A structural feature unique to coral reefs across the world has been found to play a critical role in helping to slow waves down.
James Cook University’s Dr Stephanie Duce led a team of researchers from the University of Sydney and the Victorian Department of Environment, Land, Water and Planning to measure waves as they moved across structures known as ‘spurs and grooves’ on reefs in Moorea, French Polynesia, and One Tree Reef on the southern Great Barrier Reef.
The new research, published in the journal of Geomorphology, found that spurs and grooves had very high rates of wave energy dissipation, with up to 36 per cent of wave energy lost over a distance of 10m across the zone of the structures, even without waves breaking.
The structures themselves form on the reef slope where a reef meets the open ocean and Dr Duce believes the uneven surface causes the waves to lose energy as they cross, due to friction.
“Since about the 1940s, people have been curious about spurs and grooves, how they form and what role they play,” she said.
“Our study found the rates of dissipation were the greatest at sites with high live coral cover suggesting the structural complexity of live corals may increase the friction.
“We also think that tidal currents at some sites contribute to dissipation.”
The team used a series of sensors known as ‘pressure transducers’ during their study, which are able to measure the amount of water above the sensor and provide readings about four times a second.
“If you’re measuring that frequently, you can get a really good sense of the shape and size of the wave as it moves past the instrument,” Dr Duce said.
“We placed a few of these instruments out deep, right where the spurs and grooves begin across towards the shallow end where the reef crest sits.”
Dr Duce said the study noted differences in how much wave energy was dissipated at reefs in Moorea and One Tree Reef, suggesting not all spurs and grooves were alike.
“One Tree Reef is quite a remote and pristine reef with really high coral cover,” she said.
“It also has bigger tides than Moorea, and we found the waves were dissipated more effectively by the spurs and grooves there.
“We think it’s potentially related to the corals and if the spurs are covered by corals, they’re going to be rougher and potentially cause more friction on the waves.
“But the other factor is tidal currents might use the grooves to escape, becoming like a highway for water to rush out on an outgoing tide.”
Dr Duce said her team’s research could better inform the future construction of breakwaters, in an effort to mimic the effect of spurs and grooves in slowing down waves before they reach the mainland.
“Spurs and grooves are really nature’s answer to the ultimate breakwater system,” she said.
“They can last for thousands of years and can even survive cyclones, so we know they must be fairly resilient and in tune with their environment to last that long.”
Dr Duce said further study was needed to better understand and model the hydrodynamics of spurs and grooves and the important role they play in reef dynamics and coastal protection.
“There’s a lot of scope to visit different spurs and grooves systems and deploy more instruments,” she said.
“Critically, we need to deploy them for longer and getting those measurements in high wave energy events, such as storms, would be ideal.”
