Architected Reefs Could Protect Coastlines

Ed Brown

An MIT team is hoping to fortify coastlines with “architected” reefs — sustainable, offshore structures that are engineered to mimic the wave-buffering effects of natural reefs, while also providing pockets for fish and other marine life to live. (Image: Courtesy of the researchers, edited by MIT News)

Storms contain a lot of energy, which can cause coastal erosion and damage when they reach unprotected coastlines. According to Michael Triantafyllou, Professor in Ocean Science and Engineering in the Department of Mechanical Engineering at the Massachusetts Institute of Technology, there are islands in the Pacific that are surrounded by reefs that protect them from much of that damage.

“We looked at the structure of these reefs and we found some similarities to what we had been doing in fluid mechanics. That led us to the idea of trying to make artificial reefs that we could architect and build in a very directed way, using principles we had derived from our earlier work on ocean structures used for oil extraction,” said Triantafyllou.

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, there’s a lot of variability in these structures, and no standard for engineering them. 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. Triantafyllou’s idea was that a carefully designed structure could be much more effective.

The structure was inspired by the team’s work on blowout preventers — cylindrical valves that are used to seal off oil and gas wells to prevent them from leaking. Their tests showed that the structure’s cylindrical arrangement generated a high amount of drag and appeared to be especially efficient in dissipating high-force flows of oil and gas. They wondered whether the same arrangement could dissipate another type of flow: ocean waves?

To test that idea, they designed a series of one-foot-diameter cylinders spaced about one foot apart to form a fence-like structure. In tests performed on the model, they found that their reef could dissipate 10 times more energy than any existing reef.

The team has constructed their cylinders with a sustainable cement that tests showed was inert to the ocean. “We don't want to poison the ocean,” said Triantafyllou.

They further improved their design by using artificial intelligence algorithms to try various configurations using computational and experimental methods. This led them to add four long slats, each attached to the cylinder in a way that leaves space for water to flow through the resulting structure. They found that 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.

“When you put things on the bottom of the ocean, life tends to aggregate there,” said Triantafyllou. So, inspired by coral reefs, they carefully designed a pattern of “voxels,” or microstructures, to be molded into the cement to create pockets in which fish and other animals could live.

Their next step will be to design a larger structure that they can test in a real-world setting. They are looking to work with a town on the Massachusetts coast. Once they have found a workable location, they will start designing the structure for that specific site. Since the cylinders sit on the ocean floor, the design will have to be adjusted for that particular location, and because the coast changes from year to year, measurements will be taken to make sure that things haven't shifted since the area was last mapped.

“We have funding from the National Oceanic Atmospheric Administration through a seed grant for this next stage,” said Triantafyllou. “We will now start adding details on how a large structure will look. Also, we have people working with towns because we want to work with stakeholders so that we can explain to them what we're doing, and they can give us feedback.”

This article was written by Ed Brown, Associate Editor, SAE Media Group. For more information, visit here .