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Introduction: One
can’t miss the sight of Tetrapods while taking a walk along
famous marine
drive in Mumbai. For many, these mysterious looking objects are a piece
of art;
while for others these are a symbol of assault on nature. We may love
them or
hate them, but, surely we cant ignore Tetrapods.
What
are Tetrapods?: The
word “Tetrapod" (taken from Greek) means "four-legged"
— hence
in English it means "four-legged animal”. In coastal
engineering, a
Tetrapod is a four-legged concrete structure intended to prevent
coastal
erosion. However, the term is now used to refer to any of the concrete
blocks
that come in a variety of configurations, with three to eight legs.
Use
of Tetrapods to prevent coastal erosion falls under the category of
"Hard
Stabilization" Technique. (Click
Here
to know more about "Hard Stabilization"
& "Soft Stabilization" techniques)
How
Tetrapods work?: The
Tetrapod's shape is designed to dissipate the force of incoming waves
by
allowing water to flow around rather than against it and to reduce
displacement
by allowing a random distribution of Tetrapods to mutually interlock.
Tetrapods
were designed to remain stable under even the most extreme weather and
marine
conditions, and when arranged together in lines or heaps, they create
an
interlocking, porous barrier that dissipates the power of waves and
currents.
Earlier
barrier material
used in breakwaters, such as boulders and conventional concrete blocks,
tended
to become dislodged over time by the force of the ocean constantly
crashing
against them. Tetrapods
and similar structures are often numbered so any
displacement that occurs can be monitored through satellite photographs
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| Placement
of Tetrapods in
breakwater |
A
brief History of Tetrapods: Prior
to World War II, this kind of "coastal breakwater armoring" was
primarily accomplished using rocks and boulders, and sometimes concrete
cubes.
Then, in 1950,
the Laboratoire Dauphinois d'Hydraulique in Grenoble, France
(now known as Sogreah), began making tetrapods, as we now know them,
for
coastal defense. The concept took off and engineering firms worldwide,
inspired
by the Tetrapods, began creating many similar concrete structures for
use in
breakwaters
After
about Eight years of tetrapod making its debut, Americans created
something
called the Tribar
that looks like a huge concrete trivet. These shapes were
followed by the Modified Cube (U.S., 1959), the Stabit (U.K., 1961),
the Akmon
and the Tripod (Netherlands, 1962), the Cob (U.K., 1969), the Dolos
(South
Africa, 1963), the Antifer Cube (France, 1973), the Seabee (Australia,
1978),
the Shed (U.K., 1982), the Accropode (France, 1980), the Haro (Belgium,
1984),
the Hollow Cube (Germany, 1991), the Core-Loc and the A-Jack (U.S.,
1996 and
1998, respectively), the Diahitis (Ireland, 1998) and the Samoa Block
(U.S.,
2002).
In
Japanese, the kanji characters for Tetrapod mean "wave-dissipating
blocks.
One Japanese company, Osaka- and Tokyo-based Fudo Tetra Corporation,
has a line
of 18 different blocks that range in size from half a ton (90-cm high
and 1
meter in width) to 80 tons (5 meters high and 6 meters wide). TETRAPOD is in
fact a registered trademark held by Fudo Tetra, but it is also a
generic term
used, written in lower-case letters, to refer to any of the concrete
blocks
that come in a variety of configurations, with three to eight legs.
Use
of Tetrapods in Japan: Japan
is the most prolific
user of Tetrapods with nearly
50 percent of its 35,000 kilometer coastline
having been covered or somehow altered by Tetrapods and other forms of
concrete. On almost every beach in Japan
you are likely to see endless
piles of tetrapods intended to prevent coastal erosion.
Japan
stretches a total of
2,900 km from north to south, and comprises more than 3,900 islands.
But its
maximum land width is just 320 km, so it's easy to see why every meter
lost
might be a meter lamented.
Tetrapods,
which are supposed to retard beach erosion, are big business in Japan.
Three
different ministries — of Transport, of Agriculture, Forestry
and Fisheries,
and of Construction — annually spend 500 billion yen each,
sprinkling tetrapods
along the coast.
Their
manufacture and dispersal create jobs for Japanese citizens and
contracts for
construction companies. It is estimated that Because of the
proliferation of
Tetrapods, tourists to the Hawaii-like island of Okinawa
often find it difficult to find pristine beaches and unaltered
shoreline,
especially in the southern half of the island.
The
principal Japanese  manufacturer
is Fudo Tetra Corporation. But
Fudo Tera Corporation and others in the coastal-armoring industry do
not usually produce tetrapods themselves. Rather, they lease huge steel molds
(shown in the figure on the right) to clients who pour
concrete into the
molds and cure the blocks on site. This reduces the expenses that would
be
incurred pouring the concrete at one location then shipping the blocks
far away
to another.
Criticism
of use of Tetrapods in checking erosion:
Critics,
argue that in addition
to posing a danger to swimmers, surfers and boaters,
tetrapods actually accelerate beach erosion by disrupting the natural
processes
that shape the coastal environment. Tetrapods alter ocean
currents and disrupt
the natural cycles of erosion and deposition that form and reshape
coasts. It
turns out that wave action on tetrapods wears the sand away faster and
causes
greater erosion than would be the case if the beaches had been left
alone.
Tetrapods
are also often criticized for ruining the traditional coastal scenery.
The
Japanese government has begun work of removing some wave-dissipating
blocks
from many coastal districts to preserve more of Japan's
coastal scenic beauty. This
is being done in pursuit of an utsukushii kuni (beautiful
nation) policy
launched by the Ministry of Land, Infrastructure and Transport .
It
is now agreed that excessive use of “Hard stabilization”
techniques ( like
placement of Tetrapods ) should be avoided & that these should
be used only
in areas where erosion is strictly unacceptable, such as where a
highway,
railway or human settlement is in danger. Elsewhere, soft stabilization (
like
beach nourishment, sand dune stabilization) can be used when money
allows, and
in other areas nature can be left to take its course. (Click
Here
to know more about "Hard Stabilization"
& "Soft Stabilization" techniques)
End
Note: There
are many  who appreciate Tetrapods from
aesthetics point of view. The attraction
of tetrapods lies in their contrast with nature. The material
of a tetrapod is
concrete and its shape is formal, which can't be found in nature. The
tetrapod
is a symbol of artificiality. Setting hundreds of tetrapods on a big
scale that
matches that of nature is simply art. No wonder that many
Tetrapod lovers
use mini-tetrapods as door-stoppers or as accessory-holders.
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