New guidelines for coral reef restoration aiming to reduce the risk of flooding in tropical coastal communities have been set out in a new study that simulated the behavior of ocean waves travelling over and beyond a range of coral reef structures. Published in Frontiers in Marine Science, these guidelines hope to optimize restoration efforts not only for the benefit of the ecosystem, but also to protect the coast and people living on it.
"Our research reveals that shallow, energetic areas such as the upper fore reef and middle reef flat, typically characterized by physically-robust coral species, should be targeted for restoration to reduce coastal flooding," says Floortje Roelvink, lead author on the paper and researcher at Deltares, a Dutch research institute. "This will benefit both coral ecosystems and human coastal populations that rely on them for tourism, fisheries, and recreation."
Coral reefs help to sustain the economy of 500 million people in tropical coastal communities and can offer protection from wave-driven flooding and coastal erosion, especially in the face of climate change. Reef restoration, which involves coral planting and reef management to improve the health, abundance, and biodiversity of the ecosystem, has been suggested as a way of reducing flood risk.
"Our research can help guide the design of coral reef restorations to best increase the resiliency of coastal communities from flooding," said Curt Storlazzi, U.S. Geological Survey research geologist and project lead. "Such information can increase the efficiency of coral restoration efforts, assisting a range of stakeholders in not only coral reef conservation and management, but also coastal hazard risk reduction."
"Although we know that coral reefs can efficiently attenuate ocean wave energy and reduce coastal flooding, knowledge of specifically where to locate and design coral reef restorations on specific types of reefs is lacking," explains Ap van Dongeren, coastal morphology specialist at Deltares and project co-lead. "We were keen to fill this knowledge gap because the costs and practical constraints of reef recovery efforts necessitate an approach to design and restoration that produces the most benefit for all."
Reef by design
To first understand the range of naturally occurring reef shapes, such as fringing reefs, straight sloping reefs, convex reefs and reefs with an offshore shelf, the researchers analyzed a database of over 30,000 coral reef profiles across the U.S., including those in the Mariana, Hawaii, and Virgin Islands. Using these reef profiles, they numerically "designed" reef restorations to be both feasible from an operational and ecological perspective and to have an expected beneficial impact on coastal flooding.
The researchers established that reef restorations should not be placed too deep because of operational constraints and limit on the wave reduction efficiency. Restorations should also not be too shallow, to prevent the drying of reef restorations and reef degradation due to thermal intolerance.
Different types of coral restorations were also investigated - "green", entailing solely outplanting corals, or "gray-green hybrid" restorations, entailing emplacement of structures (such as ReefBalls) and then outplanting corals on top of them. The team then used a numerical model to simulate waves travelling over both the restored and unrestored coral reef profiles to see how far those waves ran up the coast, providing an indication of the effect of the different reef restorations on coastal flooding.
"We hope this study will motivate others to continue and expand on this research, among others by conducting field and laboratory experiments to validate our findings," concludes Roelvink.