Credit: MIT News

MIT Engineers Design Artificial Reef To Protect Coastlines & Marine Species

April 3, 20249 months ago Steve Hanley 0 Comments

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Most of us don’t spend a lot of time thinking about reefs. If we give them any thought at all, it is because we are boat owners who worry about finding one accidentally and damaging the hull. But a reef serves a number of very important functions. For one, it provides shelter to young marine life from predators. For another, it helps absorb the power of ocean waves so they don’t destroy coastlines.

But reefs, which are made up of billions of tiny organisms, are dying all around the world because the temperature of the water is rising and because the ocean is becoming more acidic, dissolving the hard calcium deposits those tiny organisms secrete that form the structure of the a reef.

The ultimate solution is to stop doing the things humans do that damage the reefs, but that would mean interrupting our comfortable lifestyles and we can’t have that, now can we? In the absence of doing the right thing, the clever engineers at MIT have devised a way to make an artificial reef that serves many of the same purposes as the real thing.

The Architected Reef

MIT calls it an “architected” reef — a sustainable offshore structure that is engineered to mimic the wave buffering effects of a natural reef while also providing pockets of safety for fish and other marine life. The reef design centers on a cylindrical structure surrounded by four slats like rudders. When this structure meets an incoming wave, it breaks it into turbulent jets that ultimately dissipate most of the wave’s energy. The team has calculated that the new design could reduce as much wave energy as an existing reef while using 10 times less material.

The researchers plan to fabricate each cylindrical structure from sustainable cement (MIT is also a leader in making low or zero emissions cement), which they would mold in a pattern of “voxels” that can be automatically assembled to provide pockets for fish to explore and other marine life to settle in. The cylinders can be connected to form a long, semi-permeable wall. The engineers say those walls can be installed along a coastline about half a mile from shore. Based on initial experiments with lab-scale prototypes, the architected reef could reduce the energy of incoming waves by more than 95%.

“This would be like a long wave breaker,” says Michael Triantafyllou, a professor in Ocean Science and Engineering in the Department of Mechanical Engineering. “If waves are 6 meters (20 feet) high coming toward this reef structure, they would be ultimately less than a meter (3.3 feet) high on the other side. So, this kills the impact of the waves, which could prevent erosion and flooding.”

Details of the architected reef design are reported today in the journal PNAS Nexus. In the introduction, the authors write, “Recent studies show that wave storms have intensified as a result of climate warming. For example, in 2023 severe storms battered California, damaging infrastructure and forcing people to flee away from the coast because the intensity of the storms has significantly increased since the 1970s. Using nearly a century of data, it was found that the occurrence of extreme significant wave height events during the 1996–2016 epoch is about twice that recorded between 1949 and 1969. Combined with sea level rise, it is projected that by the end of the current century, even moderate wave storms will produce coastal impacts comparable to recent extreme winter wave events.”

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, tires, and stones. However, the variability in the types of artificial reefs means there are no standards for engineering such structures. In addition, the designs deployed today tend to have a low wave dissipation per unit volume of material used. In other words, it takes an enormous amount of material to absorb enough wave energy to adequately protect coastal communities.

Dissipating Wave Energy

The MIT team focused on 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. “But architected reefs don’t depend on temperature, so they can be placed in any water to protect more coastal areas.”

The new architected reef 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 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 — cylindrical valves used to seal off oil and gas wells to prevent them from leaking in offshore installations.

The tests conducted by the two teams showed that the cylindrical arrangement of the structure generated a high amount of drag, making it especially efficient at dissipating high force flows of oil and gas. That led them to consider whether the same arrangement could dissipate the energy in ocean waves.

The researchers began to experiment with the general structure in simulated water flows, tweaking its dimensions and adding certain elements to see how waves changed their behavior in response. This iterative process resulted in an optimized geometry consisting of 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 said.

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 were 3D-printed from plastic. Each test cylinder measured about 1 foot wide and 4 feet 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 added.

They then identified an ideal pattern of “voxels,” or micro-structures, to fabricate the reefs while creating pockets in which fish could live. This voxel geometry resembles individual egg cartons, stacked end to end. “These voxels still maintain a big drag while allowing fish to move inside,” Ferrandis said.

Artificial Reef Research Continues

Now the researchers are fabricating cement voxel structures and assembling them into a lab-scale architected reef which they will test under various wave conditions. They envision the voxel design will be modular and scalable to any desired size. It should also be 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,” said 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 will not be small,” Triantafyllou emphasized. “They will be about a mile long and about 5 meters (17 feet) tall, and will cost something like 6 million dollars 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.”

Actually, $6 million a mile is ridiculously inexpensive compared to the cost of conventional coastal restoration techniques, which usually involve placing sand along coastlines to absorb the forces created by ocean storms when they occur. The artificial reef is not only less expensive, it is a more durable solution than beach replenishment and sand restoration. This is truly a remarkable engineering solution to a problem that will become more acute as global overheating contributes to more frequent and more powerful ocean storms.