Novel tool sheds light on coral reef erosion

Evan Lubofsky | Woods Hole Oceanographic Institution
Monday, May 18, 2020

"UTube" wave abrasion simulator could aid coastal planning in low-lying Marshall Islands

Read this at Woods Hole Oceanographic Institution

Jessica Olson, a gym teacher from Springdale, Arkansas, felt like she was standing in the middle of the central Pacific Ocean, peering out at the turquoise-blue waters raging from every direction as far as the eye could see. It was if she were on a ship, but in reality, Olson was inside a friend's house on low-lying Majuro Atoll in the Marshall Islands, watching powerful waves gnaw away at a seawall in front.

“The waves crashed into the seawall, up over the house, and into the main street,” recalls Olson, a native of Majuro who moved to the U.S. mainland as a child. "We thought, wow, this is kind of insane.”

In recent decades, coastal flooding in the Marshall Islands has increased in both frequency and magnitude, due to a combination of large, storm-driven wave events and rising seas. In some cases, low-lying homes and businesses have been swallowed up, forcing some residents to leave the region.

Coral reefs are among the last lines of defense against coastal flooding. In a region where it’s hard to find land more than ten feet above sea level, reefs provide a critical natural barrier. They dissipate much of the destructive energy from large waves before sweeping onto shore.

Unlike other tropical areas where reports of coral disease and bleaching are far too common, many reefs throughout the Marshalls—particularly those in the outer and less-populated atolls—are still vibrant and teeming with life. They have managed to avoid the pressures of overfishing, pollution, and other human impacts that have decimated reefs on other islands across the Pacific.

Reef erosion, however, is a potential threat in the Marshall Islands due to the large waves from increased tropical storms and cyclones. When these storms barrel through, they churn up waves, smashing sediments—containing calcium carbonate fragments of shell-building organisms and large pieces of broken coral—against reefs at velocities high enough to cause erosion.

Olson says that while she has seen the damage waves can cause on shore, she hasn’t considered how they may be impacting coral reefs.

A UTube for coral studies

Olson’s critical question is one that James Bramante, a recent graduate of the MIT-WHOI Joint Program in Oceanography, has been grappling with. Bramante has been studying the potential impacts of waves on coral reef systems in the Marshall Islands, and says wave-driven abrasion of coral reefs can influence how far waves might travel past reefs and onto shore.

“We know waves are a major driver of coastal change, but the underlying dynamics of sediment transport and reef erosion are poorly understood,” he says.

To help bridge the knowledge gap, Bramante has developed a novel system for estimating the rate at which sediments erode reefs—one that could help island communities at risk predict and plan for coastal inundation. He’s built a massive U-shaped wave abrasion simulator—aptly called a UTube—consisting of two 13-foot PVC tubes running up each side. These are connected by a 10-foot see-through acrylic chamber running along the floor, in which roughly ten handfuls of gravel are spread out along the bottom lining of “erodible” urethane foam, which acts as a simulated coral reef substrate.

“We used this particular foam since the material will erode at roughly similar rates to the framework of a coral reef, which is made up of dead corals and coralline algae tightly welded together,” says Bramante. “We measure erosion rates simply by weighing the foam before and after our experiments.”

Bramante demonstrates the contraption in a dimly-lit, warehouse-like bay on the MIT campus in Cambridge, Mass. First, he climbs a steel ladder adjacent to the UTube system and runs a hose into one of the vertical PVC tubes until it’s nearly filled up with water. Then, he switches on two thrusters which force the water down the tube, through the chamber, and up into the second tube. Gravity then sends the water back through the system up into the first tube, where the thrusters force it back down again. The cycle repeats itself.

The wave abrasion simulator agitates gravel along a foam lining beneath to estimate how quickly storm-driven waves will erode corals. (Video by James Bramante, © Woods Hole Oceanographic Institution)

Through the clear acrylic chamber along the bottom, you can see the water flow increasing back and forth as bits of gravel begin to coalesce into little piles. After a handful of cycles, the U-Tube starts churning and the water becomes torrent-like as it moves from tube to tube. The stronger circulation pushes the individual piles of gravel into a single large mound, which agitates the foam substrate in a scouring, back-and-forth fashion.

“The wave energy we’re able produce here in our lab at MIT is similar to what real reefs are exposed to thousands of miles away in the Marshall Islands,” says Bramante.

Breaking it down

In initial testing, Bramante and his team have discovered that the “reef” erodes a lot faster than he expected.

“I thought I’d see erosion rates of meters per century,” he says, “but it is more like meters per year.”

Bramante points out, however, that this doesn’t mean coral reefs in the Marshall Islands are eroding—or will erode—at these rates. In fact, according to a new study that he co-authored with WHOI geologists Jeff Donnelly and Andrew Ashton, the experiment results support the hypothesis that wave-driven abrasion deepens channel-like “grooves” in reef surfaces over time, and that these grooves may keep abrasive sediment away from actively growing coral, thus protecting it.

“However, if coral health declines further,” says Bramante, “abrasion may overwhelm the reef’s ability to compensate, which would render reefs less effective at buffering wave events in the future.”

Science into action

Bramante’s end game is to create equations describing the underlying physics of wave-driven abrasion and feed them into predictive models that coastal planners can use to estimate the potential impacts of future storms. The models could help the Marshallese—as well as those in other low-lying areas such as the Cook Islands and Maldives—protect remaining infrastructure and make decisions about when and where to relocate resources and people.

“Abrasion is another reef stressor that could be exacerbated by climate change,” says Bramante. “The more we can learn about the role of sediment dynamics in reef health and maintaining atoll island shorelines, the more we can help protect these vulnerable communities.”

Olson agrees. She feels that without this knowledge, decisionmakers in the Marshall Islands can’t really sit down and determine how to best protect their submerging shorelines. “I don’t think the majority of people really know how and why these things are happening,” she says.

But she also feels a great sense of gratitude that the tiny tropical paradise she calls home has gained the attention of researchers like Bramante who are trying to help the Marshallese stay one step ahead of the encroaching ocean.

“It is very heartwarming that research like this was carried out with our small island nation in mind,” she says.

Funding for this research was provided by the WHOI Ocean Ventures Fund, the MIT EAPS’s Student Research Fund, and the Strategic Environmental Research and Development Program (SERDP).

Story Image: A vibrant coral reef in the Marshall Islands teems with life, but storm-driven waves from increased storms and cyclones could cause these natural coastal barriers to erode in the future. (Photo by Luiz A. Rocha, California Academy of Sciences)