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Scientists say mangrove forests
can reduce impact of tsunamis

By T V Padma, SciDev.Net

Dense mangrove forests growing along the coasts of tropical and sub-tropical countries can help reduce the devastating impact of tsunamis and coastal storms by absorbing some of the waves' energy, say scientists. When the tsunami struck India's southern state of Tamil Nadu on 26 December 2004, for example, areas in Pichavaram and Muthupet with dense mangroves suffered fewer human casualties and less damage to property compared to areas without mangroves.

But the scientists also warn that the unique coastal tropical forests are among the most threatened habitats in the world. This is due to population growth and unsustainable economic development including deliberate land reclamation for urban and industrial development, widespread shrimp farming, and chemical pollution.

"We have observed that mangroves often served as a barrier to the fury of water," said Mr M S  Swaminathan, head of the M. S. Swaminathan Research Foundation (MSSRF) in Chennai, India.

MSSRF scientists found, for example, that in October 1999, mangrove forests reduced the impact of a 'super-cyclone' that struck Orissa on India’s east coast, killing at least 10,000 people and making 7.5 million homeless.

More than 15 years ago, MSSRF launched a programme to restore India's vanishing mangrove forests. One success story is the Joint Mangrove Management project, supported by the India-Canada Environment Facility with funding from the Canadian International Development Agency. Implemented in six mangrove areas along the east coast of southern India between 1996 and 2003, the project helped restore 1,447 hectares of degraded mangrove forest.

The foundation adopted a three-pronged strategy. The first goal was to conserve and regenerate mangroves along India's east coast. The second involved identifying and transferring salt tolerance genes from mangroves species to crops like rice and mustard growing in coastal areas. Thirdly, the foundation has been raising awareness among local communities about impending storms and about safe fishing zones and days.

Jeff McNeely, chief scientist of the Switzerland-based World Conservation Union (IUCN), also voiced concern this week about rapidly disappearing mangrove forests that offer protection against events like tsunamis.

Mr McNeely told the Agence France Press news agency that over the past several decades, many mangroves have been cleared to grow shrimp ponds by, among others, outsiders granted concessions from governments to set up shrimp farms, who lacked the long-term knowledge of why the forests should have been saved.

According to the US-based Earth Island Institute's Mangrove Action Project, mangrove forests once covered three-quarters of the coastlines of tropical and sub-tropical countries, but only half of that area remains intact today.

The Mangrove Action Project says vast tracts of mangroves have been cleared to make way for shrimp farms in developing countries, and that national governments have been unable to adequately regulate the industry. Multilateral agencies are also supporting shrimp farming projects without paying attention to social and ecological security, says the organisation.

Shrimp farming alone caused a loss of 65,000 hectares of mangroves in Thailand, according to a 2002 paper by V. P. Upadhyay and colleagues in the journal Current Science. In Indonesia, Java has lost 70 per cent of its mangrove area, Sulawesi 49 per cent, Sumatra 36 per cent. Globally the rate of decline in mangrove forest cover is 2 to 8 per cent each year, said the paper.

In India, large stretches of mangrove forest have been severely degraded in almost all areas where they are found.

As well as acting as a barrier against tsunamis, cyclones and hurricanes, mangrove forests provide society with a range of other 'ecological services'. These include preventing coastal erosion, protecting coral reefs from silting up, and providing a source of timber, food and traditional medicines.


Tsunamis are often not taller than normal wind waves, but they are much more dangerous. Wind waves (top diagram) come and go without flooding higher areas while a tsunami (bottom diagram) runs quickly over the land as a wall of water. Even a tsunami that looks small can be dangerous.

Tsunamis
Tsunamis are giant sea waves that are produced by submarine earthquake or slope collapse into the seabed. Tsunamis can travel thousands of miles at 300-600mph with very little loss of energy. They reach the coast with devastating impact on shoreline communities. Successive crests can arrive at intervals of every 10 to 45 minutes and wreak destruction for several hours.

Factors of vulnerability include:
 • Settlement located in low-lying coastal areas (direct impact);
• Lack of tsunami resistant building;
• Lack of timely warning system and evacuation plan.

Main causes of morbidity and mortality:
Deaths occur principally by drowning. Injuries and trauma are few and occur from battering by debris. Short and long term mental health effects are observed.