This website uses cookies primarily for visitor analytics. Certain pages will ask you to fill in contact details to receive additional information. On these pages you have the option of having the site log your details for future visits. Indicating you want the site to remember your details will place a cookie on your device. To view our full cookie policy, please click here. You can also view it at any time by going to our Contact Us page.

Artificial coral reef could protect fish from climate change destruction

27 March 2024

An egg box-shaped structure could provide a safe habitat for marine life, and help protect coastal communities, by significantly reducing the effects of extreme weather.

Image: Courtesy of the researchers, edited by MIT News
Image: Courtesy of the researchers, edited by MIT News

The beautiful, gnarled, ‘nooked-and-crannied’ reefs that surround tropical islands serve as a marine refuge and natural buffer against stormy seas. 

But as the effects of climate change bleach and break down coral reefs around the world, and extreme weather events become more common, coastal communities are left increasingly vulnerable to frequent flooding and erosion.

An MIT team is now hoping to fortify coastlines with ‘architected’ reefs – sustainable, offshore structures engineered to mimic the wave-buffering effects of natural reefs, while also providing pockets for fish and other marine life.

The team’s reef design centres on a cylindrical structure surrounded by four rudder-like slats. The engineers found that when this structure stands up against a wave, it efficiently breaks the wave into turbulent jets that ultimately dissipate most of the wave’s total energy. 

The team has calculated that the new design could reduce as much wave energy as existing artificial reefs, using 10 times less material.

The researchers plan to fabricate each cylindrical structure from sustainable cement, which they would mould in a pattern of ‘voxels’ that could be automatically assembled, and would provide pockets for fish to explore and other marine life to settle in. 

The cylinders could be connected to form a long, semipermeable wall, which the engineers could erect along a coastline, about half a mile from shore. 

Based on the team’s initial experiments with lab-scale prototypes, the architected reef could reduce the energy of incoming waves by more than 95 percent.

“This would be like a long wave-breaker,” says Michael Triantafyllou, the Henry L. and Grace Doherty Professor in Ocean Science and Engineering in the Department of Mechanical Engineering. 

“If waves are 6m high coming toward this reef structure, they would be ultimately less than a metre high on the other side. So, this kills the impact of the waves, which could prevent erosion and flooding.”

Leveraging turbulence
Some regions have already erected artificial reefs to protect their coastlines from encroaching storms. These structures are typically sunken ships, retired oil and gas platforms, and even assembled configurations of concrete, metal, tyres, and stones. 

However, there’s variability in the types of artificial reefs that are currently in place, and no standard for engineering such structures. 

What’s more, the designs that are deployed tend to have a low wave dissipation per unit volume of material used. That is, it takes a huge amount of material to break enough wave energy to adequately protect coastal communities.

The MIT team instead looked for ways to engineer an artificial reef that would efficiently dissipate wave energy with less material, while also providing a refuge for fish living along any vulnerable coast.

“Remember, natural coral reefs are only found in tropical waters,” says Triantafyllou, who is director of the MIT Sea Grant.
 “We cannot have these reefs, for instance, in Massachusetts. But architected reefs don’t depend on temperature, so they can be placed in any water, to protect more coastal areas.”

The new effort is the result of a collaboration between researchers in MIT Sea Grant, who developed the reef structure’s hydrodynamic design, and researchers at the Center for Bits and Atoms (CBA), who worked to make the structure modular and easy to fabricate on location. The team’s architected reef design grew out of two seemingly unrelated problems. 

CBA researchers were developing ultralight cellular structures for the aerospace industry, while Sea Grant researchers were assessing the performance of blowout preventers in offshore oil structures – cylindrical valves that are used to seal off oil and gas wells and prevent them from leaking.

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

The researchers began to play with the general structure in simulations of water flow, tweaking its dimensions and adding certain elements to see whether and how waves changed as they crashed against each simulated design.

This iterative process ultimately landed on an optimised geometry: a vertical cylinder flanked by four long slats, each attached to the cylinder in a way that leaves space for water to flow through the resulting structure. 

They found this setup essentially breaks up any incoming wave energy, causing parts of the wave-induced flow to spiral to the sides rather than crashing ahead.

“We’re leveraging this turbulence and these powerful jets to ultimately dissipate wave energy,” Ferrandis says.

Standing up to storms
Once the researchers identified an optimal wave-dissipating structure, they fabricated a laboratory-scale version of an architected reef made from a series of the cylindrical structures, which they 3D-printed from plastic. Each test cylinder measured about 1ft wide and 4ft tall. 

They assembled a number 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 passing through the architected reef.
“We saw the waves reduce substantially, as the reef destroyed their energy,” Triantafyllou says.

The team has also looked into making the structures more porous, and friendly to fish. They found that, rather than making each structure from a solid slab of plastic, they could use a more affordable and sustainable type of cement.

“We’ve worked with biologists to test the cement we intend to use, and it’s benign to fish, and ready to go,” he adds.

They identified an ideal pattern of ‘voxels’ or microstructures, that cement could be moulded into, in order to fabricate the reefs while creating pockets in which fish could live. This voxel geometry resembles individual egg cartons, stacked end to end, and appears to not affect the structure’s overall wave-dissipating power.

“These voxels still maintain a big drag while allowing fish to move inside,” Ferrandis says.

The team is currently fabricating cement voxel structures and assembling them into a lab-scale architected reef, which they will test 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 various offshore locations. 

“Now we’re simulating actual sea patterns, and testing how these models will perform when we eventually have to deploy them,” says Anjali Sinha, a graduate student at MIT who recently joined the group.

Going forward, the team hopes to work with beach towns in Massachusetts to test the structures on a pilot scale.
“These test structures would not be small,” Triantafyllou emphasises. 

“They would be about a mile long, and about 5m tall, and would cost something like $6 million per mile. So, it’s not cheap. But it could prevent billions of dollars in storm damage. And with climate change, protecting the coasts will become a big issue.”

Print this page | E-mail this page