|Introduction||How Tetrapods work||Brief History|
|Use in Japan||Criticism||End Note|
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”.
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.
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.
(Photographs below show 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).
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.
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:
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 on “Hard Stabilization” & “Soft Stabilization” to know more about these techniques)
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.
from standard technical literature & various websites including: