April 15, 2009

Mangroves Save Lives In Storms

A new study of storm-related deaths from a super cyclone that hit the eastern coast of India in 1999 finds that villages shielded from the storm surge by mangrove forests experienced significantly fewer deaths than villages that were less protected.

The study, conducted by researchers at the University of Delhi and Duke University, analyzed deaths in 409 villages in the poor, mostly rural Kendrapada District of the Indian state of Orissa, just north of the cyclone's landfall.

"Our analysis shows a clear inverse relationship between the number of deaths per village and the width of the mangroves located between those villages and the coast," said Jeffrey R. Vincent, Clarence F. Korstian Professor of Forest Economics and Management at Duke's Nicholas School of the Environment.

"Taking other environmental and socioeconomic factors into account, villages with wider mangroves suffered significantly fewer deaths than ones with narrower or no mangroves," Vincent said. "We believe this is the first robust evidence that mangroves can protect coastal villages against certain types of natural disasters."

Vincent conducted the analysis with Saudamini Das of the University of Delhi's Swami Shradanand College. Their findings appear in a paper in this week's online early edition of The Proceedings of the National Academy of Sciences.

Mangroves are dense forests of trees and shrubs that grow in brackish, low-lying coastal swamps in the tropics and subtropics. In 1944, mangroves covered nearly 31,000 hectares of land in Kendrapada District and the average village had 5.1 kilometers of mangroves between it and the coast. Since then, nearly half the area has been cleared, mostly for rice production. Today, the average width of mangroves between the villages and the coast has shrunk to 1.2 kilometers.

The 1999 storm, which made landfall on Oct. 29, killed nearly 10,000 people, more than 70 percent of whom drowned in its surge.

Using statistical models, Das and Vincent predicted there would have been 1.72 additional deaths per village within 10 kilometers of the coast if the mangrove width had been reduced to zero.

"This is a measure of the life-saving impact of the mangroves that remained in 1999," Vincent said. "It implies that they cut the death toll by about two-thirds."

Although mangroves evidently saved fewer lives than an early warning issued by the Indian government, the retention of the remaining mangroves in Orissa is economically justified, Vincent believes, even without considering the many other environmental benefits they provide, such as acting as nurseries for economically and environmentally vital fisheries, or sites for ecotourism. Previously published estimates of Indians' willingness to pay to reduce the risk of accidental deaths are higher than Das and Vincent's estimate of the cost of saving lives in Orissa by retaining mangroves.

The study does not assess mangroves' ability to reduce deaths from tsunamis, Vincent cautions, because of key differences in their wave dynamics. Storm surges have shorter wave lengths than tsunamis, and relatively more of their energy is found near the surface of the water. Mangroves' ability to protect villages against tsunamis has been a point of controversy in the scientific and policy communities since the 2004 Indian Ocean tsunami. Das and Vincent designed their study to overcome criticism leveled at studies examining the effects of mangroves on that disaster.

The study was supported by the South Asian Network for Development and Environmental Economics (SANDEE), with research facilities provided by the Institute of Economic Growth in Delhi, India.

By Tim Lucas, Duke University


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