Coral reefs can reduce coastal flooding by as much as 30% if restorers focus on building up specific reef locations that sap waves of their destructive power, which could help protect millions in coastal communities worldwide.

These findings, published May 10 in Frontiers in Marine Science, build on prior research that shows how coastal reefs can reduce flooding. But instead of looking at how an entire reef can break waves, the authors decided to look at how they could optimize coral restorations to reduce flood risk efficiently, potentially solving a problem that threatens millions in coastal tropical communities. 

"If you look at the practical side of it, you cannot restore your whole reef — or at least from a financial perspective, that's not always feasible — so then you would really want to target locations that are most effective in reducing your flooding at your reef coasts," said lead author Floortje Roelvink, a researcher in reef hydrodynamics at Deltares, a Netherlands-based water research institute. "That's how this started. We really wanted to have a look at: Where is [restoration] the most effective? And how can we pinpoint these locations?"

Such projects are underway around the world. In the U.S., the National Oceanic and Atmospheric Administration and others launched a decades-long project to restore seven "iconic reefs" in the Florida Keys, where live corals are rapidly disappearing. And in Indonesia, the world's biggest coral restoration program is using steel structures as a base for regrowing coral fragments.

Besides underpinning important ecosystems, reefs can also reduce wave energy reaching the shore. This occurs when a coarse reef surface creates friction underneath a wave as it forms, preventing it from reaching as high as it otherwise would. But the effectiveness of each reef depends greatly on the variation of its shape.

To find out how different reef types and restorations could weaken flooding, Roelvink and her colleagues drew on a global dataset of 30,000 reef profiles to select four common types of profile for analysis: "fringing," "convex," "three-slope" and "linear." Each of these has its own unique properties that make it better or worse at reducing waves and flooding. Fringing reefs, for example, have wide and shallow reef flats, which are horizontal surfaces on the upper level of a reef, where waves lose energy faster than at reefs with smaller, deeper features. 

Using a computer model, the researchers simulated the effect of different-sized waves as they traveled over each reef type, using different restoration configurations. They found that, in general, shallow restorations would be most effective at dampening waves, though the effects are relative depending on the shape of each reef. 

Where fringing reefs typically protect coastlines quite well due to their strong wave-dampening capabilities, three-slope reefs are quite vulnerable to flooding because their overall profile includes a steep fore reef and a deep reef flat, neither of which dampen incoming wave energy, according to Roelvink. But with a restoration that decreases the depth above the restoration, the three-slope reef can reduce wave runup by 26% to 30%. 

Future reef restoration projects may need to consider using stronger coral species than what are typically grown in coral nurseries, which tend to be weaker but faster to grow. New types of coral are being invented, as well; researchers in Japan recently created a new type of resilient coral cell that can withstand and even grow in stressful lab conditions. 

"Our main message is that we have these more optimal locations, but often these locations are in regions where there is a relatively high wave energy, a lot of wave action," Roelvink said in an interview with The Academic Times. "That's why you would also need to pay attention to what types of corals you want to put back on the reef, and that asks for resilient corals that can withstand these wave forces."

For Roelvink, the study can provide guidelines for coastal communities interested in combining reef restoration with flood protection. At the same time, it could show state actors a new means to mitigate flooding on a larger scale than the community level, which could potentially lead to funding for additional reef restoration and the other ecological benefits that entails, such as providing habitat for fish.

"We want to show this sort of nature-based solution as an efficient way of increasing your coastal safety, so you can you create multiple ecosystem services with these restorations, and we want to show that they're actually also really effective on the coastal protection value, and how they can be most effective," said Roelvink.

Roelvink is currently investigating how coral islands develop in response to sea-level rise over long periods of time. For example, waves may deposit sediments as they travel over reefs, and this deposition could promote reef growth with sea-level rise.

"It's not yet a perfect answer," Roelvink said. "I think it's also an invitation for others to join in this research of optimizing your restorations and putting forward this idea that coral reef restorations are indeed effective for coastal protection."

The study, "Coral reef restorations can be optimized to reduce coastal flooding hazards," published in the May 10 issue of Frontiers in Marine Science, was authored by Floortje E. Roelvink, Deltares; Curt D. Storlazzi, United States Geological Survey; Ap R. van Dongeren, Deltares and IHE Delft Institute for Water Education; and Stuart G. Pearson, Deltares and Delft University of Technology.