April 24, 2024

Engineer-designed artificial reef could protect marine life and reduce storm damage

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An MIT team hopes to strengthen coastlines with “architected” reefs—sustainable offshore structures that are designed to mimic the wave-protection effects of natural reefs while also providing pockets for fish and other marine life. Credit: Massachusetts Institute of Technology

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An MIT team hopes to strengthen coastlines with “architected” reefs—sustainable offshore structures that are designed to mimic the wave-protection effects of natural reefs while also providing pockets for fish and other marine life. Credit: Massachusetts Institute of Technology

The beautiful twisted and jagged reefs that surround the tropical islands serve as a marine refuge and natural protection against stormy seas. But as the effects of climate change bleach and destroy coral reefs around the world, and extreme weather events become more common, coastal communities are increasingly vulnerable to frequent flooding and erosion.

An MIT team now hopes to strengthen coastlines with “architected” reefs—sustainable offshore structures designed to mimic the wave-buffering effects of natural reefs while also providing pockets for fish and other marine life. The study is published in the journal PNAS Nexus.

The team’s reef design revolves around a cylindrical structure surrounded by four rudder-shaped slats. Engineers discovered that when this structure resists a wave, it efficiently breaks the wave into turbulent jets that ultimately dissipate most of the wave’s total energy. The team calculated that the new design could reduce as much wave energy as existing artificial reefs, while using 10 times less material.

The researchers plan to manufacture each cylindrical structure from sustainable cement, which they would mold into a pattern of “voxels” that could be assembled automatically and provide pockets for fish to explore and other marine life to settle.

The cylinders could be connected to form a long, semi-permeable wall, which engineers could erect along the coast, about half a mile offshore. Based on the team’s initial experiments with lab-scale prototypes, the engineered reef could reduce incoming wave energy by more than 95%.

“This would be like a long wavebreaker,” says Michael Triantafyllou, Henry L. and Grace Doherty Professor in Ocean Science and Engineering in the Department of Mechanical Engineering. “If the waves were 6 meters high coming towards this reef structure, they would be less than a meter high on the other side. So this kills the impact of the waves, which could prevent erosion and flooding.”

Triantafyllou’s co-authors at MIT are Edvard Ronglan SM ’23; graduate students Alfonso Parra Rubio, Jose del Auila Ferrandis and Erik Strand; research scientists Patricia Maria Stathatou and Carolina Bastidas; and Professor Neil Gershenfeld, director of the Center for Bits and Atoms; along with Alexis Oliveira Da Silva from the Paris Polytechnic Institute, Dixia Fan from Westlake University and Jeffrey Gair Jr.

Taking advantage of the turbulence

Some regions have already built artificial reefs to protect their coasts from invading storms. These structures are typically sunken ships, retired oil and gas platforms, and even configurations assembled from concrete, metal, tires, and rocks. However, there is variability in the types of artificial reefs currently in existence and no standards for engineering such structures.

Furthermore, the implemented projects tend to have low wave dissipation per unit volume of material used. In other words, an enormous amount of material is needed to break down enough wave energy to adequately protect coastal communities.

Instead, the MIT team looked for ways to design an artificial reef that would efficiently dissipate wave energy with less material, while also providing a refuge for fish living along any vulnerable coastline.

“Remember that natural coral reefs are only found in tropical waters,” says Triantafyllou, director of MIT Sea Grant. “We can’t have these reefs, for example, in Massachusetts. But engineered reefs are not dependent on temperature, so they can be placed in any water, to protect more coastal areas.”

MIT researchers test the wave-breaking performance of two artificial reef structures at the MIT Tow Tank. Credit: Edvard Ronglan and others

The new effort is the result of a collaboration between researchers at MIT Sea Grant, who developed the hydrodynamic design of the reef structure, and researchers at the Center for Bits and Atoms (CBA), who worked to make the structure modular and easy to manufacture in the local. .

The team’s architected reef design arose from two seemingly unrelated problems. CBA researchers were developing ultralight cellular structures for the aerospace industry, while Sea Grant researchers were evaluating the performance of blowout prevention devices in offshore petroleum structures – cylindrical valves used to seal oil and gas wells and prevent leaks.

The team’s tests showed that the structure’s cylindrical arrangement generated a lot of drag. In other words, the structure appeared to be especially efficient at dissipating high-force oil and gas flows. They asked themselves: could the same arrangement dissipate another type of flow, in sea waves?

The researchers began playing with the overall structure in water flow simulations, adjusting its dimensions and adding certain elements to see if and how the waves changed as they collided with each simulated design.

This iterative process ultimately arrived at an optimized geometry: a vertical cylinder flanked by four long slats, each fixed to the cylinder in a way that leaves room for water to flow through the resulting structure. They found that this configuration essentially breaks up any incoming wave energy, causing parts of the wave-induced flow to spiral sideways rather than colliding forward.

“We are taking advantage of this turbulence and these powerful jets to ultimately dissipate wave energy,” says Ferrandis.

Facing storms

Once the researchers identified an ideal wave-dissipating structure, they fabricated a lab-scale version of an engineered reef made from a series of cylindrical structures, which they 3D printed from plastic. Each test cylinder measured about 30 centimeters wide and 1.2 meters high.

They assembled a series of cylinders, each spaced about a foot apart, to form a fence-like structure, which they then lowered into a wave tank at MIT. They then generated waves of various heights and measured them before and after they passed over the engineered reef.

“We saw the waves reduce substantially as the reef destroyed its energy,” says Triantafyllou.

The team also sought to make the structures more porous and easier to fish. They discovered that instead of making each structure from a solid slab of plastic, they could use a more affordable and sustainable type of cement.

“We worked with biologists to test the cement we intend to use, and it is benign for fishing and ready for use,” he adds.

They identified an ideal pattern of “voxels,” or microstructures, into which cement could be molded in order to fabricate the reefs and, at the same time, create pockets where fish could live. This voxel geometry resembles individual egg cartons, stacked end to end, and appears not to affect the overall wave dissipation power of the structure.

“These voxels still maintain a lot of drag while allowing the fish to move inward,” says Ferrandis.

The team is currently manufacturing cement voxel structures and assembling them into a laboratory-scale engineered reef, which will be tested under various wave conditions. They envision that the voxel design could be modular, scalable to any desired size, and easy to transport and install in multiple offshore locations.

“We are now simulating real sea patterns and testing the performance of these models when we eventually have to deploy them,” says Anjali Sinha, an MIT graduate student who recently joined the group.

In the future, the team hopes to work with coastal cities in Massachusetts to test the structures on a pilot scale.

“These test structures would not be small,” emphasizes Triantafyllou. “They would be about a kilometer long and about 5 meters high and would cost somewhere around 6 million dollars per kilometer. So it’s not cheap. But it could prevent billions of dollars in storm damage. And with climate change, protecting coastlines will become a big problem.”

More information:
Edvard Ronglan et al, Architected materials for artificial reefs to increase energy dissipation from storms, PNAS Nexus (2024). DOI: 10.1093/pnasnexus/pgae101

Diary information:
PNAS Nexus

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