Tsunami mitigation by mangroves and coastal forests

The role of mangroves in providing coastal protection against the actions of waves, wind and water currents in general, is well known. But the extent to which mangrove green belts contribute to saving lives against large tsunamis, such as the recent one in Asia, depends on a number of factors including the height and velocity of the tsunami, the topography and orientation of the coastline, the width of the forest and - to a lesser extent - the height, density and species composition of the forest.

As widely reported, extensive areas of mangroves can reduce the loss of life and damage caused by tsunamis by taking the first brunt of the impact and by dissipating the energy of the wave as it passes through the mangrove area. On the other hand, narrow strips of mangroves, when uprooted or snapped off at mid-trunk and swept inland, can cause extensive property damage and loss of life.

Examples of the positive effects of mangroves and other coastal forests (Casuarina, Pandanus, coconuts etc) in absorbing or reducing the strength of the recent tsunami include the following:

• In Tamil Nadu, India, the trees standing in the front lines were damaged but well established forests such as the Pichavaram mangrove forest acted as a protective belt slowing down the waves and protecting around 1 700 people living in hamlets built inland between 100 to 1 000 meters from the mangroves.
• In Malaysia, in areas where the mangrove forests were intact, there was reduced damage, as reported by the Penang Inshore Fishermen Welfare Association.
• In Indonesia, the death toll in the island of Simeuleu, located close to the epicentre was relatively low, partly due to mangrove forests that surrounded the island.
• Large mangrove forests mitigated the force of the tsunami in Phang Nga province (Thailand) where the inland territories were only slightly damaged.
• Low impact was observed behind a 100m mangrove strip in Medilla (Sri Lanka, RUK area), behind an elevated beach with coconuts and behind a tall sand dune stabilized by a Casuarina plantation.

Similar examples exist of the positive role played by mangroves and coastal forests as buffer zones during past natural hazards, such as the 1999 cyclone in the eastern coastal state of Orissa (India); the 1991 cyclone and tidal wave in Bangladesh; and the typhoon Wukong in Viet Nam in 2000.

On the other hand, mangroves and other coastal trees were uprooted and snapped off at mid trunk level and caused extensive damage when swept inland by a tidal wave in PNG in 1998.