CORAL REEFS ARE THE RAINFORESTS OF THE OCEANS, thriving ecosystems with the most extensively biodiverse species of plants and animals on the planet. Despite occupying less than one percent of the ocean floor, they are home to a quarter of all marine life, providing food, shelter, resting, and breeding grounds for millions of species.
These formidable structures can also absorb up to 97 percent of wave energy, playing a critical role in protecting coastal infrastructure and livelihoods from the impacts of storms, waves, and currents. However, these natural barriers are increasingly threatened by climate change and warming ocean temperatures that exacerbate global coral bleaching events. More than half of the world’s coral reefs have died and 90 percent are projected to perish within the next century.
With 1,700 coastal military installations around the world, the US Department of Defense (DoD) has been seeking long-lasting solutions to these climate threats. In 2022, its Defense Advanced Research Projects Agency (DARPA) awarded the University of Hawai‘i a $25-million, five-year grant to leverage nature-based solutions to develop novel coastal protection capabilities.
An Innovative, Multifaceted Solution
As part of the DARPA Reefense program, UH is developing a groundbreaking technology called Rapid Resilient Reefs for Coastal Defense (R3D), spearheaded by the Applied Research Laboratory at the University of Hawai‘i (ARL at UH) in partnership with University of Hawai‘i at Mānoa’s School of Ocean and Earth Science and Technology, Florida Atlantic University, Scripps Institute of Oceanography, University of California San Diego, The Ohio State University, and Makai Ocean Engineering.
R3D is a complex, multidisciplinary project that is not only designing novel structures to dissipate wave energy, lowering flood risks and coastal erosion, but is also integrating techniques and technologies that aid the rapid settlement and growth of resilient coral communities well into the future.
Key components of the R3D project:
A Robust Foundation
The coastal engineering team developed an array of thin-walled, porous structures that gradually pulls energy out of the wave in the form of turbulence that is generated through flow-separation. This unique design creates spilling versus plunging waves, that exert less force on the structures. Results from tests on scaled-down models at the Oregon State University Large Wave Flume indicate wave energy reduction of over 80 percent.
The large reef crest structures, which will bear the brunt of the larger waves, will be anchored to the seabed to prevent them from moving during larger-wave events. The back reef structures, which will rest in calmer environments, will be stabilized using their own weight.
Self-Healing Properties
In order to support the natural growth of a coral reef ecosystem, R3D incorporates various technologies that support rapid seeding and growth of coral and other reef-building organisms.
Hawai‘i Institute for Marine Biology (HIMB) Professors Josh Madin and Rob Toonen are working on developing techniques and technologies for maximizing coral settlement and growth, including:
- Coral growth modules with genetically diverse coral fragments to accelerate coral growth while enhancing overall resilience of the outplanted coral community; and
- Coral settlement modules, which are domes designed to geometrically mimic the complexity of natural coral reefs, with specific crevice sizes and depths that dramatically increase larval coral settlement, and provide areas for oysters, sponges and other cryptic fauna to settle and grow, while providing refuge for herbivorous fish.
They are also working with other university project partners on incorporating the following elements:
- Underwater Zooplankton-Enhancing Light Apparatus to increase heterotrophic feeding, shown to accelerate coral growth and increase larvae post-settlement survival;
- Intricate micro-structures and biomaterials like slippery surfaces to prevent harmful algal growth that hinder new coral fragment growth, and incorporating naturally sourced chemical cues to enhance coral settlements; and
- Acoustic enhancement techniques that mimic the sound of healthy coral reefs to attract coral and fish larvae during key reproduction periods.
Climate Adaptation Capabilities
Another team at HIMB led by Toonen and Coral Resilience Lab Principal Investigator Crawford Drury, is also implementing various techniques to help coral reefs survive warming ocean conditions and prevent bleaching, including:
- Using cutting-edge technology to identify predictive biomarkers of thermally tolerant traits to prioritize propagation efforts of thermally resilient coral; and
- Developing rapid stress testing protocols for determining thermal tolerance of coral larvae, juveniles, and fragments.
They are also working with The Ohio State University on exploring cost-effective ways to supplement feeding to aid recovery of bleached corals.
“This project builds on over a decade of research at HIMB, and being able to collaborate with other universities allows us to build even stronger and more sustainable reef structures to protect our coastal roads, runways, and neighborhoods,” said Toonen.
Comprehensive monitoring protocols are also being implemented to ensure structural stability, and to track reef health and resilience over time.
Deploying the Structures
The team anticipates launching the full 50-meter array of structures at Ulupa‘u, off the Kailua Bay side of Marine Corps Base Hawai‘i in late 2024 or early 2025.
The array will be placed in relatively smooth areas of degraded reefs, leveraging the natural shape of the seafloor to reduce its size and carbon footprint. The structures will sit just below the water’s surface, preserving the areas’ natural aesthetics. Designed to resemble the flow of a natural coral reef, the R3D structures will minimize disruptions to water and sediment flow, while significantly increasing wave reduction compared to the existing reef.
Broader Impact and Applications
“One of the most valuable aspects of this project is that we are taking all of the lessons that we are learning and developing a robust template for how to implement this work elsewhere,” said Joshua Levy, R3D technical program manager. “This includes customizing surveying techniques and technology designs that best mimic the area’s physical environment and natural genetic diversity.”
The DoD is looking to eventually deploy components of this work at other military installations in the Pacific. The R3D team is also exploring potential applications at other vulnerable areas like ‘Ewa and Ka‘a‘awa on O‘ahu.
“The value of bringing together a team of experts from such a wide variety of disciplines and backgrounds to address one of the Pacific region’s most pressing concerns goes beyond the R3D project itself,” said Jones. “Establishing a critical mass of students, early career folks and senior experts who are energized and focused on climate adaptation and coastal resilience will help us deal with one of the great challenges of this century. “