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In coastal engineering, a tetrapod is a four-legged concrete structure used as armour unit on breakwaters. 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 photos.
The unit was originally developed in 1950 by Laboratoire Dauphinois d'Hydraulique in Grenoble, France (now Sogreah ). They are no longer protected by a patent, and are widely used all over the world, produced by many contractors.
The Tetrapod inspired many similar concrete structures for use in breakwaters, including the Modified Cube (U.S., 1959), the Stabit (U.K., 1961), the Akmon (Netherlands, 1962), the Dolos (South Africa, 1963), the Seabee (Australia, 1978), the Accropode (France, 1981), the Hollow Cube (Germany, 1991), the A-jack (U.S., 1998), and the Xbloc (Netherlands, 2001), among others. In Japan, the word Tetrapod is often used as a generic name for wave-dissipating blocks including other types and shapes.
Tetrapods have been shown to offer little advantage compared to other concrete armour units. Layering and the amount of tetrapods have shown to have no appreciable effect on the stability of the tetrapod structures although stability increases with the size of the tetrapod pad.
Tetrapods have also been criticized for causing more damage than they prevent because they alter ocean currents and disrupt the natural cycles of erosion and deposition that form and reshape coasts. Concrete coastal installations can also be lethally dangerous to swimmers and surfers, as well to shipping and recreational boaters.
New shore protection innovations have shown that there are better methods that are less obtrusive and more environmentally friendly than concrete armor structures. Alternate placements of sand and mix sediments and modifications of incident wave conditions through the use of reefs are examples of those.
See also 
- Artificial reef
- Breakwater (structure)
- Coastal management
- Coastal erosion
- Ocean surface wave
- al.], P.F. Lagasse ... [et (2007). Countermeasures to protect bridge piers from scour. Washington, D.C.: Transportation Research Board. ISBN 0-309-09909-9.
- Hesse, Stephen. "The Japan Times environment columnist". DON'T YOU JUST LOVE 'EM . . . TETRAPODS. The Japan Times. Retrieved 13 May 2011.
- Zimmerman, ed. by Claus (2005). Environmentally friendly coastal protection : [proceedings of the NATO Advanced Research Workshop on Environmentally Friendly Coastal Protection Structures, Varna, Bulgaria, 25-27 May 2004] ([Online-Ausg.] ed.). Dordrecht: Springer. ISBN 978-1-4020-3299-8.
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