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Forecast predicts global increase in coastal overtopping

When waves, tides or storm surges breach natural or artificial barriers, whether dunes or a flood wall, it is called coastal overtopping. Photo courtesy of the government of Guadeloupe
When waves, tides or storm surges breach natural or artificial barriers, whether dunes or a flood wall, it is called coastal overtopping. Photo courtesy of the government of Guadeloupe

June 18 (UPI) -- Coastal overtopping is expected to accelerate during the coming decades, exposing more of Earth's coastline to the risk of flooding.

Coastal overtopping, or wave overtopping, occurs when waves, storm surges or extreme tides breach natural or artificial coastal barriers, whether a reef or flood wall.

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As sea levels rise and coastal storms become more frequent and intense, the risk of coastal overtopping predictably increases.

To anticipate global increases in coastal overtopping, researchers combined satellite images with sophisticated models.

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The results, published Friday in the journal Nature Communications, suggest coastal overtopping will accelerate through the end of the century, especially in the tropics.

Scientists began by estimating the number of global submersion events between 1993 and 2015.

Using satellite data, researchers measured the slope of coastlines around the world, as well as the highest points along a coastline.

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Armed with timestamped measurements of local sea levels, researchers estimated how often and for how long coastal defenses were at risk of being overtopped.

"The combination of tides and episodes of large waves is the main contributor to episodes of coastal overflow," study coordinator Rafaël Almar said in a press release.

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"We identified hot-spots, where the increase in risks of overtopping is higher, such as in the Gulf of Mexico, the Southern Mediterranean, West Africa, Madagascar and the Baltic Sea," said Almar, a researcher in coastal dynamics at the French Institute for Sustainable Development, IRD.

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Next, scientists used computer models to simulate coastal overtopping increases under various sea level rise scenarios.

The simulations showed increases in the risk of coastal overtopping are likely to outpace rates of sea level rise.

"The frequency of overtopping is accelerating exponentially and will be clearly perceptible as early as 2050, regardless of the climate scenario," Rafaël Almar said. "By the end of the century, the intensity of the acceleration will depend on the future trajectories of greenhouse gas emissions and therefore the rise in sea-level."

Researchers found that under the high emissions scenario, the risk of coastal overtopping could increase by 50 fold.

"As we go along the 21st century, more and more regions will be exposed to overtopping and consequent coastal flooding, especially in the tropics, north-western United States, Scandinavia and the Far East of Russia," Rafaël Almar said.

Researchers suggest additional studies are necessary to home in on more precise coastal overtopping forecasts for regions and stretches of coastline.

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