科学与发展网络(SciDev.Net)

Mangroves can provide a protective shield against destructive cyclones

Flickr/dinesh_valke

[NEW DELHI] Mangrove forests, common along tropical coasts, can provide a protective shield against destructive cyclones and reduce deaths, a study has found.

The finding follows a report published earlier this year (January) which said that mangroves were not effective against tsunamis (see Mangroves do not protect against tsunamis). It adds to a growing debate on using mangroves as bioshields in coastal areas.

The new study, conducted by scientists at the University of Delhi, India, and Duke University in the United States, analysed the 1999 'super cyclone' that ravaged Orissa state in eastern India, killing an estimated 10,000 people.

The scientists found that coastal villages in Orissa with the widest mangrove belts suffered fewer deaths, compared to those with narrower or no mangroves.

Their statistical models suggest that without mangroves, villages within ten kilometres of the coast would have suffered an average of 1.72 additional deaths.

"Statistical evidence of this life-saving effect is robust" and remains "highly significant" even after taking into account other environmental and socioeconomic factors, the report says.

The January study, however, found that 'bioshields' have negligible effects against tsunamis. Others have argued that promoting green coastal belts as a buffer against tsunamis is diverting valuable funds from effective protection measures such as developing early warning systems.

Saudamini Das, an associate professor at the University of Delhi and a co-author of the new study points out that while the new study does not address whether mangroves protect tsunami-hit areas it does clear all doubts about their effects against cyclones.

Das told SciDev.Net that there are key differences in the height and energy of the waves in tsunami and cyclone situations. Cyclones cause sea waters to rise and form a wall of water called a 'storm surge' up to eight metres high. The waves are driven by cyclone winds and their energy is concentrated near the water surface. "Mangroves can reduce the wind energy and [wave] velocity in this case," she says.

By contrast, tsunami wave heights can reach up to 20 metres and their effects on mangroves are not clear as there is no similar study covering a large sample size and taking into account other environmental and socioeconomic factors, she says.

Das also says the maximum speed of tsunami waves mangroves can withstand is yet to be studied.

The study is reported in the 20 April online edition of Proceedings of National Academy of Sciences*

*free access to users in developing countries