The scale of the 26 December 2004 Indian Ocean tsunami was almost unprecedented. In areas with the maximum tsunami intensity, little could have prevented catastrophic coastal destruction. Further away, however, areas with coastal tree vegetation were markedly less damaged than areas without. Mangrove forests are the most important coastal tree vegetation in the area and are one of the world’s most threatened tropical ecosystems
Measurement of wave forces and modeling of fluid dynamics suggest that tree vegetation may shield coastlines from tsunami damage by reducing wave amplitude and energy. Analytical models show that 30 trees per 100 m2 in a 100-m wide belt may reduce the maximum tsunami flow pressure by more than 90%. Empirical and field-based evidence is limited, however.
Cuddalore District in Tamil Nadu, India, provides a unique experimental setting to test the benefits of coastal tree vegetation in reducing coastal destruction by tsunamis. Cuddalore has a relatively straight shoreline, a fairly uniform beach profile, and a homogenous continental slope. Moreover, the shoreline comprises vegetated as well as non-vegetated areas and was documented by cloudfree pre- and post-tsunami satellite images.
The force of the tsunami impact in Cuddalore is illustrated by the central part of our study area (see Figure attached). At the river mouth, the tsunami completely destroyed parts of a village and removed a sand spit that formerly blocked the river. However, areas with mangroves and tree shelterbelts were significantly less damaged than other areas (supporting online text). Damage to villages also varied markedly. In the north, stands of mangroves had five associated villages, two on the coast and three behind the mangrove. The villages on the coast were completely destroyed, whereas those behind the mangrove suffered no destruction even though the waves damaged areas unshielded by vegetation north and south of these villages. In the south, the shore is lined with Casuarina plantations. Five villages are located within these plantations and all experienced only partial damage. The plantations were undamaged except for rows of 5 to 10 trees nearest to the shore, which were uprooted.
Our results suggest that mangroves and Casuarina plantations attenuated tsunami-induced waves and protected shorelines against damage. Human activities reduced the area of mangroves by 26% in the five countries most affected by the tsunami, from 5.7 to 4.2 million ha, between 1980 and 2000. Conserving or replanting coastal mangroves and greenbelts should buffer communities from future tsunami events. Mangroves also enhance fisheries and forestry production. These benefits are not found in artificial coastal protection structures. Coastal tree vegetation can be established for investments of U.S. /150 to U.S./2000 per ha. Mangroves, however, are suitable for planting only on coastal mudflats and lagoons, which cover ~25% of the continental coastline of the Bay of Bengal. Elsewhere, the conservation of dune ecosystems or green belts of other tree species, such as Casuarina, could fulfill the same protective role.
SCIENCE VOL 310 28 OCTOBER 2005 643 on May 16, 2008. www.sciencemag.org