of a cliff will move
the sand, including shingles, cobbles and even
boulders along in the direction of the train or
current. All around the coasts of Trinidad and
Tobago one will see examples of a coastal erosion
of one kind or another, the nature and extent of
which is determined by local factors.
In Trinidad one can see a wide range of examples
from wave, current and bioerosion around the
limestone offshore islands, to differential erosion
of coastal formations, to some spectacular examples
of beach erosion and accretion. As the energy
levels are highest on the exposed coasts coastal
formations there tend to be rocky shores and
beaches. Perhaps the most striking example to
be seen is the south coast of Trinidad.
The westerly flowing current, plus the prevailing
wind, assisting it by wave generation, is constantly
eating away at the cliff bases and transporting
the eroded material along with it.
On the Venezuelan side of the channel you see
tongues of mangroves curving out the shore into
the stream-a classic case of shoreline regression
on our side and shoreline progradation of the Venezuelan
side. Looked at it another way it is comparable
with a bend in a stream in the Northern Range,
bare rock on the outer side of the bend and sand
on the inner side of the curve.
The oldest strata in the country are to be found
in the Northern Range and in Tobago. The youngest
strata are to be found in the
southern and southwestern region of Trinidad.
The most strikingly visible areas of coastal erosion
may be seen along the south coast between Guayaguayare
and Islote Point and between and San Fernando and
La Brea and between La Brea and Cedros. Much of
the coastal formations consist of unconsolidated
sediments and sandstones.
These formations, even the sandstones, are readily
eroded by moving water, and although giving the
impression of some sort of stability in the presence
of beaches at the feet of cliffs, are in fact quite
unstable and are constantly being eroded. Standing
on the beach at the end of the Chatham Beach Road
and looking east one will see low cliffs, bare
cliffs, and much worn cliff rubble at different
parts of the beach, the red fragments of porcellanite
that could only have originated at Red Cliff far
to the east and carried far to the west of Chatham
by longshore transport in the current.
There are also several areas where there may be
seen very active erosion
of the supralittoral. Noted examples are at Guayaguayare,
Mayaro, southern Cocal, southern Columbus Bay.
The most dramatic is certainly that of Columbus
Bay particularly at Coral Point on Constance Estate.
There are also noted examples of beach or spit
accretion to be seen at northern Cocal, Erin and
spectacularly so at Icacos Point. In the case of
the Icacos peninsula there is considerable archival
evidence suggesting extreme changes near the tip
of the peninsula, starting with Columbus's naming
one particular strip of the coast Punta del Arenal,
or sand point, the Mallet map of 1797 showing an
elongate fractured elevated ridge projecting westwards,
the Cazabon painting of 1854 showing the Los Gallos
formation consisting of eight islets and a sea
arch, the 1957 aerial photography showing six islets
and the 1994 showing only four.
The sandy shore of Columbus Bay has increasingly
been eroded over the past centuries while there
has been pronounced deposition of sand at Icacos
Point, otherwise known as Punta del Arenal. The
Harry Vincent book of about 100 years ago shows
photographs of Guayaguayare Bay with noticeable
erosion of coconuts, not unlike today on the Cocal
or at Columbus Bay. In the early 1950s numerous
timber groynes were erected at Guayaguayare in
a futile attempt to halt erosion.
As suggested earlier the key to understanding
the coastal erosion
problems of the country lie in understanding the
nature of the forces applied. In the case of the
Columbus Bay/Coral Point the cause is probably
the gradual erosion of the Los Gallos formation
that has acted for centuries as a massive groyne.
Now that it is mostly gone the full force of the
Gulf of Paria gyre is being applied longshore in
Columbus bay and transporting sand toward Icacos
point where there is slack water and where the
sand is deposited.
I have framed two working hypotheses, one for
Columbus Bay and one for the southern Cocal, and
communicated these to the Institute of Marine Affairs,
with a proposal for how these could be tested using
current meters. I never did get a response.
And by the way, if there were absolutely no sea
level rise would you expect coastal erosion to
cease?
Or, does anyone think that the EMA can do anything
about the Sahara dust?
Or sea level rise? Or the coastal erosion problem?
Or the vehicle emissions problem? Or the noise
problem? Or reforest a herbaceous swamp to sequester
carbon? Or that Petrotrin, Atlantic LNG or BPtt
will cease adding the greenhouse gas carbon dioxide
to the atmosphere?
Or that the differences in sea levels between
the north and south coasts might not simply be
a standing seiche wave in the Columbus Channel?
And, would you believe that by 2071 no less than
precisely 211.7 hectares of land on a particular
part of the coast will be lost? Wow!
Trinidad
Express
Sunday, June 17th 2007
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