This study explored the effects of coastal vegetation on tsunami damage based on field observations carried out after the Indian Ocean tsunami on 26 December 2004. Study locations covered about 250 km (19 locations) on the southern coast of Sri Lanka and about 200 km (29 locations) on the Andaman coast of Thailand. The representative vegetation was classified into six types according to their habitat and the stand structures of the trees. The impact of vegetation structure on drag forces was analyzed using the observed characteristics of the tree species. The drag coefficient, including the vertical stand structures of trees, C d-all , and the vegetation thickness (cumulative trunk diameter of vegetation in the tsunami direction) per unit area, dN u (d: reference diameter of trees, N u : number of trees per unit area), varied greatly with the species classification. Based on the field survey and data analysis, Rhizophora apiculata and Rhizophora mucronata (hereafter R. apiculata-type), kinds of mangroves, and Pandanus odoratissimus, a representative tree that grows in beach sand, were found to be especially effective in providing protection from tsunami damage due to their complex aerial root structure. Two layers of vegetation in the vertical direction with P. odoratissimus and Casuarina equisetifolia and a horizontal vegetation structure of small and large diameter trees were also important for increasing drag and trapping floating objects, broken branches, houses, and people. The vertical structure also provided an effective soft landing for people washed up by the tsunami or for escaping when the tsunami waves hit, although its dN u is not large compared with R. apiculata-type and P. odoratissimus. In addition, the creeks inside mangroves and the gaps inside C. equisetifolia vegetation are assumed to be effective for retarding tsunami waves. This information should be considered in future coastal landscape planning, rehabilitation, and coastal resource management.

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Three units of free water surface (FWS) constructed wetlands treating domestic wastewater under tropical conditions were examined in terms of water quality and biomass characteristics. One unit (L2) was planted with Scirpus grossus, one with Typha angustifolia (L3), and the unplanted third (L1) served as control. Influent and effluent quality parameters: biological oxygen demand (BOD(5)), nitrate (NO(3)(-)-N), ammonium (NH(4)(+)-N), phosphorus (P), total suspended solids (TSS) and fecal coliforms were regularly measured. The average BOD(5) reductions were 37.0%, 58.5%, and 53.8% for units L1, L2, and L3, respectively. The planted units removed pollutants more effectively although there was no significant difference between the Scirpus grossus and Typha angustifolia units. Plant growth was monitored in marked quadrats by measuring shoot height and other growth parameters. The above-ground organs in L2 and L3 was harvested whenever the shoots reached maximum shoot height and formed flowers. Scirpus grossus had sustainable above-ground biomass production but Typha angustifolia could not sustain repeated harvestings with the above-ground biomass production declining significantly following four consecutive harvests.

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