Across the globe, shorelines are under threat from rising sea levels and intensifying storms. Island states and coastal cities are taking action to defend themselves, from building seawalls to dredging sand from the seabed and pumping it onto beaches.

In the Maldives, a 900-kilometer-long chain of about 1,200 islands in the Indian Ocean, the Massachusetts Institute of Technology (MIT) Self-Assembly Lab and Maldivian organization Invena are working on a more natural solution. Using submersible structures, they’re harnessing the ocean’s forces to make sand accumulate in carefully chosen spots to protect islands — and even potentially grow new ones.

Since 2019, the organizations have been running field tests in the Maldives, where the shoreline of almost every island is eroding.

The various experiments — mostly conducted in the shallows of a reef flat just south of the capital city Malé — have consisted of everything from submerging a web of rope tied into tight knots to collect sand, to using a material that transforms from textile to rigid concrete when sprayed with water to create a barrier that was placed onto the seafloor to build up sand there.

In another field experiment, a floating garden was installed above a sand bank, to explore if the roots could help stabilize already accumulated sand, and collect more.

It may not sound all that novel. After all, ideas like using mangroves for coastal defense have been around for a while. But there is serious data and technology behind the work.

The field installations start as experiments in wave tanks on MIT’s campus in Cambridge, Massachusetts. To determine how to orient the structures, and their ideal geometries, the team relies on wave and ocean current information collected by tilt sensors in the Maldives, publicly accessible tide and weather data, thousands of computer simulations, and a machine learning model trained on satellite images to predict how sand will move.

The group has been conducting experiments in wave tanks at an MIT lab since 2017, to test how different wave conditions, sand behaviors and object configurations and geometries can work together to promote sand accumulation. MIT Self-Assembly Lab + Invena

Skylar Tibbits, the founder and co-director of the Self-Assembly Lab, which focuses on materials and processes that enable the formation of objects and spaces, told CNN that he hopes the submersible structures can provide a more sustainable method than conventional engineering solutions for reinforcing eroded coastlines. “We’re using the natural force of the ocean to guide the sand,” he said.

“The sand wants to be there”

The Maldives, with an average elevation of just one meter (3.3 feet) above sea level, is the world’s lowest-lying country. Officials, resort operators and real estate developers have resorted to dredging and hard engineering solutions, building seawalls, breakwaters and groynes to try to deal with the problem.

But these interventions can be expensive, difficult to maintain, and disruptive to ecosystems.

Pumping and dredging needs to be repeated every few years. Seawalls and other infrastructure can even exacerbate the erosion they’re intended to prevent or fix, especially when the design or construction is shoddy, or ideas are copied and pasted from elsewhere, without factoring in local considerations.

Paul Kench, a coastal geomorphologist at the National University of Singapore not involved in MIT and Ivena’s work, has seen evidence of this. His research has shown that structures from seawalls to boat harbors can make erosion worse and degrade reef productivity. “The kinds of engineering solutions that we tend to use on continental coastlines shouldn’t really go anywhere near a reef island,” he said, but “people tend to use them, because that’s what they know.”

MIT Self-Assembly Lab and Invena’s use of local data is working with natural forces rather than against them, Tibbits argues, “so the sand wants to be there.” With each field experiment, the group says it is advancing its understanding of what materials, configurations, and construction techniques can make sand accumulate in the simplest, most cost-effective, sustainable, long-lasting and scalable way.