Breaking Waves: Mangroves shielded parts of coast from tsunami

Along a strip of India’s southeastern coastline, forests protected certain villages from last December’s tsunami there, while waves wiped out neighboring settlements that weren’t sheltered by vegetation. Those observations provide some of the best evidence yet that forests can guard coastal communities from the ravages of an angry sea.

IN HARM’S WAVE. The December 2004 tsunami knocked down casuarina trees adjacent to the Indian Ocean but left trees farther inland—and the villages behind them—standing. V. Selvam/Science

Mangroves, which grow in tidal areas, are widely perceived to protect land against major storms and waves. Laboratory models and anecdotes from the field support that notion, but little rigorous, real-world evidence exists.

For ecologist Finn Danielsen of the Copenhagen-based nonprofit organization Nordic Agency for Development and Ecology and his colleagues, the hunt for such evidence began last Dec. 27, the day after the Indian Ocean tsunamis killed more than 180,000 people.

A 21-kilometer length of coastline in Cuddalore, India, identified from satellite images, provides a “unique experimental setting,” says Danielsen. Because it’s straight and has largely uniform offshore features, he says, the waves probably had the same energy along the entire stretch of coastline.

“In areas with maximum tsunami intensity, little could have prevented catastrophic destruction,” he says. But the 4- or 5-meter waves that washed into Cuddalore were modest enough for vegetation to make a difference.

Two villages situated along the shore and unprotected by mangroves were obliterated, Danielsen’s team reports in the Oct. 28 Science. Three other villages that were behind a screen of mangroves that was hundreds of meters thick survived.

A few kilometers away, villages that were protected by plantations of casuarina trees also survived the tsunami with minimal damage. Local residents planted those trees, which grow on dry land, after experiencing a destructive cyclone 2 decades ago, Danielsen says.

“This was not entirely a natural disaster,” Danielsen says. Mangroves have been lost in the region because of overexploitation of their wood and tree removal for agriculture, he says. Elsewhere, the creation of shrimp farms and fish ponds has played a major role in the destruction of mangroves.

The work by Danielsen’s group shows that mangrove forests have “an extremely important role as a buffer between land and sea,” says oceanographer Björn Kjerfve of Texas A&M University in College Station. “Undisturbed mangroves in a belt close to the sea … shelter people who live on the other side.”

Kjerfve says that he’s less convinced that casuarina trees can act as a physical barrier against the sea because they cover ground less densely and are more easily uprooted.

“Mangroves can function as a living dyke,” says mangrove ecologist Farid Dahdouh-Guebas of Vrije Universiteit Brussel in Belgium. “Local people are very much aware of the benefits.” Dahdouh-Guebas and his colleagues have conducted surveys of people living near mangrove forests in India, Sri Lanka, and other regions.

In the aftermath of the tsunamis, the researchers observed that relatively intact mangrove forests in Sri Lanka had mitigated wave damage and that few trees were destroyed. Mangrove forests that had been degraded by excessive human use provided substantially less protection and were heavily damaged by the waves, the scientists reported in the June 21 Current Biology.

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