Coastal processes around a Maldivian island
The coastal processes are of
course dependant on the local configuration, but the main
forcings can be identified: oceanic swell, wind waves from
the South-West monsoon and wind waves from the North-East
monsoon.
The oceanic swell is mostly
from the South-East, even though on the western side of the
archipelago it is rather from the west. In this respect, it
is highly influenced by the configuration of the atolls and
the bigger channels that separates them. On the island inside
the atoll it can still have an importance if there is a discontinuity
in the atoll ring, which allows the main swell to be diffracted
inside the atoll. It is usually the most important factor
as the periods are the longest and they carry more energy
and their influence reaches deeper places. In fact, most of
the energy from those waves is dissipated when they crash
on the reef crest. In many places, they are the main forcing
of the current on the reef flat and around the island, thus
determining most of the sediment transport: to prove it, the
sediment found on the island is usually comprised of big chunks
of coral rubble taken of the reef on the side exposed to the
oceanic swell, which are getting transported along the coast
and getting eroded and therefore their size is getting reduced
as we move along the beach. Eventually when we reach the protected
side, we find a nice sandy beach. The oceanic swell probably
has a big influence on the creation of islands as in many
atolls in the Maldives, the side the most exposed to the oceanic
swell has got the most numerous and the longest islands.
The wind waves created by the
South-West monsoon is the second most important factor as
the wind from the south west are usually more important than
the one during the North-East monsoon. They don't carry the
same energy as the oceanic swell, having shorter periods.
Still the atolls are generally large enough to enable those
wind waves to be generated inside the atoll. When they reached
the western and northern sides of the atoll, they are usually
consequent on windy days, and the reefs on the inside of the
atoll are generally a bit lower, these waves comes all the
way to the coast and have a major role in the sediment resuspension
and transport.
The wind waves created by the
wind from the North-West monsoon are the least energetic factor,
even though they are locally important. They often contribute
in increasing the effects of the swell during the North-West
monsoon. As we can see, the coastal processes are highly dynamic.
They can be considered as a system where the oceanic swell
is the principal factor and where the alternation of the monsoon
causes the wind induced waves to either counter the effects
of the swell or increase them. It is therefore not surprising
that a sand bank is often shifting from one side of an island
to the other according to the monsoon.
Problems arising when developing an island
Most islands inhabited
by people or developed for some commercial or industrial purpose
are facing a huge erosion problem on the one hand and a nutrient
input problem on the other hand. These remain two of the major
constraints that should be studied when planning for the development
of an island. Little serious studies have been made and little
litterature is available on the subject in the Maldives, but
people have been learning from some of the mistakes made in
the past but there is a long way before a prediction can be
really reliable.
The first thing that happened
historically is that the massive corals were removed from
the reef to provide material for construction of houses. By
doing this, the islanders were actually removing the natural
barrier constituted by those corals to the oceanic swell.
The waves were then hitting the coast with more energy, creating
erosion. Now coral mining is regulated and prohibited in certain
areas. Still, the problem remains in some place.
The second mistake that was
made was to build in-fill jetties that would hinder sediment
transport. A place that doesn't receive the sediment anymore
but is still exposed to the conditions that was removing it,
is bound to get depleted and the beach to vanish. This also
occurs in some place when a harbor has been built. Several
methods have been tried out to try and counter this adverse
effect.
The first step that was taken
was to construct jetties on pillars to allow for sediment
transport and currents to flow underneath, but some in-fill
jetties have been retained and methods have been implemented
to try and stabilize the coastline. The main ones were groynes,
breakwaters, containing walls or beach replenishment programs.
Some adverse effects to those
structures have been noticed locally. And more effects in
other places where corals and sand were sometimes mined to
provide material for those structures or for the replenishment
programs have never been monitored. In situ, drawbacks are
short term, during the construction phase and long term. The
structures must usually be modified several times before a
correct disposition is adopted when any good result is ever
achieved at all. A good study of the conditions and consequences
of such structures should be undertaken to avoid mistakes
in the future.
As far as the nutrient input
problem is concerned, it has been noticed that often sea grass
beds would appear once population settles on an island, but
little is known about this phenomenon. It is now necessary
for new resorts to have a sewerage treatment plant, but to
date no requirement about the quality of the effluent has
been given.
